Welcome, Gellman. Your story reminds me of the time, way back in the 1950's, when my mother "cleaned up" my room and burned all my comic books while I was away at camp. As an adult, I became a comic book collector (probably a response to the trauma) and had to spend hundreds (thousands?) of dollars to reacquire some of the books she burned. When people don't share your enthusiasm for a hobby, they sure can cause you a lot of grief !!
Hi Doug, The guys over at gearslutz pointed me to your excellent video. I have one of these amps that has been sitting in my house for the last 35+ years. My stepfather bought it for me when I was 15. I'm 60 now, he's 95. Loved hearing about the history behind it and clearly worth keeping. Thank you so much for sharing!
You're a lucky guy, L47, and your stepfather had great taste in amps. It's nice to hear about one-owner amps of any sort, let alone a great one like the PR. I hope you continue to enjoy it for many more years :)
....that our amps were made in accordance with the AA-764 schematic, while the majority of PR's were made in accordance with the AA-1164 schematic, containing a few minor "upgrades" (as described above). Another possibility is that the tube charts were ordered and installed haphazardly, as happened so often in the Fender shop, because they never realized that some day a bunch of fanatics (i.e. us) would analyze every detail of their actions. As far as rarity goes, there must be far fewer......
.....of the AA-764 PR's, simply because large, elaborate Internet sites (totally dedicated to fanatical evaluation of every Fender amp model and serial number) do not seem to mention their existence. Yet a bunch of us have them. It's a real enigma and can drive you nuts if you let it.....and I generally do. Thanks for your interest and inquiry, Dave.
I just found this video. Thank you so much for clarifying this. I have been trying to figure it out. Great help and video. Not dry at all. Just Great video...
Yes, indeed, Dave. I didn't even mention the GZ-34 vs 5U4GB rectifier difference, which is significant. The GZ-34 is far superior, IMHO. Thanks again for your input.
Aloha, Uncle Doug. This puzzle solver has finally come in firsthand handy for me, as my good buddy has gifted to me a ‘67 PR that features the AA764 chart. He’ll be hand delivering it to me in a few weeks and I may have additional questions for you in the near future, depending on what we find inside. Chances are you already provided all requisite info in your previous videos, so I’ll be sure to perform my due diligence before knocking on the door. But if needed, I may drop a note via Patreon message, if that’s an option (I’m not quite sure). Anyway, I just wanted to thank you for all that you do and have done to share your knowledge with us over the years. You are tops, sir ✌️👌🤙
Greetings, D. What a wonderful gift !! For a Patreon patron, I would be glad to offer additional advice, if the videos do not answer all your questions.
My 1962 BIRD GOLDEN EAGLE 4/25 uses the GZ34 rectifier. It's was one of the best amplifiers in England all through the 60-70's. 25 Watts can be VERY loud.
Doug. I much appreciate your clarification of the nomenclature for Fender amps here. I really did think the AA764 type of nomenclature designated the design. And I do see now that ... that - plus the amp type - does in fact designate the design based on amp type and date/revision of the design. It's always really good to understand source info !! Thanks again !!
.....and for throwing nothing away. Check the values I listed in my response to Dave Gray (1164 values are listed second) and check to see whether you have a GZ34 (764) or a 5U4 (1164) rectifier. As far as value, I wouldn't think the outdated tube chart would have much effect. My VG Price Guide shows a range of $875 to $1150 for Silverface PR's depending on age and condition. Thanks for your inquiry, Clint.
I have one of these AA764 Princeton reverbs. Thanks for clearing up a little confusion when I was recently trying to date the amp. the only listing I could find were for AA1164
Thank you, Uncle Doug! It took me a while to find you, but this cleared up my dilemma in dating my (fairly rare, I guess) '65 Princeton Reverb-amp AA764 design rev, and provided a valuable schematic to take to my amp tech.
You're welcome, Bill. I hope the information was useful and of interest. I have posted a similar video regarding the Gibson BR-9/GA-9 which may also be of interest.
I've worked on several of these and noticed much the same about schematics, so I drew my own from one listed as AA764. I noticed that there were several minor variations in the circuitry, so every one I touch turns in that circuit. FWIW, twice I have seen them with 5U4 rectifiers and shorted primary windings on the power transformer. This causes a change in turns ratio and an increase in secondary voltage output. The one I now use myself when I finally figured it out had 585 DC volts, blowing ALL, and I mean ALL the capacitors. Most of these amps had 2 wire power cords, so I used the ground polarity switch for a stand by. I also forgo tube rectifiers entirely and use silicon rectifiers ... pretty easy to put two of those in an old octal tube base.
Gary: I just checked a site called The Fender Amp Field Guide and it has a very detailed section on the silverface Pro Reverb, including all the cabinet dimensions. Again, I can't send you a link to it, but it's readily available on the Internet.
Thank you!!! I just bought one of these mystical AA764 PR's. I was surprised to see it still had the original iron in it. Mine has an old Mullard GZ34 in it and I'm guessing that may have saved the original transformers from certain death. Thanks for the info!
You're quite welcome. Yes, it's just a date, not a model designation. You're lucky to have both the original TF's and a Mullard GZ34.....my favorite rectifier :) The PT could probably handle a 5U4.....but the word "probably" always interferes with my ability to sleep well at night. Again, congratulations on a great acquisition.
Just purchased a 1970 princeton reverb. Amp is untouched, all original inside the chassis. Still has 2 prong cord. Transformer date codes are late 1969. Pots are 69 and 70. Serial number 1970. Tube chart says AA764 and has the gz34 shown. Amp came with a 5u4gb rectifier in it, not to say someone didn't change the tubes out as it had two different brand 6v6's. Frustrating not to know what is correct here. If the tube chart was from some other amp as some have said, what amp would that be? I have a 1969 champ aa764 and the the reddish brown caps are the same in the princeton, not the blue molded ones.
I had a 1968 Fender Princeton Reverb head. I used to by-pass the reverb tank and use that extra tube pre-amp as a monstrous distortion / overdrive. It sounded great. The tubes would brightly glow purple. Was that safe to do?
Hi Uncle Doug, My 1965 Vibrolux Reverb has a tube chart with AA864 on it. I have never been able to find this schematic.... I wonder if you have ever seen one? It has the GZ 34 rectifier tube. Rgds!
Gary: I have never personally owned a Pro Reverb and have no access to one, but I did notice an old Craigslist ad from Santa Fe, NM for one. I tried to send you a link to it, but YT won't allow it. You could contact him and, if it's not sold already, perhaps he could provide accurate measurements of the cabinet. At the very least, the ad has some good photographs which may be of assistance. Also, you could try this tactic with other CL and Ebay sellers.
Greetings, Dave. According to conventional dating charts, your amp was made in 1966 (one year older than mine), and should have a AA1164 (Nov, '64) tube chart and chassis......yet it, like mine, has a AA764 (July, '64) tube chart. If you look at the 764 and 1164 schematics side-by-side, there are several minor differences: PV on the 12AT7 (355V vs 400V), cap between Vibrato pedal and 12AX7 plate (.02uf vs .01uf), electrolytic cap just after the biasing diode (50uf vs 25uf), etc. My guess is...
Here's another twist on the rectifier confusion. I have a '67 Princeton Reverb with the AA764 circuit and the tube chart says GZ34 but someone hand wrote in pencil 5AU4. The tube that is installed is a 5AU4. I'm sure that it is all factory original including the pencil change because I acquired the amp from the original owner 40 years ago, and he barely used it as a kid. Based on what Uncle Doug says, I'm not sure if I should leave the stock 5AU4 in there or switch it out for the highly praised GZ34. What do you think?
Are you sure that that tube isn't a 5AR4? Because a 5AR4 is simply another name for a GZ34, they are the exact same thing. I think one is a British designation and the other is the American designation, I just can't remember which is which.
Greetings, NYC. The GZ34 filament only draws 1.9A while the 5U4 draws 3A, so the 5U4 would be a 58% greater current draw on the 5V winding of the PT. Most PT's have some leeway in their current ratings, so you could probably get away with it. The GZ34 is more efficient that the 5U4, so the 5U4 would indeed be a little "saggier" at lower volumes. Here is a link to a great site and discussion of your amp: fenderguru.com/amps/super-reverb He mentions the rectifier switch.
What do you think about what the author of the tube guitar amp guide book thing said? He said that the standby awitch is actually a superstition made up because fender started to use them. The electrons before warming up don't harm the tubes.
I agree that in the vast majority of guitar amps, the Standby Switch is really not helpful. Standby Switches originated in radio transmitters, where plate voltages are greater than 1000V, to preheat the cathodes and reduce "cathode stripping"; but in guitar amps, with plate voltages generally less than 500V, they are not needed. Also, since most Standby Switches are in the DC plate voltage circuit, they tend to arc and wear out quickly, and if you have a tube rectifier (i.e. GZ34), it provides a "built-in" delay in high voltage that protects the tubes. Lots of knowledgeable people recommend not using the Standby Switch at all in guitar amps, and from a technical standpoint, they are probably correct.
In the 5F6-A Bassman, Fender introduced a DC coupled cathode follower into a solid state rectified amp. At switch-on, before the tube begins to draw current, the grid of the cathode follower is at B+ voltage while the cathode is at ground potential, which can lead to arcing between grid and cathode, often destroying the tube. It's possible that the standby switch was introduced as a fix for this problem, although a neon bulb between grid and cathode would have been a solution which did not rely on the user to follow the correct procedure when powering up the amp. The Valve Wizard recommends a reliability mod for DC coupled cathode followers: valvewizard.co.uk/dccf.html
Hi Uncle Dough. I'm going to take on a quite deep restoration of my Fender Princeton AA764 version. Need to do a full recap of all the electrolitics, including the multitap can-type one, the power transformer and the rectifier valve, which in my case has been replaced by a full wave diode rectifier bridge. I'm thinking of using a Hammond made 291AEX power transformer as replacement, as I live in Italy ad we run our mains @230V AC. As per rectifier , after watching your video, I think I should go with a GZ34. Do you suggest any particular brand/type? Also here in Italy it is quite difficult to get a replacement for the filter cap. I wouldn't like to use external caps and I'd like to retain original look as much as I can. I saw on other videos you are using caps made by Ge Distribution but they are quite expensive to get, with shipping and import taxes. Any suggestion you could give to me is very welcome. Thank you
A full wave bridge rectifier requires a special power transformer without a center-tapped high voltage winding. You would be better off installing a proper ClassicTone 40-18019 Fender Princeton power transformer with a GZ-34 rectifier tube. They also have international models with 110VAC and 220VAC primary windings.....check their site. You can retain the capacitor can on the outside of the chassis and then replace all the capacitors with individual axial electrolytics hidden within the chassis. Good luck.
@@UncleDoug Thank for your answer, Sir. As per capacitor, I know schematic requires a 4 x 20uF can cap. Would a 20/20/20/30 be a better option, even for cosmetic reasons? Thank you
@@dreamlessfull I'm not sure why the latter would be more "cosmetic" than the 4 x 20uF, but yes, you can use a somewhat higher value cap in the first position of the power supply chain.
@@UncleDougtrue, I could use 4 electrolitics and keep the disconnected can cap, but I'd like to keep the number of modern components inside the chassis at minimum Thank for your advice, I've learned a lot from your vids
Have you heard the Morgan PR12? It’s one of the best sounding amps I’ve heard. It is apparently based on the Princeton Reverb, but with a 12 inch speaker, and modified to provide better bass, and it appears to be solid state rectified because I looked on the website at the specs and there are only 3 12ax7s and 2 6v6s... no mention of a rectifier tube. How hard would it be to modify one like that uncle Doug?
Thanks, Uncle Doug, very helpful information to have. I wound up here while trying to research the best tube replacements to buy and had no idea my AA764 was the subject of such scandal! A couple of questions if you have a moment: My tube chart calls for a single "7056" (or "7058", hard to tell) and that differs from the BlueGlow schematic, which mentions a 7025. Any thoughts? Next (and apologies if you've already mentioned this elsewhere), may I ask what brand of tubes you would purchase as replacements? Thanks for considering, and I appreciate your time. Jake
Uncle Doug, Thanks for the reply. I did some more looking and I think it must be a 7058. Apparently they do wind up in Fender amps from time to time. Anyway, this number is from the tube chart itself, so I think it's legit (even though I had some trouble reading it initially, I'm positive it doesn't say 7025). Interesting...
I have to point out that with the link you gave, right on the schematic image there's a notice that says this AA764 schematic was photoshopped from an AA1164 schematic to match the AA764 Princeton Reverb amp. And it goes on to say that there is no original AA764 Princeton Reverb schematic to their knowledge. Anyways, I think your video's are awesome. I've watched many of them multiple times.
Greetings, AA, and thanks for your very kind appraisal of my YT channel. This statement was added to the schematic in the past few months. As you can see from the video at 2:39, no such statement was evident at the time of the video. It does explain, however, why this appears to be the only source for a AA764 schematic. It also would appear to confirm that the AA764 chassis did exist, with or without an available schematic. Thanks for bringing this to my attention. I will amend the video description to reflect this new information.
Uncle Doug No problem Doug. Glad I could help. That's strange that there doesn't seem to be a true schematic for it. I wonder what the story is behind that.
It appears that the AA764 circuit was only utilized for about 4 months before it was (slightly) altered to the AA1164 form. Maybe only a very few copies of the earlier schematic were ever made during this brief period, and none have yet been discovered and posted on the Internet. Somewhere, in a dusty file cabinet in Southern California, one is probably just waiting to be found......Let's hope :)
Hey Uncle Doug & Rusty, I hope your all doing well. Could you use a GZ34 in a Fender Champ amp, or is the 5Y3GT better suited for the Champ power requirements? The GZ34 sounds like the way to go if you can. I'm doing research for the Champ build. Thank you for all the great videos, time & effort. Your amps are absolutely beautiful and sound fantastic. Your craftsmanship is awesome. Take care, be good, C.
+Cass Virgillo You're welcome, Cass, and thanks for the very kind comments. Since most Fender Champs are factory biased at the ragged edge of spontaneous combustion, changing to a (much more efficient) GZ-34 might be like throwing gasoline on a smoldering log......but, if you were to quickly adjust the bias to a reasonable level (and checked all circuit voltages) after the substitution, it would probably be a good, anti-sag, PT-current-reducing, move. Let us know what happens.
Bbendfender Your power transformer will love it, due to reduced current draw, Bb, but your output tube plate dissipation will go up due to increased plate voltage. You'll probably have to re-bias them. Also, the amp will become less "saggy", with current responsiveness more like solid state rectification.
It's a fairly simple procedure, David. Simply increase the value of one or more of the capacitors in the oscillation loop.....i.e. if the cap value is .02uf, then substitute a .03uf. Higher value caps charge and discharge more slowly, thus slowing down the tremolo speed.
Uncle Doug. Thanks for the vids. I'm learning a lot. I have a 1974 Silverface Princeton Non-Reverb. Can I use a GZ34 instead of the 5U4Gb? The amp has very low power. I was hoping this would help. What else can I check/try to increase volume/power? Thanks!
You're welcome, Diane. Yes, you can replace the 5U4 with a GZ34 and the result will be higher plate voltage and possibly higher plate dissipation, so you should re-bias the amp. It's possible that your 6V6 output tubes are showing their age and getting weak. If you have never replaced them, it might be time to do so.....and another good reason to re-bias (necessary with the new tubes). Good luck.
blueglow link now states that AA764 Princeton Reverb schematic was Photoshopped. I provided a scan of my original AA1164 Princeton Reverb schematic with GZ34 rectifier to a website called Schematic Heaven years ago. The original in my possession was mailed to me directly from Fender in the 1960s. As far as I know, any copy of the GZ34 version of the Princeton Reverb schematic has been disseminated from that source. Free use was assumed and welcomed by me. Enjoy !
I have a 1967 Princeton Date code QD April 67. Under indicator bulb is only the number 68..no other text...most say Patent date or other...never seen this on other..is this rare?
Dear Sir, first off, it´s always a pleasure watching your videos, I´ve learnt a lot from them. I would like to know if there is a way of getting in contact with you. I would like to discuss a Fender Princeton Amp/no reverb. Best regards, Roger
Greetings, Roger. Much as I would like to provide personal consultations, the large bulk of correspondence I receive does not allow for it. Please feel free to post your questions here and perhaps I can help.
hi, thanks for posting this... its really interesting and helpfull... I am very confused though, as my PR tube chart states it is a AA764 circuit, but it shows a 5U4 rectifier tube... any hints on this? which schematic should I use... also, which power and output transformer replacements would you recommend to restore and improve these amps???
You're welcome, Sergio. Normally the AA1164 chassis uses a 5U4 and the AA764 uses a GZ34. If the tube chart in your amp says "AA764" and shows a diagram of a "5U4" rectifier, then I would go with the tube chart and keep the 5U4. Other than the rectifier tube, there is very little difference (2 capacitor values) between the two circuits, so it really doesn't matter which one you use. I would not suggest replacing either transformer unless the original has an open winding (i.e. it's "burned out"). If you do need a replacement PT or OT, then check Triode USA: store.triodestore.com/transformers.html They have good quality transformers at very reasonable prices. Good luck !!
It's impossible to make any absolute statements about the circuits at that time, Doug. I have seen all sorts of circuit variations, alterations, and combinations....probably depending on what parts were available on the day of manufacture.
Uncle Doug, today I acquired a 1968 Princeton Reverb (in near mint condition!) with the AA764 tube chart. The amp sounds great but there is one issue. The vibrato, or tremolo, is weak. Here is what's really odd: If you turn the amp on and strike a chord, the vibrato sounds great. But then the effect fades and it's hard to tell it's even on. What would cause the vibrato to sound strong only when the amp is initially fired up? Thanks!
There are two portions of the tremolo circuit which could be responsible, Cal. The three capacitors in the oscillation loop could be out of spec, and/or the oscillator tube (half of a 12AX7) could be failing. The tube is the easiest to check, simply replace it and see if the problem subsides.
Thanks for your video! I have a 1970 Fender Princeton Reverb and my tube chart AA764 that is in good shape shows a 5U4 rectifier tube... It had the Torres bias mod done to it and recapped the tech gave me a solid state rectifier plug and a 5U4 tube. I used it with the SS one for years and recently plugged an old hammond 5U4 into the amp and it got really distorted. Not in a bad way it just had a lot more gain when turned up loud. I read below that you told someone else to follow the chart, but after watching your video I was just wondering what you thought... Thanks!
Greetings, Tim. It seems strange that a 5U4 could outdo a SS rectifier.....generally, it's the other way around. My advice would be to go with whatever works best for you. Perhaps for practicing at home, the SS would be best.....while you might want the additional volume and saturation for a public performance. If you really want some additional gain, try a GZ34 rectifier. Best of luck.
Doug, thanks so much for the journey through these little gems as well as continuing to mentor a whole boatload of us out here. A little '65 BF Champ has rekindled my interest in a PR, although it'll end up having to be the reissue; good ones are probably pretty spendy. (The PR still is a wonderful little 1x10 2x 6V6 package that can't be beat.) I really appreciate your discussion of the different rectifiers; for someone who's sensitive to it the difference can be noticed in the playing. I see that the reissue is using a 5AR4 instead of the 5U4 from the AA1164 circuit, probably a good thing. However, do you have any sense as to whether the inefficient 5U4 may have been used specifically because it has such a difference in rise time (or something) and, perhaps, is more touch-sensitive? Just curious because the dynamics of my little Champ (with its 5Y3) have me a bit spoiled that way. Thanks!
Greetings, Wes, and thanks so much for your kind words :) I tend to rank the 5U4 at the bottom of the rectifier pile. It's a current hog, not particularly efficient, and rather saggy. To be honest, I think they used it because it was readily available and relatively inexpensive. If possible I convert to GZ34, but the high output combined with high 120+VAC receptacle voltage can yield some scary plate voltages. The 5Y3 is in the middle for me.....ideal for all rather low wattage circuits like your little Champ. The GZ34/5AR4 is at the top......low current draw, relatively high output (high efficiency), and very slow start up due to the indirectly heated cathode, which really babies the amplification tubes. You really don't need a Standby switch any more. The high efficiency yields minimal sag, almost like diode rectification. For those who appreciate sag, however, I guess the 5U4 would be the way to go.
Uncle Doug One other thing on the misconception you correctly pointed out regarding whether the schematic number relates to a specific circuit. The 764 can't be the circuit for the tone stack for both the Champ and the PR because the resistors bound to the bass pots have vastly different tonal results - 15k (Champ, punchy) and 6.8k (PR, sweet, kinda scooped). Would Rusty agree the solution is that we should always have one of each? :)
Rusty votes for a third alternative: moist and meaty....like a T-Bone steak :) Thanks for the input, Wes. For more confusion, how about the "Design and Circuits Patented 68" on the control panel of BF PR's? I'm sure this refers to some obscure legal action or filing number, but it sure looks strange :)
I used the link for the Schematic - and it says it's a photo-shopped conversion of the AA1164 , so maybe your initial comments were correct. The PRRev. AA764 never existed - maybe it was the stamp they used that hadn't been changed.
Strangely enough, the statement about it being photo-shopped was added AFTER I posted the video and link. I have copies of the AA764 schematic both ways, with and without the notice. The addition of this notice does indeed cast doubt on the actual existence of any legitimate AA764 schematic......or chassis.
@@UncleDoug But it still remains that the Tube Chart with the stamp AA764 shows the GZ34 and the AA1164 Tube Chart has the 5U4GB. Peculiar to say the least :-)
@@UncleDoug Maybe even more peculiar is this SF PR on Reverb - 1968 :-) reverb.com/item/7825642-vintage-1968-fender-princeton-reverb-silver-face-tube-amplifier-aa764-circuit-a-true-sonic-gem
Hi Uncle Doug. I have a '66 with the AA764 chart, and it came to me with a 5U4GB rectifier. I'm considering a swap to the GZ34. Before I do, can you tell me how the differences in the AA1164 circuit address the potentially harmful characteristics of the 5U4GB rectifier if used in the AA764 circuit? Many thanks. Great videos.
Greetings, Jonathan. I don't believe that Fender made any modifications for the 5U4 rectifier in the AA1164 chassis. In the schematics, it appears that both chassis use the same power transformer, even though the 5U4 draws over 50% more filament current than the GZ34. I assume that the 5V winding of this PT is rated at 3A, so it is capable of handling the 5U4 and would therefore be way under-stressed when dealing with the GZ34 at only 1.8A. Your AA764 chassis would probably be OK with a 5U4. It's a common substitution, since 5U4's tend to be more readily available and cheaper than GZ34's, but in addition to drawing more filament current, the 5U4 is much less efficient than the GZ34, provides lower plate voltages, has more "sag", and does not provide slow warm up to protect your tubes. Bottom line: the GZ34 is a superior rectifier in all respects.
This *is* interesting. I had asked you for a link to a good readable copy of an AA1164 Princeton reverb, and you sent one from I think TheVintageSound web site. My brother found another one, that says "AA1164 Princeton Reverb" on top, but it has a GZ34 rectifier tube and the power transformer is 340 0 340v and a spot check, the other components look the same as well.
Variations exist in schematics. The GZ34 rectifier is generally found in the AA764 circuit, but can be used in the AA1164 as well. The GZ34 is a more efficient rectifier, so plate voltages will be higher (output tubes must be rebiased if rectifiers are changed). It offers a slower warmup, to protect the tubes, and it has lower current demands from the power transformer. It is overall quite superior to the 5U4.
Thanks for the info, Uncle Doug. Glad I somehow "accidented" my way to the GZ34 version of the schematic. Wow much better tube! As an aside, a little worried that the schematic I have might have a fatal error/typ'o (short or missing component or value is wrong) but maybe I can find a different source for the schematic and compare component values and placement.
If I had not found your youtube channel, and all the great and detailed information on projects you've worked on, and tutorials, I don't think I would start an amp building project. Thanks!
+Ilya Shadrin If your PR is lacking reverb intensity, check (or replace) the 12AT7 driver tube, be sure that the cathode bypass caps on the 12AT7 and 12AX7 are in good shape, check (or replace) the 12AX7 recovery tube, and be sure the tank transducers are in good shape. The only reason for a PR to be lacking reverb intensity is if some component(s) is/are old, weak, or failing. Good luck.
Greetings, Clint. If you have a 1970's PR with a AA-764 tube chart (and circuit) then it really adds to the mystery surrounding this issue. If the circuit was modified in Nov. of '64 (1164), why would the factory still be installing the long-outdated 764 charts and circuits (?) in 70's amps? I have a strong feeling that your circuit matches the 1164 schematic and the tube chart is an old one they had laying around the shop. Fender was famous for haphazard use of whatever was handy.......
Actually it's my workshop guitar, Marstyn, a 1979 Matsumoku Vantage Performer with MMK-45 humbucker pickups. The guitar output and tone controls are usually set on the high end......8 or 9. The amp was probably at Vol 4, Treb & Bass 5, Rev 0, Sp 5, Int 5.....I'm just guessing, since it was a long time ago, but this is probably correct.
Wow. Facts and data. Rare on the internet, and even more rare in amp discussions. I'm thinking of building a Princeton Reverb, as it seems to be the Fender amp with the most headroom that will not blow out a Celestion Blue. In your opinion, was the PR AA1164 circuit an improvement over the AA764, or should I go with the original? Thanks.
Thanks, Jeff. I prefer the AA764, because it uses the GZ34 rectifier, which is vastly superior to the 5U4. Other than that, there are very few differences between the two circuits other than a couple of capacitor values. Good luck with your project :)
Jeff, the AB763 chassis of the Deluxe Reverb came with a GZ34 rectifier, so it would probably work in other versions of the DR (i.e. AB868). One thing to watch is the plate voltage will go up with the GZ34, and you would have to rebias the output tubes.
I have a Princeton Reverb Amp, Listed as AA764. The tube chart is as follows GZ34, Two 6v6GT, two 12AX7, 1 12AT7 and 1 7025. However the tube slot contains a 12AX7 instead of the 12 AT7. is the 12AX7 a substitute for the 12AT7? I notice the amp is now getting hot and the transformer may be going out. any comment is appreciated.
Hi Doug- looking at the schematic you posted (thanks!) I see there is a notice that says "This drawing was photoshopped from a scan of an original AA1164 schematic. The only details change were the AA764 designation, the bias capacitor from its original 25u 50V value and one tremolo capacitor from 0.01 to 0.02. To my knowledge there is no known example of an original AA764 Princeton Reverb schematic." I did not see this disclaimer on your printout in the video, though (must have been updated later on?). I assume the photoshopped schematic is valid/ correct, but why is this such a difficult schematic to find? Thanks for all the videos
I see no such notice on either of the AA764 schematics I have on file, KP, but do acknowledge that this is a virtually impossible schematic to find.....although the circuit itself is not at all uncommon. It continues to be a mystery to which I have found no final, satisfactory answer.
So what's the business with people including amp makers today saying that the standby switch is just a remnant from the past and not really needed? Has the common way to design amps changed? Is it because of the rectifier diodes still shooting the full load on the tubes once the standby switch is turned? Something else?
Hey Doug. I have a Princeton Reverb Silverface with a AA764 tube chart and the gz34 rectifier specified on the label, yet i thought this schematic only came in blackface Princetons. Can you give me some info? Thank you.
Greetings, JD. According to the Fender Guru, your Silverface 1969 PR should have an AB1270 chassis with a 5U4 rectifier: fenderguru.com/amps/princeton-reverb However, I have seen all sorts of variations in chassis and circuits during this period. CBS had just taken over and it appears that they were probably using whatever they had on hand to build amps.....so a left-over AA764 chassis (with GZ34 rectifier) may have been installed in a 1969 Silverface cabinet. Other than the rectifier, the two circuits are virtually identical (2 small cap differences) so it's really not a huge issue. One other explanation might be that you do have an AB1270 chassis but they put the wrong tube chart in the cabinet, and someone followed the tube chart when they replaced the rectifier and installed a GZ34. If you really want to be sure, download both schematics and see which one matches your circuit. As you will see, other than the 2 small caps, they are virtually identical. Good luck.
Thanks so much for the quick response Doug! Actually with further research it turns out it is actually a 7th week of 68 amp.. i read it wrong.. And it has the aluminum around the mesh, which according to fender guru makes it a 68 silver. Would that make it more normal that it would have the aa764 circuit? Thanks again, you got my sub
I think it is not only possible but quite likely that an early '68 Silverface would have a chassis from the previous year's BF model. Fender probably had a bunch of AA764 chassis left over from the Blackface years, and it would have been financially unwise not to use them up. After all, if you compare schematics, there is virtually no difference between the BF and SF chassis....so why not use them? This use of BF parts in the "Drip Edge" '68 Silverfaces (like yours) is why they command higher prices than the later Post-CBS SF models. You're welcome, JD, and thanks for subscribing :)
Uncle Doug 1f you work on enough Fender products you see all kinds of overlaps in the styling and little cosmetic details. I recently acquired a late 60's silver face Bandmaster head, thinking that I'd do the black face conversion on it and have a righteous amp for low bucks. then when I looked inside, lo and behold, it was an AB763 circuit! Now all I have to do is replace the caps, throw in some fresh tubes, and re-bias it. WOOHOO! YEAH BABY SCORE!!!
The schematic that you link to CLEARLY states right on it that it’s been photoshopped to say aa764 and added in a gz34. There is no such circuit. Maybe you could update this to avoid further confusion?
The schematic upon which the video is based has no such disclaimer, and I made the justifiable assumption that it was legitimate. Later, it became evident that the schematic had indeed been photoshopped, as clearly stated on the schematic which is linked in the video description. To avoid further confusion, I will add a statement to the video description explaining the situation.
I believe the PR is a Class AB amp, double-ended and grid-biased. Class A amps are most often single-ended and cathode biased.....like the Fender Champ.
There are Internet sites that provide accurate dating info on Fender amps based on the chassis numbers, Joe. Also, if there is a tube chart in the cabinet, it should have a rubber-stamped two letter code that tells the year and month of manufacture. I have two of these PR's, 1966 and 1967. I can't recall which one was in the video.
The amp is really dirty to the point where you can not read the serial numbers. The tube chart has been damaged as well. I can read the top of the chart showing the AA764 circuit, but you have been very helpful thank you.
The serial numbers are usually engraved into the chassis rear wall, Joe. If dirt obscures them, then use some 409 on a rag, or even some fine steel wool to expose them. You're welcome. Best of luck.
I have a Silver face Princeton Reverb amp with a Tube chart Label that indicates the AA764. I note your description speaks specifically of the black face model. Serial number of my amp is A 19337 which suggests 1967 or 1968. Uncle Doug, I could send a pic of the tube chart label is you need it. contact me if you wish.
Thanks for the input, Pete. Common sense would dictate that the AA764 circuit should have been long since replaced by the AA1164 by the time the Silverfaces were made....and yet, as your tube chart indicates, apparently they were not. Is the rectifier tube in your amp a GZ34 or a 5U4? Another possibility is that they printed up a whole bunch of AA764 tube charts and then kept using them for several years, even with the newer circuits, until they finally ran out. Yet they never seemed to print out any schematics for the AA764. Very strange. I don't recall ever seeing a AA1164 tube chart in a PR.....has anyone else seen one? If so, in what year was the PR made ??
Uncle Doug unfortunately, I am electronically challanged! so unless you tell me how to check it out, I cant answer you. I checked out all the tubes but dont either of the markings. I did have the unit in a shop and they replaced some of the tubes but as I said, I dont know what to look for. obviously I dont read schematics well neither. lol
Uncle Doug I also have a 1969 Fender Princeton Reverb with the AA764 tube chart. It also has the GZ34 rectifier tube. I thought it may have been the wrong tube chart, so I pulled the tube and it's GZ34.
Another good example of the mysterious AA764 circuit.....no original schematic available and it continues to appear for several years after it was (apparently) replaced by the AA1164 circuit.
Leo didn't like it because it over taxed the power transformer. Iy was brought back during the early CBS years. Best thing to do?? Put in deluxe transformers and tap the unused cap in the cap can.
@@UncleDoug uhh not compleatly accurate about the cap can ,,, it could be a replacement can with 40, 20, 20, 20. The only thing that stops me from realising my amp building dreams is the wife!!!! She won't even allow me an electric guitar!! Ahhhrgggh. Can't live with em , can't kill em. ( I'm joking) I'd like to replace some if the furniture with amps!!! It would make the living room more pleasant!!!
Welcome, Gellman. Your story reminds me of the time, way back in the 1950's, when my mother "cleaned up" my room and burned all my comic books while I was away at camp. As an adult, I became a comic book collector (probably a response to the trauma) and had to spend hundreds (thousands?) of dollars to reacquire some of the books she burned. When people don't share your enthusiasm for a hobby, they sure can cause you a lot of grief !!
I have a 1966 Fender Princeton reverb that I purchased in 1978. It has the GZ34. I'm finally getting around to rebuilding it , Thanks Uncle!
Hi Doug,
The guys over at gearslutz pointed me to your excellent video. I have one of these amps that has been sitting in my house for the last 35+ years. My stepfather bought it for me when I was 15. I'm 60 now, he's 95. Loved hearing about the history behind it and clearly worth keeping. Thank you so much for sharing!
You're a lucky guy, L47, and your stepfather had great taste in amps. It's nice to hear about one-owner amps of any sort, let alone a great one like the PR. I hope you continue to enjoy it for many more years :)
Your videos are quickly becoming my "go to" videos
That's good to hear, AB :)
....that our amps were made in accordance with the AA-764 schematic, while the majority of PR's were made in accordance with the AA-1164 schematic, containing a few minor "upgrades" (as described above). Another possibility is that the tube charts were ordered and installed haphazardly, as happened so often in the Fender shop, because they never realized that some day a bunch of fanatics (i.e. us) would analyze every detail of their actions. As far as rarity goes, there must be far fewer......
.....of the AA-764 PR's, simply because large, elaborate Internet sites (totally dedicated to fanatical evaluation of every Fender amp model and serial number) do not seem to mention their existence. Yet a bunch of us have them. It's a real enigma and can drive you nuts if you let it.....and I generally do. Thanks for your interest and inquiry, Dave.
I just found this video. Thank you so much for clarifying this. I have been trying to figure it out. Great help and video. Not dry at all. Just Great video...
Thanks, NYG. You're welcome :)
Yes, indeed, Dave. I didn't even mention the GZ-34 vs 5U4GB rectifier difference, which is significant. The GZ-34 is far superior, IMHO. Thanks again for your input.
Aloha, Uncle Doug. This puzzle solver has finally come in firsthand handy for me, as my good buddy has gifted to me a ‘67 PR that features the AA764 chart. He’ll be hand delivering it to me in a few weeks and I may have additional questions for you in the near future, depending on what we find inside. Chances are you already provided all requisite info in your previous videos, so I’ll be sure to perform my due diligence before knocking on the door. But if needed, I may drop a note via Patreon message, if that’s an option (I’m not quite sure). Anyway, I just wanted to thank you for all that you do and have done to share your knowledge with us over the years. You are tops, sir ✌️👌🤙
Greetings, D. What a wonderful gift !! For a Patreon patron, I would be glad to offer additional advice, if the videos do not answer all your questions.
My 1962 BIRD GOLDEN EAGLE 4/25 uses the GZ34 rectifier. It's was one of the best amplifiers in England all through the 60-70's. 25 Watts can be VERY loud.
Thank goodness there are people out there willing to create and share schematics
Yes, indeed. It's rare when I can't find a particular schematic somewhere on the Internet.
Doug. I much appreciate your clarification of the nomenclature for Fender amps here. I really did think the AA764 type of nomenclature designated the design. And I do see now that ... that - plus the amp type - does in fact designate the design based on amp type and date/revision of the design. It's always really good to understand source info !! Thanks again !!
You're welcome, Donald.
.....and for throwing nothing away. Check the values I listed in my response to Dave Gray (1164 values are listed second) and check to see whether you have a GZ34 (764) or a 5U4 (1164) rectifier. As far as value, I wouldn't think the outdated tube chart would have much effect. My VG Price Guide shows a range of $875 to $1150 for Silverface PR's depending on age and condition. Thanks for your inquiry, Clint.
I have one of these AA764 Princeton reverbs. Thanks for clearing up a little confusion when I was recently trying to date the amp. the only listing I could find were for AA1164
You're welcome, Trem......glad the video was helpful.
Thank you, Uncle Doug! It took me a while to find you, but this cleared up my dilemma in dating my (fairly rare, I guess) '65 Princeton Reverb-amp AA764 design rev, and provided a valuable schematic to take to my amp tech.
Glad the video was helpful, Tim :)
thank Uncle Doug ! once again, your simple, clear explanation cleared up the same issue I was having with a build.
Glad to hear it, David :)
You're welcome, Bill. I hope the information was useful and of interest. I have posted a similar video regarding the Gibson BR-9/GA-9 which may also be of interest.
I've worked on several of these and noticed much the same about schematics, so I drew my own from one listed as AA764. I noticed that there were several minor variations in the circuitry, so every one I touch turns in that circuit. FWIW, twice I have seen them with 5U4 rectifiers and shorted primary windings on the power transformer. This causes a change in turns ratio and an increase in secondary voltage output. The one I now use myself when I finally figured it out had 585 DC volts, blowing ALL, and I mean ALL the capacitors. Most of these amps had 2 wire power cords, so I used the ground polarity switch for a stand by. I also forgo tube rectifiers entirely and use silicon rectifiers ... pretty easy to put two of those in an old octal tube base.
Thanks for your input, Mr. B.
a great and humble appreciation for your teachings - thank you!
Uncle Doug is simply the best!! If I lived close I would be arrested for stalking his every amp video!!
Thanks, James. You're welcome to stalk them from the comfort of your own home ;)
Gary: I just checked a site called The Fender Amp Field Guide and it has a very detailed section on the silverface Pro Reverb, including all the cabinet dimensions. Again, I can't send you a link to it, but it's readily available on the Internet.
Thank you!!! I just bought one of these mystical AA764 PR's. I was surprised to see it still had the original iron in it. Mine has an old Mullard GZ34 in it and I'm guessing that may have saved the original transformers from certain death. Thanks for the info!
You're quite welcome. Yes, it's just a date, not a model designation. You're lucky to have both the original TF's and a Mullard GZ34.....my favorite rectifier :) The PT could probably handle a 5U4.....but the word "probably" always interferes with my ability to sleep well at night. Again, congratulations on a great acquisition.
Saved me a lot of trouble… Thanks Uncle Doug 🙏🏻
Glad to hear it, David.
Just purchased a 1970 princeton reverb. Amp is untouched, all original inside the chassis. Still has 2 prong cord. Transformer date codes are late 1969. Pots are 69 and 70. Serial number 1970. Tube chart says AA764 and has the gz34 shown. Amp came with a 5u4gb rectifier in it, not to say someone didn't change the tubes out as it had two different brand 6v6's. Frustrating not to know what is correct here. If the tube chart was from some other amp as some have said, what amp would that be? I have a 1969 champ aa764 and the the reddish brown caps are the same in the princeton, not the blue molded ones.
The GZ34 is correct, but a 5U4 may have been installed to reduce the high plate voltage. have no idea about the caps.
The Tubestore.com has all the schematics from old to new including both versions of this amp.
I had a 1968 Fender Princeton Reverb head. I used to by-pass the reverb tank and use that extra tube pre-amp as a monstrous distortion / overdrive. It sounded great. The tubes would brightly glow purple. Was that safe to do?
It doesn't sound like a healthy situation, Mark. Did the tubes last very long ??
Hi Uncle Doug, My 1965 Vibrolux Reverb has a tube chart with AA864 on it. I have never been able to find this schematic.... I wonder if you have ever seen one? It has the GZ 34 rectifier tube.
Rgds!
I have not seen a AA864 schematic, Alvaro. The changes between it and later circuits are not huge, so a later schematic will probably work for your,
Gary: I have never personally owned a Pro Reverb and have no access to one, but I did notice an old Craigslist ad from Santa Fe, NM for one. I tried to send you a link to it, but YT won't allow it. You could contact him and, if it's not sold already, perhaps he could provide accurate measurements of the cabinet. At the very least, the ad has some good photographs which may be of assistance. Also, you could try this tactic with other CL and Ebay sellers.
I just scored one of these in excellent condition sans footswitch for $750. Even has the Mullard GZ34.
If it's a blackface PR, you scored a major coup, BR......in good, original shape they tend to cost about three times what you paid.
I have one of these AA764 amps I would like to sell and your video gave me some info on it. Thanks.
You're welcome, BW. Good luck with the sale.
Greetings, Dave. According to conventional dating charts, your amp was made in 1966 (one year older than mine), and should have a AA1164 (Nov, '64) tube chart and chassis......yet it, like mine, has a AA764 (July, '64) tube chart. If you look at the 764 and 1164 schematics side-by-side, there are several minor differences: PV on the 12AT7 (355V vs 400V), cap between Vibrato pedal and 12AX7 plate (.02uf vs .01uf), electrolytic cap just after the biasing diode (50uf vs 25uf), etc. My guess is...
Here's another twist on the rectifier confusion. I have a '67 Princeton Reverb with the AA764 circuit and the tube chart says GZ34 but someone hand wrote in pencil 5AU4. The tube that is installed is a 5AU4. I'm sure that it is all factory original including the pencil change because I acquired the amp from the original owner 40 years ago, and he barely used it as a kid. Based on what Uncle Doug says, I'm not sure if I should leave the stock 5AU4 in there or switch it out for the highly praised GZ34. What do you think?
Are you sure that that tube isn't a 5AR4? Because a 5AR4 is simply another name for a GZ34, they are the exact same thing. I think one is a British designation and the other is the American designation, I just can't remember which is which.
Thanks Doug. Yes I'd always heard they are the same circuit. That myth is still kicking around on the web 9 years later
You're welcome, John.
Would going from a GZ34 in a 64 Super Reverb to a 5U4GB tax the amp parts anymore? Would it likely achieve more sag at lower volumes?
Greetings, NYC. The GZ34 filament only draws 1.9A while the 5U4 draws 3A, so the 5U4 would be a 58% greater current draw on the 5V winding of the PT. Most PT's have some leeway in their current ratings, so you could probably get away with it. The GZ34 is more efficient that the 5U4, so the 5U4 would indeed be a little "saggier" at lower volumes. Here is a link to a great site and discussion of your amp: fenderguru.com/amps/super-reverb He mentions the rectifier switch.
Uncle Doug That site is where my search started. Guess Ill be posting my results after giving it a try..
What do you think about what the author of the tube guitar amp guide book thing said? He said that the standby awitch is actually a superstition made up because fender started to use them. The electrons before warming up don't harm the tubes.
I agree that in the vast majority of guitar amps, the Standby Switch is really not helpful. Standby Switches originated in radio transmitters, where plate voltages are greater than 1000V, to preheat the cathodes and reduce "cathode stripping"; but in guitar amps, with plate voltages generally less than 500V, they are not needed. Also, since most Standby Switches are in the DC plate voltage circuit, they tend to arc and wear out quickly, and if you have a tube rectifier (i.e. GZ34), it provides a "built-in" delay in high voltage that protects the tubes. Lots of knowledgeable people recommend not using the Standby Switch at all in guitar amps, and from a technical standpoint, they are probably correct.
Uncle Doug I see. Thanks for the facts.
In the 5F6-A Bassman, Fender introduced a DC coupled cathode follower into a solid state rectified amp. At switch-on, before the tube begins to draw current, the grid of the cathode follower is at B+ voltage while the cathode is at ground potential, which can lead to arcing between grid and cathode, often destroying the tube. It's possible that the standby switch was introduced as a fix for this problem, although a neon bulb between grid and cathode would have been a solution which did not rely on the user to follow the correct procedure when powering up the amp. The Valve Wizard recommends a reliability mod for DC coupled cathode followers: valvewizard.co.uk/dccf.html
Hi Uncle Dough. I'm going to take on a quite deep restoration of my Fender Princeton AA764 version. Need to do a full recap of all the electrolitics, including the multitap can-type one, the power transformer and the rectifier valve, which in my case has been replaced by a full wave diode rectifier bridge. I'm thinking of using a Hammond made 291AEX power transformer as replacement, as I live in Italy ad we run our mains @230V AC. As per rectifier , after watching your video, I think I should go with a GZ34. Do you suggest any particular brand/type? Also here in Italy it is quite difficult to get a replacement for the filter cap. I wouldn't like to use external caps and I'd like to retain original look as much as I can. I saw on other videos you are using caps made by Ge Distribution but they are quite expensive to get, with shipping and import taxes. Any suggestion you could give to me is very welcome. Thank you
A full wave bridge rectifier requires a special power transformer without a center-tapped high voltage winding. You would be better off installing a proper ClassicTone 40-18019 Fender Princeton power transformer with a GZ-34 rectifier tube. They also have international models with 110VAC and 220VAC primary windings.....check their site. You can retain the capacitor can on the outside of the chassis and then replace all the capacitors with individual axial electrolytics hidden within the chassis. Good luck.
@@UncleDoug Thank for your answer, Sir. As per capacitor, I know schematic requires a 4 x 20uF can cap. Would a 20/20/20/30 be a better option, even for cosmetic reasons? Thank you
@@dreamlessfull I'm not sure why the latter would be more "cosmetic" than the 4 x 20uF, but yes, you can use a somewhat higher value cap in the first position of the power supply chain.
@@UncleDougtrue, I could use 4 electrolitics and keep the disconnected can cap, but I'd like to keep the number of modern components inside the chassis at minimum
Thank for your advice, I've learned a lot from your vids
@@dreamlessfull Then I guess you have no other option than to buy a "very expensive" can capacitor. Good luck with the repairs.
Have you heard the Morgan PR12? It’s one of the best sounding amps I’ve heard. It is apparently based on the Princeton Reverb, but with a 12 inch speaker, and modified to provide better bass, and it appears to be solid state rectified because I looked on the website at the specs and there are only 3 12ax7s and 2 6v6s... no mention of a rectifier tube. How hard would it be to modify one like that uncle Doug?
The modifications you mention would be easy, Alan, but you would need a schematic to ensure that all alterations were incorporated into a clone.
Thanks, Uncle Doug, very helpful information to have. I wound up here while trying to research the best tube replacements to buy and had no idea my AA764 was the subject of such scandal!
A couple of questions if you have a moment:
My tube chart calls for a single "7056" (or "7058", hard to tell) and that differs from the BlueGlow schematic, which mentions a 7025. Any thoughts?
Next (and apologies if you've already mentioned this elsewhere), may I ask what brand of tubes you would purchase as replacements?
Thanks for considering, and I appreciate your time.
Jake
The 7056 is a 7-pin tube, and I have never seen it used in Fender amp. If your tube socket is 9-pin, it is probably either a 7025/12AX7 or a 12AT7.
Uncle Doug, Thanks for the reply. I did some more looking and I think it must be a 7058. Apparently they do wind up in Fender amps from time to time. Anyway, this number is from the tube chart itself, so I think it's legit (even though I had some trouble reading it initially, I'm positive it doesn't say 7025).
Interesting...
I have to point out that with the link you gave, right on the schematic image there's a notice that says this AA764 schematic was photoshopped from an AA1164 schematic to match the AA764 Princeton Reverb amp. And it goes on to say that there is no original AA764 Princeton Reverb schematic to their knowledge. Anyways, I think your video's are awesome. I've watched many of them multiple times.
Greetings, AA, and thanks for your very kind appraisal of my YT channel. This statement was added to the schematic in the past few months. As you can see from the video at 2:39, no such statement was evident at the time of the video. It does explain, however, why this appears to be the only source for a AA764 schematic. It also would appear to confirm that the AA764 chassis did exist, with or without an available schematic. Thanks for bringing this to my attention. I will amend the video description to reflect this new information.
Uncle Doug
No problem Doug. Glad I could help. That's strange that there doesn't seem to be a true schematic for it. I wonder what the story is behind that.
It appears that the AA764 circuit was only utilized for about 4 months before it was (slightly) altered to the AA1164 form. Maybe only a very few copies of the earlier schematic were ever made during this brief period, and none have yet been discovered and posted on the Internet. Somewhere, in a dusty file cabinet in Southern California, one is probably just waiting to be found......Let's hope :)
Hey Uncle Doug & Rusty, I hope your all doing well. Could you use a GZ34 in a Fender Champ amp, or is the 5Y3GT better suited for the Champ power requirements? The GZ34 sounds like the way to go if you can. I'm doing research for the Champ build. Thank you for all the great videos, time & effort. Your amps are absolutely beautiful and sound fantastic. Your craftsmanship is awesome. Take care, be good, C.
+Cass Virgillo You're welcome, Cass, and thanks for the very kind comments. Since most Fender Champs are factory biased at the ragged edge of spontaneous combustion, changing to a (much more efficient) GZ-34 might be like throwing gasoline on a smoldering log......but, if you were to quickly adjust the bias to a reasonable level (and checked all circuit voltages) after the substitution, it would probably be a good, anti-sag, PT-current-reducing, move. Let us know what happens.
Special Thanks for the schematic and your video :)
Joël PAGES You're welcome, Joel. Glad you liked it :)
Can I substitute a GZ34 for a 5U4 in a '73 and '79 SFPR? If not, what changes do I need to make to make it work?
Bbendfender Your power transformer will love it, due to reduced current draw, Bb, but your output tube plate dissipation will go up due to increased plate voltage. You'll probably have to re-bias them. Also, the amp will become less "saggy", with current responsiveness more like solid state rectification.
Have you ever come across a way to slow down the tremolo in a Princeton Reverb (mine is a reissue). Maybe a different value pot for the speed?
It's a fairly simple procedure, David. Simply increase the value of one or more of the capacitors in the oscillation loop.....i.e. if the cap value is .02uf, then substitute a .03uf. Higher value caps charge and discharge more slowly, thus slowing down the tremolo speed.
Uncle Doug. Thanks for the vids. I'm learning a lot. I have a 1974 Silverface Princeton Non-Reverb. Can I use a GZ34 instead of the 5U4Gb?
The amp has very low power. I was hoping this would help. What else can I check/try to increase volume/power? Thanks!
You're welcome, Diane. Yes, you can replace the 5U4 with a GZ34 and the result will be higher plate voltage and possibly higher plate dissipation, so you should re-bias the amp. It's possible that your 6V6 output tubes are showing their age and getting weak. If you have never replaced them, it might be time to do so.....and another good reason to re-bias (necessary with the new tubes). Good luck.
blueglow link now states that AA764 Princeton Reverb schematic was Photoshopped. I provided a scan of my original AA1164 Princeton Reverb schematic with GZ34 rectifier to a website called Schematic Heaven years ago. The original in my possession was mailed to me directly from Fender in the 1960s. As far as I know, any copy of the GZ34 version of the Princeton Reverb schematic has been disseminated from that source. Free use was assumed and welcomed by me. Enjoy !
Very interesting. Thanks, Larry :)
I have a 1967 Princeton Date code QD April 67. Under indicator bulb is only the number 68..no other text...most say Patent date or other...never seen this on other..is this rare?
There are several different versions of the control panel plate, Brian, but I don't recall seeing one with just the number "68".
Dear Sir, first off, it´s always a pleasure watching your videos, I´ve learnt a lot from them. I would like to know if there is a way of getting in contact with you. I would like to discuss a Fender Princeton Amp/no reverb. Best regards, Roger
Greetings, Roger. Much as I would like to provide personal consultations, the large bulk of correspondence I receive does not allow for it. Please feel free to post your questions here and perhaps I can help.
hi, thanks for posting this... its really interesting and helpfull... I am very confused though, as my PR tube chart states it is a AA764 circuit, but it shows a 5U4 rectifier tube... any hints on this? which schematic should I use... also, which power and output transformer replacements would you recommend to restore and improve these amps???
You're welcome, Sergio. Normally the AA1164 chassis uses a 5U4 and the AA764 uses a GZ34. If the tube chart in your amp says "AA764" and shows a diagram of a "5U4" rectifier, then I would go with the tube chart and keep the 5U4. Other than the rectifier tube, there is very little difference (2 capacitor values) between the two circuits, so it really doesn't matter which one you use.
I would not suggest replacing either transformer unless the original has an open winding (i.e. it's "burned out"). If you do need a replacement PT or OT, then check Triode USA: store.triodestore.com/transformers.html They have good quality transformers at very reasonable prices. Good luck !!
Uncle Doug I have a very old Fender Princeton Reverb that needs looked at!!
Do you still work on amps?
Message me on FB: Uncle Doug's Vintage Amps. I can help you.
Do all of the AA764 models have the GZ34 and while only SOME of the AA1164 have it? thanks
It's impossible to make any absolute statements about the circuits at that time, Doug. I have seen all sorts of circuit variations, alterations, and combinations....probably depending on what parts were available on the day of manufacture.
Uncle Doug, today I acquired a 1968 Princeton Reverb (in near mint condition!) with the AA764 tube chart. The amp sounds great but there is one issue. The vibrato, or tremolo, is weak. Here is what's really odd: If you turn the amp on and strike a chord, the vibrato sounds great. But then the effect fades and it's hard to tell it's even on. What would cause the vibrato to sound strong only when the amp is initially fired up? Thanks!
There are two portions of the tremolo circuit which could be responsible, Cal. The three capacitors in the oscillation loop could be out of spec, and/or the oscillator tube (half of a 12AX7) could be failing. The tube is the easiest to check, simply replace it and see if the problem subsides.
I have fender super reverb SF 1969 ab568, but in the chassis they wrongly wrote ab763...
I'm not familiar with chassis being marked with circuit ID, Ali, but misrepresentation is not unusual with these early amps.
@@UncleDoug thanks for your kind reply
Thanks for your video!
I have a 1970 Fender Princeton Reverb and my tube chart AA764 that is in good shape shows a 5U4 rectifier tube...
It had the Torres bias mod done to it and recapped the tech gave me a solid state rectifier plug and a 5U4 tube.
I used it with the SS one for years and recently plugged an old hammond 5U4 into the amp
and it got really distorted. Not in a bad way it just had a lot more gain when turned up loud.
I read below that you told someone else to follow the chart, but after watching your video I was just wondering what you thought...
Thanks!
Greetings, Tim. It seems strange that a 5U4 could outdo a SS rectifier.....generally, it's the other way around. My advice would be to go with whatever works best for you. Perhaps for practicing at home, the SS would be best.....while you might want the additional volume and saturation for a public performance. If you really want some additional gain, try a GZ34 rectifier. Best of luck.
Uncle Doug, if you made these tube/rectifier changes, would you need to re-bias the output tubes?
Doug, thanks so much for the journey through these little gems as well as continuing to mentor a whole boatload of us out here. A little '65 BF Champ has rekindled my interest in a PR, although it'll end up having to be the reissue; good ones are probably pretty spendy. (The PR still is a wonderful little 1x10 2x 6V6 package that can't be beat.)
I really appreciate your discussion of the different rectifiers; for someone who's sensitive to it the difference can be noticed in the playing. I see that the reissue is using a 5AR4 instead of the 5U4 from the AA1164 circuit, probably a good thing. However, do you have any sense as to whether the inefficient 5U4 may have been used specifically because it has such a difference in rise time (or something) and, perhaps, is more touch-sensitive? Just curious because the dynamics of my little Champ (with its 5Y3) have me a bit spoiled that way.
Thanks!
Greetings, Wes, and thanks so much for your kind words :) I tend to rank the 5U4 at the bottom of the rectifier pile. It's a current hog, not particularly efficient, and rather saggy. To be honest, I think they used it because it was readily available and relatively inexpensive. If possible I convert to GZ34, but the high output combined with high 120+VAC receptacle voltage can yield some scary plate voltages. The 5Y3 is in the middle for me.....ideal for all rather low wattage circuits like your little Champ. The GZ34/5AR4 is at the top......low current draw, relatively high output (high efficiency), and very slow start up due to the indirectly heated cathode, which really babies the amplification tubes. You really don't need a Standby switch any more. The high efficiency yields minimal sag, almost like diode rectification. For those who appreciate sag, however, I guess the 5U4 would be the way to go.
Uncle Doug
Thanks for much for the feedback! :)
Uncle Doug
One other thing on the misconception you correctly pointed out regarding whether the schematic number relates to a specific circuit. The 764 can't be the circuit for the tone stack for both the Champ and the PR because the resistors bound to the bass pots have vastly different tonal results - 15k (Champ, punchy) and 6.8k (PR, sweet, kinda scooped). Would Rusty agree the solution is that we should always have one of each? :)
Rusty votes for a third alternative: moist and meaty....like a T-Bone steak :) Thanks for the input, Wes. For more confusion, how about the "Design and Circuits Patented 68" on the control panel of BF PR's? I'm sure this refers to some obscure legal action or filing number, but it sure looks strange :)
I used the link for the Schematic - and it says it's a photo-shopped conversion of the AA1164 , so maybe your initial comments were correct.
The PRRev. AA764 never existed - maybe it was the stamp they used that hadn't been changed.
Strangely enough, the statement about it being photo-shopped was added AFTER I posted the video and link. I have copies of the AA764 schematic both ways, with and without the notice. The addition of this notice does indeed cast doubt on the actual existence of any legitimate AA764 schematic......or chassis.
@@UncleDoug But it still remains that the Tube Chart with the stamp AA764 shows the GZ34 and the AA1164 Tube Chart has the 5U4GB. Peculiar to say the least :-)
@@UncleDoug Maybe even more peculiar is this SF PR on Reverb - 1968 :-)
reverb.com/item/7825642-vintage-1968-fender-princeton-reverb-silver-face-tube-amplifier-aa764-circuit-a-true-sonic-gem
Hi Uncle Doug. I have a '66 with the AA764 chart, and it came to me with a 5U4GB rectifier. I'm considering a swap to the GZ34. Before I do, can you tell me how the differences in the AA1164 circuit address the potentially harmful characteristics of the 5U4GB rectifier if used in the AA764 circuit? Many thanks. Great videos.
Greetings, Jonathan. I don't believe that Fender made any modifications for the 5U4 rectifier in the AA1164 chassis. In the schematics, it appears that both chassis use the same power transformer, even though the 5U4 draws over 50% more filament current than the GZ34. I assume that the 5V winding of this PT is rated at 3A, so it is capable of handling the 5U4 and would therefore be way under-stressed when dealing with the GZ34 at only 1.8A.
Your AA764 chassis would probably be OK with a 5U4. It's a common substitution, since 5U4's tend to be more readily available and cheaper than GZ34's, but in addition to drawing more filament current, the 5U4 is much less efficient than the GZ34, provides lower plate voltages, has more "sag", and does not provide slow warm up to protect your tubes. Bottom line: the GZ34 is a superior rectifier in all respects.
Good work!!
Thanks, Chris.
This *is* interesting. I had asked you for a link to a good readable copy of an AA1164 Princeton reverb, and you sent one from I think TheVintageSound web site. My brother found another one, that says "AA1164 Princeton Reverb" on top, but it has a GZ34 rectifier tube and the power transformer is 340 0 340v and a spot check, the other components look the same as well.
Variations exist in schematics. The GZ34 rectifier is generally found in the AA764 circuit, but can be used in the AA1164 as well. The GZ34 is a more efficient rectifier, so plate voltages will be higher (output tubes must be rebiased if rectifiers are changed). It offers a slower warmup, to protect the tubes, and it has lower current demands from the power transformer. It is overall quite superior to the 5U4.
Thanks for the info, Uncle Doug. Glad I somehow "accidented" my way to the GZ34 version of the schematic. Wow much better tube! As an aside, a little worried that the schematic I have might have a fatal error/typ'o (short or missing component or value is wrong) but maybe I can find a different source for the schematic and compare component values and placement.
If I had not found your youtube channel, and all the great and detailed information on projects you've worked on, and tutorials, I don't think I would start an amp building project. Thanks!
You're welcome. Good luck with your project.
Hello Uncle! I have Princeton, but want more reverb intensity as it is on Deluxe reverb. Can you tell how can i archieve this?
+Ilya Shadrin If your PR is lacking reverb intensity, check (or replace) the 12AT7 driver tube, be sure that the cathode bypass caps on the 12AT7 and 12AX7 are in good shape, check (or replace) the 12AX7 recovery tube, and be sure the tank transducers are in good shape. The only reason for a PR to be lacking reverb intensity is if some component(s) is/are old, weak, or failing. Good luck.
Greetings, Clint. If you have a 1970's PR with a AA-764 tube chart (and circuit) then it really adds to the mystery surrounding this issue. If the circuit was modified in Nov. of '64 (1164), why would the factory still be installing the long-outdated 764 charts and circuits (?) in 70's amps? I have a strong feeling that your circuit matches the 1164 schematic and the tube chart is an old one they had laying around the shop. Fender was famous for haphazard use of whatever was handy.......
thanks, this is very helpful.
You're welcome, Jon :)
Thanks!
Checking out schematics it appears the AA764 is just an AA964 with the reverb unit added.
That would make sense, since the AA964 is the Princeton model and the AA764 is the Princeton Reverb.
Uncle Doug By the way...What are you playing on the intro, Strat? Whay are the settings guitar & amp?
Actually it's my workshop guitar, Marstyn, a 1979 Matsumoku Vantage Performer with MMK-45 humbucker pickups. The guitar output and tone controls are usually set on the high end......8 or 9. The amp was probably at Vol 4, Treb & Bass 5, Rev 0, Sp 5, Int 5.....I'm just guessing, since it was a long time ago, but this is probably correct.
Wow. Facts and data. Rare on the internet, and even more rare in amp discussions.
I'm thinking of building a Princeton Reverb, as it seems to be the Fender amp with the most headroom that will not blow out a Celestion Blue. In your opinion, was the PR AA1164 circuit an improvement over the AA764, or should I go with the original?
Thanks.
Thanks, Jeff. I prefer the AA764, because it uses the GZ34 rectifier, which is vastly superior to the 5U4. Other than that, there are very few differences between the two circuits other than a couple of capacitor values. Good luck with your project :)
Uncle Doug Thanks much. Do you know if a GZ34 would work in a Deluxe Reverb? If so, would it drop in, or need a rectifier redesign?
Jeff, the AB763 chassis of the Deluxe Reverb came with a GZ34 rectifier, so it would probably work in other versions of the DR (i.e. AB868). One thing to watch is the plate voltage will go up with the GZ34, and you would have to rebias the output tubes.
I have a Princeton Reverb Amp, Listed as AA764. The tube chart is as follows GZ34, Two 6v6GT, two 12AX7, 1 12AT7 and 1 7025. However the tube slot contains a 12AX7 instead of the 12 AT7. is the 12AX7 a substitute for the 12AT7? I notice the amp is now getting hot and the transformer may be going out. any comment is appreciated.
It is listed as a 80 watt
Hi Doug- looking at the schematic you posted (thanks!) I see there is a notice that says "This drawing was photoshopped from a scan of an original AA1164 schematic. The only details change were the AA764 designation, the bias capacitor from its original 25u 50V value and one tremolo capacitor from 0.01 to 0.02. To my knowledge there is no known example of an original AA764 Princeton Reverb schematic."
I did not see this disclaimer on your printout in the video, though (must have been updated later on?). I assume the photoshopped schematic is valid/ correct, but why is this such a difficult schematic to find?
Thanks for all the videos
I see no such notice on either of the AA764 schematics I have on file, KP, but do acknowledge that this is a virtually impossible schematic to find.....although the circuit itself is not at all uncommon. It continues to be a mystery to which I have found no final, satisfactory answer.
Interesting. Well thanks again!
So what's the business with people including amp makers today saying that the standby switch is just a remnant from the past and not really needed? Has the common way to design amps changed? Is it because of the rectifier diodes still shooting the full load on the tubes once the standby switch is turned? Something else?
Standby switches cause more harm than good, Yup. Read what the Valve Wizard has to say about them: www.valvewizard.co.uk/standby.html
Hey Doug. I have a Princeton Reverb Silverface with a AA764 tube chart and the gz34 rectifier specified on the label, yet i thought this schematic only came in blackface Princetons. Can you give me some info? Thank you.
More info, the number 606 - 9 - 07 , making it an early 69.
Greetings, JD. According to the Fender Guru, your Silverface 1969 PR should have an AB1270 chassis with a 5U4 rectifier: fenderguru.com/amps/princeton-reverb
However, I have seen all sorts of variations in chassis and circuits during this period. CBS had just taken over and it appears that they were probably using whatever they had on hand to build amps.....so a left-over AA764 chassis (with GZ34 rectifier) may have been installed in a 1969 Silverface cabinet. Other than the rectifier, the two circuits are virtually identical (2 small cap differences) so it's really not a huge issue.
One other explanation might be that you do have an AB1270 chassis but they put the wrong tube chart in the cabinet, and someone followed the tube chart when they replaced the rectifier and installed a GZ34. If you really want to be sure, download both schematics and see which one matches your circuit. As you will see, other than the 2 small caps, they are virtually identical. Good luck.
Thanks so much for the quick response Doug! Actually with further research it turns out it is actually a 7th week of 68 amp.. i read it wrong.. And it has the aluminum around the mesh, which according to fender guru makes it a 68 silver. Would that make it more normal that it would have the aa764 circuit? Thanks again, you got my sub
I think it is not only possible but quite likely that an early '68 Silverface would have a chassis from the previous year's BF model. Fender probably had a bunch of AA764 chassis left over from the Blackface years, and it would have been financially unwise not to use them up. After all, if you compare schematics, there is virtually no difference between the BF and SF chassis....so why not use them? This use of BF parts in the "Drip Edge" '68 Silverfaces (like yours) is why they command higher prices than the later Post-CBS SF models. You're welcome, JD, and thanks for subscribing :)
Uncle Doug 1f you work on enough Fender products you see all kinds of overlaps in the styling and little cosmetic details. I recently acquired a late 60's silver face Bandmaster head, thinking that I'd do the black face conversion on it and have a righteous amp for low bucks. then when I looked inside, lo and behold, it was an AB763 circuit! Now all I have to do is replace the caps, throw in some fresh tubes, and re-bias it. WOOHOO! YEAH BABY SCORE!!!
The schematic that you link to CLEARLY states right on it that it’s been photoshopped to say aa764 and added in a gz34. There is no such circuit. Maybe you could update this to avoid further confusion?
The schematic upon which the video is based has no such disclaimer, and I made the justifiable assumption that it was legitimate. Later, it became evident that the schematic had indeed been photoshopped, as clearly stated on the schematic which is linked in the video description. To avoid further confusion, I will add a statement to the video description explaining the situation.
Thanks uncle Doug. You are highly respected for good reason. Keep up the great work. And thank you for all you share…
@@johnraham_AfterlifeStudios Thank you for reminding me of this confusing issue. I hope the added note resolves any uncertainty.
Uncle doug,a friend told me a princeton reverb is a "class a"amp like a vox.Is that true?
I believe the PR is a Class AB amp, double-ended and grid-biased. Class A amps are most often single-ended and cathode biased.....like the Fender Champ.
The Vox AC30 is not a Class A amp, it's Class AB.
thanks uncle doug. i just picked up this amp with this circuit and really hard to date. what year is this amp is it a 1966?
There are Internet sites that provide accurate dating info on Fender amps based on the chassis numbers, Joe. Also, if there is a tube chart in the cabinet, it should have a rubber-stamped two letter code that tells the year and month of manufacture. I have two of these PR's, 1966 and 1967. I can't recall which one was in the video.
The amp is really dirty to the point where you can not read the serial numbers. The tube chart has been damaged as well. I can read the top of the chart showing the AA764 circuit, but you have been very helpful thank you.
The serial numbers are usually engraved into the chassis rear wall, Joe. If dirt obscures them, then use some 409 on a rag, or even some fine steel wool to expose them. You're welcome. Best of luck.
what would be your choice of rectifier tube. would the jj 5r4 be your choice or
and its a 66
Do you happen to have the layout for this particular amp? Thanks!
Yes, it's available on the Internet. Simply do a search for "Fender Princeton Reverb AA764 Layout".
Thanks
You're welcome, Luis.
I have a Silver face Princeton Reverb amp with a Tube chart Label that indicates the AA764. I note your description speaks specifically of the black face model. Serial number of my amp is A 19337 which suggests 1967 or 1968. Uncle Doug, I could send a pic of the tube chart label is you need it. contact me if you wish.
Thanks for the input, Pete. Common sense would dictate that the AA764 circuit should have been long since replaced by the AA1164 by the time the Silverfaces were made....and yet, as your tube chart indicates, apparently they were not. Is the rectifier tube in your amp a GZ34 or a 5U4?
Another possibility is that they printed up a whole bunch of AA764 tube charts and then kept using them for several years, even with the newer circuits, until they finally ran out. Yet they never seemed to print out any schematics for the AA764. Very strange.
I don't recall ever seeing a AA1164 tube chart in a PR.....has anyone else seen one? If so, in what year was the PR made ??
Uncle Doug unfortunately, I am electronically challanged! so unless you tell me how to check it out, I cant answer you. I checked out all the tubes but dont either of the markings. I did have the unit in a shop and they replaced some of the tubes but as I said, I dont know what to look for. obviously I dont read schematics well neither. lol
No problem, Pete. If the amp works well, then just enjoy it. The exact nature of the circuit is secondary to its function. Thanks for your input :)
Uncle Doug I also have a 1969 Fender Princeton Reverb with the AA764 tube chart. It also has the GZ34 rectifier tube. I thought it may have been the wrong tube chart, so I pulled the tube and it's GZ34.
Another good example of the mysterious AA764 circuit.....no original schematic available and it continues to appear for several years after it was (apparently) replaced by the AA1164 circuit.
The GZ34 is great.
Thanks, BVA, I'm glad to hear that others agree with my love for this fine rectifier tube :)
Leo didn't like it because it over taxed the power transformer. Iy was brought back during the early CBS years. Best thing to do?? Put in deluxe transformers and tap the unused cap in the cap can.
Thanks for the info, Russell.
@@UncleDoug uhh not compleatly accurate about the cap can ,,, it could be a replacement can with 40, 20, 20, 20. The only thing that stops me from realising my amp building dreams is the wife!!!! She won't even allow me an electric guitar!! Ahhhrgggh. Can't live with em , can't kill em. ( I'm joking) I'd like to replace some if the furniture with amps!!! It would make the living room more pleasant!!!
@@russellesimonetta3835 I understand completely, Russell ;)