Late-50's Gibson GA-6 Amplifier ....with some Very Unusual Problems
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- Опубліковано 10 лют 2014
- This video features a late-1950's (pre "Lancer") Gibson GA-6 amp that was brought to me for repair. The symptoms: wildly excessive volume, intermittent hum, and rather shrill tone. I must admit that I've never before encountered an amp that could literally blow the cone out of its speaker at a volume setting of 3, so this was an unusual challenge. Also, along the way, several pitfalls, cautionary tales, and examples of idiocy were discovered that should prove quite amusing. It all ended well, however. I hope you enjoy watching how it was done.
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Thanks for watching !!!
UPDATE: Although the schematic shows a 2.2K cathode bias resistor for the phase inverter, a 1.5K was originally installed in this amp. Changing it to 2.7K provided significant additional volume reduction. I believe a big part of the high volume problem was due not to the replacement speaker, but to excessive gain in the phase inverter.
I have owned several Gibsons of this vintage and almost all are very loud low on the dial, say, around 2 or 3 it gets almost too loud. You might be right about some of the bad consistency on the part of Gibson's assembly (which is often questionable), but I think a lot of their amps are just naturally loud, even when all components are there and functioning as they should.
This amp would appear to support your theory, Brad. If it's a common trend, then perhaps Gibson intended it, assuming that customers would gravitate toward louder amps. I haven't seen many ads for Gibson amps during this period of time. I wonder if they ever promoted this characteristic?
Hi Uncle Doug,
Just bought a 1958 ga-6 like this one last week and it had the original instruction booklet with the schematic. It's the missing link between the first version and the Lancer. It has a 1.5k cathode bias resistor for the phase inverter. Where can I send it to you. Hope it will answer some of your questions.
I'm a new fan of your videos BTW.
This is a link to the schematic from the instruction booklet I uploaded:
flic.kr/p/rb85PV
BTW my 1958 ga-6 as the original JENSEN P12R and seems to be loud too but sounds great with lots of bass.
Thanks for the schematic, Pierre. If your GA-6 seems very loud also, then it's probably a design characteristic of this amp rather than a malfunction.
love your sense of humor.
I think you would be a excellent teacher
Thanks, AJ. I was a classroom teacher for almost 20 years :)
Uncle Doug ...without a doubt... is one of the best instructional / tutorial sites anywhere on the web....clear...concise...unbelievably thorough and informative....not a bit pretentious or pedantic....just clean and to the point. Regardless of what technology is being addressed.... every instructional techno video "presenter" could take a big lesson on how it should be done right here. You are the rare and exceptional preceptor !. (I don't know why I commented on this particular video....I could have done this on any one of them...I guess it just dawned on me how great these all are!) Thanks Doug! Your greatly appreciated
Thanks so much for your very kind assessment, William. Favorable comments like this are also greatly appreciated......and make it all worthwhile :)
Uncle Doug You are the very definition of a... "random act of kindness".....it's evident you don't have to do any of this....you just want to ....or ...need to... pass it on. A Quote ."Men must be taught as if you taught them not,..... and things proposed....as things forgot" ............. Alexander Pope..............(Oh Hell...that is pretentious....I broke my own rule!.) Where is Rusty when you need him....( Using a sly exit! Hah!. Hah! )......Bill...(Plant Facility Manager...God Help me!)
Greetings, Bill, and thanks for your kind words. Rusty and I simply enjoy sharing our hobby with others; and, in so doing, we get to meet all sorts of very nice people (yourself included).....it's the best paying non-job we ever had :) ^. .^
I agree. Uncle Doug has helped to take the mystery out of these amps and how they work. I am using the videos now for my own instruction so that I will build my own amp when I am ready.
Greetings, Brian. It's great to hear that the videos are helpful. Best of luck with your amp project....be sure to let us know how it turns out.
man oh man your great!! thanks for making videos like these..i love it!!
You're welcome, UGM. It's great to hear that you are enjoying them :)
Hello Uncle Doug!! I’m finding these videos extremely helpful and informative.. of course I realize this vid didn’t have Rusty to give you an honest pre-evaluation.. did pulled it off 👀!! Thanks so much for all of the time and work in production you put in them!!
You're welcome, Luth......and thanks for the nice comments.
Awesome repair, and great explanation about your findings.
+RODALCO2007 Thanks, Rod. Rusty and I really appreciate your kind words :)
You and Rusty work well as a team! You do good work !!
Enjoyed the video, as usual !!
Thanks, Kal. I have to give Rusty credit for sniffing out most of the problems with this amp. I just follow behind him and fix the things he points out :) ^. .^
Uncle Doug:) :) Good job !
Great video Doug and thanks for the tips really appreciate them. Was great to watch this again and make a note about the potentiometer resistor mod. All the best to Rusty and Jack. Colin
+mixolydian2010 You're welcome, Colin. Best regards from Rusty and the gang :)
Thanks Uncle Doug for the festivities..!! I missed the parade and smoke show
You're welcome, Rick. Dirty Harry is Rusty's favorite !!!
I love you Uncle Doug, Not only you are a Genius but you're also funny, the way you describe the problems is so hilarious!
Thanks so much, RS, but Rusty is actually the one in charge.....I simply provide him with plenty of single-malt scotch and clean up after he finishes each new project :)
Enjoyed watching. thanks.
You're welcome, Grady. I appreciate your time and interest.
Thanks for such an informative video.
+SiliconSet You're welcome, SS :)
Uncle Doug -- Another first rate video. So entertaining and informative. My complements to your producer Rusty as well.
Thanks!
Bill Norman Thanks so much, Bill. Rusty and I really appreciate your kind words :)
I admire all your videos,also you have a smashing accent this side of the pond!!!
Thanks, Colin. It took over 70 years to perfect my West Texas accent.....so I'm glad it's appreciated :)
I enjoyed watching your video . I have been around vintage electronics such as citizens band radios . I never really learned how to repair them . but ,,, I have brought a few back from the dead by replacing burnt parts in them . great video . Thank You for posting it . Ray in Pa.
You're quite welcome, Ray. I'm glad to hear that you enjoyed the video :)
I think the reason Gibson used such a high value resistor on one channel was for use with a crystal mike or pickup. The low end freq response could suffer working into a 47K resistor.
Another SUPER video. I learn a lot about these wonderful circuits thanks to you. Keep them coming. Maybe explain how EQ Circuts work in amps. I never got that.. You did touch on it here!
Thanks so much, Jorma. It's always good to hear from you. The tone control in most of these old amps is often quite simple: a pot that offers the high frequency portion of the signal a (variable-resistance) path to ground through a low-value capacitor (in this case .005uf). High frequencies pass easily through a low-value cap, while bass does not. It simply removes more or less treble from the signal, which is why the volume goes down as you "turn up" the bass.....actually you aren't increasing the bass, you are only eliminating more treble. This is not a very good system, particularly at both extremes of the pot setting. "Active" tone controls are much better. They actually boost the desired frequency, rather than simply eliminating the undesired one. Perhaps this would be a good topic for a video. Thanks for the suggestion.
That would be very awesome. I am interested in the Fender, Marshall, Vox slopes and how they work. Its a topic that always eluded me. an understanding of Active EQ would be super valuable too.
Thank you for all of this time and effort!
Hi Uncle Doug, just wanted to say I appreciate all the videos I've watched and learned heaps from. I wondered if you use an osilloscope in your restorations? If so I am sure there's lots you could teach us. I very much enjoy your presentations and hope you'll keep them coming. Kind regards from the UK.
Greetings, DD, and thanks for your very kind remarks. I did indeed use an oscilloscope in my recent video regarding Photoresistor Tremolos. I will probably use it again in future videos, where appropriate. Thanks for watching and best regards from Texas :)
Thank you Rusty. As always great info, work and craftsmanship comes together. Also some thoughts on repairmen i guess :-). I guess I'll start biding...
Jan Larsen Fredriksen You're welcome, Jan, and thanks for the kind words. I'm lucky that Rusty is able to sniff out incompetence :)
You're the man.
Thanks, Bill :)
Subscribed. I just started working on tube gear last week. Formerly I'd only repaired low voltage or solid state stuff, nothing older than late 70's or fewer than 200 parts. Dunno why I was always hesitant to learn about or work on tubes. After watching some of your videos I wish I'd gotten into tube gear earlier.
Welcome and thanks for subscribing, MM. I think you'll find tube gear to be much more "soulful" than SS.....especially the earlier, vintage items.
Good honest work. Nice simple amp, as it should be in my opinion.
Thanks, WN :)
Those Astrons at 4:33! Also probably because the heat from the tube filament would rise up through them. I can't understand why 'tubes-down' orientation is so common! It also seems to take a toll on phenolic tube sockets.
Positioning the chassis at the top of the amp, with tubes hanging downward, allows for easy access to the top-mounted controls. The early vintage amps with floor-mounted chassis had hard-to-access controls at the back, rear of the cabinet.
This amp sounds incredible!
Thanks, MD. I should have included a segment in the video in which the owner, who is a good guitarist, tested the amp. Even my neighbors were impressed (it was still pretty loud). It reminded me of a really nice Fender tweed Deluxe, and sounded so good that I am considering building a clone.
You should hear my 1956 one with the wallpaper style covering
Hi Uncle Doug, I can't seem to get enough of your videos! I have a question about an amp on another video that used a pentode in the preamp stage. I am building a harmonica amp and I would like to experiment with a pentode. Do you remember which amp that was? Thanks in advance and keep up the good work.
I have featured several amps with pentode pre-amps, Christopher. One that comes to mind is the Mystery Amp......check out the videos.
I've seen the Astron-style caps in a few Ampeg amps...I had one in the shop recently where the rectifier tube was red-plating 3 seconds after power-up, then the fuse would blow...turns out that both caps were wide open! These caps remind me of the one's used in the all-American 5 radios.
I agree, Frank. The sealed, light-yellow Astrons hold up well, but the brown cardboard, duplex electrolytic Astrons don't. I can't recall ever seeing a vintage one that wasn't either shorted, open, or way off its proper value.
Astrons were in alot of the early amps. Rickenbacker, Ampeg, Gibson, etc. When I test them they usually fail. Even the yellow coated ones. They just don't like aging. How did you test out the tone caps? do you load test them? If they were new, do you think you would have had to nurse the circuit like you did? Thank You for another awesome video!
Greetings, Sam, and thanks for your kind appraisal of the video. I haven't had that much trouble with the sealed Astrons. I tested the caps for proper capacitance, but not under load. Although the video did not show it, I operated the amp at a volume setting of 10 several times while testing it, and it held up well. My camera microphone doesn't cope well with excessive volume.....it all becomes a roar.....so I tend to only record at reasonable volume levels.
The "elephant in the room" is why the volume was so ridiculously high in the first place (without the distortion you might expect if there was some component failure), and on both channels (both pots could not have failed to exactly the same degree....and they both checked out). I wonder if the replacement Jensen speaker is way more efficient than the old one? I'll ask the customer if the problem arose after the speaker change and what the impedance of the original speaker was. Stay tuned.
Greetings, Force. The speaker currently in the amp is an almost new Jensen P12R rated at 25 watts. The amp, with two medium-biased 6V6's is rated at only 14 watts more or less, so the Jensen should be a good speaker for the amp. I believe the original speaker was also a P12R or something close, rated at 25 watts, and according to the owner, it was destroyed by the high volume of the amp.....so apparently the speaker is not the problem.
Beautiful work, Doug, thanks for the vid. If you don't mind me asking, what kind of cap testers do you use?
You're welcome, Micheal. Thanks for the compliment. For capacitance, I simply use my shop multimeter. For ESR, I use an Anatek Blue ESR meter.
Hey there uncle Doug, I enjoy your videos. Learned a lot also, still learning. Keep'em coming. I think Ol' Rusty might need some Geritol to help him keep up though. I have several old tube amps I am going to repair. I am just learning the old 50/60/s tech. stuff. I have an old Silvertone 1471, I am recapping now. It looked a lot like the amp in this video, with extra stuff hanging. Might need some help Buddy, Thanks Again.
You're welcome, James. Rusty and I are glad the videos are helpful. Good luck with your amp projects :)
Brilliant!
Thanks !!!
Fascinating issues.
Good job You are a good man
Thanks, Bob :)
Oh, I've seen a lot of this nasty pseudo-repairing job around here, It makes repairing a lot harder.
Weird issue about the crazy volume level; I would say it can be something wrong with the gain of the phase inverter (floating paraphase, old-fashioned arrangement) , It could be driving the output stage too high even with a small input signal.
I think a good speaker, with better sensitivity, would make the amp louder, but not this much!
Great video, you did a great job. Always enjoy watching.
Thanks, Nixo. You're right about the speaker. I finally got an opportunity to ask the owner about the original (damaged) speaker, and it was also a Jensen P12R, so different efficiency is not an issue. I find your suggestion re the phase inverter to be quite interesting. I wonder if adding some resistance to the line inputting the signal to the PI grid might not reduce the tendency to overdrive. Also, perhaps altering the cathode bias resistor value. I will try this and report the result.
You certainly know your stuff uncle Doug. ....awesome video
Thanks so much, Daniel. This amp was a bit of a challenge, but it came out really well. I'm glad you enjoyed the video.
I enjoy all your videos......I learned so much over the last few weeks from you with the explanation videos and am very grateful.
That's good to hear, Daniel. I'm glad the videos are helpful.
I still wonder how do tubes short, guessing the spot welding of the wires from the grids may break and then the wires are free to wander and cause trouble.
For that botched capacitor job, I would have expected more hum while it still worked at the beginning. Perhaps the cap that was left in was in better shape when the _'repair'_ was done.
Also wonder how much did Rusty make of the replaced parts.
+MisterTalkingMachine The innards of most tubes are in close proximity and a bit spindly and somewhat fragile, subject to displacement and breakage due to impact and vibration.....especially when hot. Rusty used the spare parts to make a Muzak system for his doghouse :)
Wow, what an improvement. Really enjoying your videos even though I don't have the first clue about electronics ( it has made me want to learn more though ) and isn't it a shame that people are willing to go to those lengths to fool people for profit.
Thanks, Mac :)
A very interesting set of problems to cure.
No kidding, Andrew, and then, just to make matters worse, the 6V6 fails. To be honest, I actually enjoy weird challenges like this....they awaken dormant brain cells (some of which have been dormant for a long, long time :)
So they justified their "rip off job" that's a new one! Good work! Love your videos! Subscribed 😃
Thanks and welcome aboard, MS :)
Cannot believe that someone who previously repaired that ‘chucked’ in some generic old caps and resistors to justify a large bill. Thats awful. Good work on your repair mate.
Thanks, Greg. It ended up sounding great and didn't cost a lot.....this time :)
Thanks for the video I would have liked to has seen the power cord installation because I have and old Gibson and the white wire is going to the fuse first I know other videos say to change it to the black wire and remove the death cap but there wired to the switch first
The white (return) wire can be connected directly to one of the power transformer primary leads, Charles. The black wire should pass through the on/off toggle, then the fuse, then the other PT primary lead. Many people prefer it to go to the fuse first and then the toggle......but it's a matter of personal preference.
Yes good work I got a RCA pa amp 35 watt very heavy I'm in Monroeville pa was wondering if you where far away?
I am REALLY far away, Dan. What did you have in mind?
Did you ever go back & re-try the 12AX7 tube after the other gain adjustments? With the 12AX7 being so popular among guitar amps, I’m wondering how much the tone changed.
The amp probably draws a lot more current from the supply circuit, now that the bass response has been improved.
That was long ago, A. I don't recall.....but my guess is that the gain may increase with the 12AX7 but the tone would remain basically the same.
Interesting seeing vacuum tubes upside down! How does a tube short?Excellent, insightful restoration! Would be nice to see who did the original rip-off repair.
Yes, most of them tend to work in virtually any position.....horizontal being the most challenging, due to "sag' of internal components and possible shorting (undesirable contact between any of them).
Hi Doug, great videos. I'm curious If you built your guitar or modded it? Thanks.
Greetings and thanks, Scott. My shop guitar is an old, original 1979 Vantage (Matsumoku) VS-695 Performer model with MMK-45 humbucker pickups. I collected vintage guitars and even considered trying to build one from scratch, but ended up becoming addicted to amplifiers instead. :))
Uncle Doug, could you do a video just on discharging filter capacitors?
+Peter D I have demonstrated it in several videos, including the tool I made to facilitate the procedure......a 100 Ohm 10W resistor in series between an insulated, pointed stylus and ground. I touch each + electrolytic filter cap lead with the grounded stylus for about 5 seconds, often doing them all twice just to be sure.
Greetings Uncle Doug, was everything ok at the OT when they replaced the old field coil with a new speaker?
Greetings to you, TB. Yes the output transformer was fine. The old, original Jensen P12R voice coil had started rubbing and making unpleasant sounds, probably due to excessive volume, so the owner replaced it with a brand new P12R. According to him (he picked the amp up yesterday) it now sounds far better than it ever did, and it now has manageable volume (to protect the new speaker), so the modifications and repair were completely successful.......Now, it's time for the next big challenge :))
I have watched all your Fender Restoration/repair videos and i really dig your channel. All these videos is so interesting to watch! Especially when someone that knows what they're doing is making the videos.
The only thing that could've been better would be if you tuned your guitar.. It always makes me cringe.
Greetings, Elix. The guitar was tuned immediately prior to the sound demo, so the problem is either with the recording method or my playing.....or both. Rusty and I apologize if audio problems are interfering with your enjoyment of the videos :) ^. .^
the problem with too high volume on 1 is the same on the Fender tweed 5e3,the pots are not wired like a divider but they just load down the grid.
An interesting observation, Mr. A. I didn't realize that this phenomenon had been seen before, and on other amps. I'll check into this. Thanks !!
It's disgusting how unscrupulous some people are prepared to be with faking repairs. It might be a good idea to take a digital photo of your tube amp before sending it in for repair - that way, any components presented to you could be identified in-place (or not as the case may be). Another REALLY interesting repair - give Rusty a pat from me!
Greetings, Stony. Rusty says, "Hi". Your idea of photographing the circuitry and speaker of your amp prior to handing it over for repair is a good one. Some chassis are not easy to remove from the cabinet, but if possible, removal and photo documentation would be great insurance against false claims for work that was really not done. Another really awful possibility would be substitution of modern parts for valuable, good-working vintage parts, so writing down (or making a pencil tracing of) the parts numbers of your original transformers and speakers is also good insurance. No doubt, the vast majority of repair techs are scrupulously honest, but the occasional "bad apple" can taint the entire barrel.....and precautions like this can detect and expose them.
Great video Uncle !! Thank Rusty for me will ya !! Your debugging comments did spark an interest in me re: the tube short !! When there is a situation like this where it seems obvious there is some major component failure(s) - is there a way to pre-check all the tubes in a non-powered mode with multimeter resistance measurements ?? Did you just measure the pins to find that short (of course - after being alerted by the burned up resistor). Other Comments ?? Do you have a reference that details "good" pin-to-pin resistance measurements for common tubes ?? Thanks again for your great work and your personal integrity !! It's very refreshing !!
Greetings, Donald. In most cases the only pins that have any continuity in a vacuum tube are the two pins connected to the filament (heater) of the tube. To be sure, you need to check a tube handbook. To be honest, tube shorts are so uncommon that I don't pre-check for them, unless I see a burned plate, screen, or cathode resistor. Once again, the trusty Current Limiter comes to the rescue. If the bulb flashes or glows brighter than usual when an amp starts up, it's time to turn it off, unplug it, drain the filter caps, and start looking for the short circuit(s). If you don't have a CL, watch my video showing how to build one, and make your own.....before you plug in another vintage amp.
Uncle Doug
Thanks again for the education Uncle Doug !! Yup - got a CL and a Variac - and use them liberally. Thinking about a tube tester - but that may be a bit excessive ! -- Don
Sounds good, Don. Best of luck :)
Great videos. Have you ever heard of sheridan gt20 guitar amp. I can't find anything on them.
Thanks, John. No, I have never heard of that brand.
@@UncleDoug it's just that it has a hum from the speaker which does not change with the volume control. It also has no sound when you plug anything into it. I think it has 2 problems caps and dc. What would u suggest?
I prefer not to troubleshoot long distance, John, but would suggest that you replace the electrolytic filter caps and see if that helps.
The GA6 I have -and all the other pre-Lancer models are grid leak bias. Very similar to the 5A3 and 5B3 Tweed TV front Deluxe. Either this was an interim model or the bias configuration was changed based on that Lancer schematic?
These amps and the similar GA 20 are beautifully dirty tone monsters. Crazy good harp amps (I'm a harp player) and can be great with guitar too. On the 'mic' channel - I personally like the option of a low impedance super hot-n-nasty channel. It's actually too hot and dirty for my harp mic.s but can scream nicely with guitar when you're looking for that sort of thing.
I'd love to see you dig in on a grid leak biased amp sometime and was a bit dissapointed this GA 6 wasn't that.There is a good discussion of this amp - with pictures of a very similar era GA6 and re-drawn schematics by Miles O'Neal, of Austin, TX here:
www.rru.com/~meo/Guitar/Amps/Gibson/GA6/.
As ever - thanks for the great work and regards to Rusty. I suppose he had a well deserved day off.
Thanks for all your detailed and interesting input, Noam.
Great job Uncle Doug. It pisses me off when incompetent, unscrupulous, so called "repairmen" resort to piss poor work, and then lie about what they didnt do with parts that were never in the amp. 11 caps? Relly? Jeeez. Im glad the customer bought it to you- knowlegeable and reputable. Keep up the great work and the great videos, sharing your experience and knowledge- it makes the world a much better place with people like you. Peace.
Thanks so much, Dave. I would like to think that the vast majority of repair people are quite honest, but the few bad ones sure get a lot of notoriety. It's sort of like airplane flights.....you never hear about the tens of thousands that land safely, only the one that doesn't. By the way, Rusty is pestering me to ask about the fine-looking pup in your icon photo......what's his (or her) name?
I agree- the bad always overshadows the good- too bad it is seen that way. It should be that good overshadows bad.
My Husky was named Max. He went with the Angels after 16 years 1.5 months on earth in Sept. 2011. I am grateful to have adopted him @ 1.5 years old. He is my hero- he saved my life after being with me only 3 weeks. Boiler backed up in my house I just bought and was spewing Co2 middle of the night- he knew it was trouble and woke me up. I miss him every day. Thanks for asking Rusty! And might I add how cute you are!
It's obvious that Max was an exceptional, loyal friend, Dave. I share in your mourning, having lost several very special pals over time. We currently have a collie, also named Max, who is getting near "the end of the trail" and I seriously dread his demise. Rusty is about 16 years old and is beginning to slow down a bit, but still loves to chase the tennis ball for as long as you're willing to throw it......and make videos, of course :)
The journey with our beloved pets is great until it ends. I find the unconditional love and happiness they give us is one of God's greatest gifts, and I can't imagine going thru life without them, even though it really hurts losing them. Tell Rusty and all your fur buddies my Husky Igloo and I say hi.
I will, Dave. Thanks, and best of everything to you and yours.
I'm not a fan of leaving the old filter caps in the amp even if you bypass them for that very reason: they leak all over the other components and just take up space. If you are a big fan of the LOOK of an old filter cap, I suggest pulling the cardboard roll off of the old caps and stuffing a new cap in the old roll and then installing it.
I only leave them if they show low ESR values, no DC leakage, and are fully functional at operating voltage.
Is that a soldered connection to ground on the new 3 wire power lead, or is it just crimped.
I don't recall, Allen. That was a long time ago.
So,was tube damage caused by the bad filter cap? Sorry I'm new at this.
Probably not, Wayne. A short within a tube is generally the result of impact or physical damage to the tube, which bends some of the internal components into close proximity. A bad filter cap, on the other hand, will generally cause too much current to flow through the power transformer and rectifier......damaging them, but not generally a tube.
What I have learned from this detective work and repair is to take pictures of the amp before handing it over to a "qualified expert" for any work.
Good idea, Stephane, and always ask for the parts that were removed, then compare them to the photo.
hi there uncle doug and your trusty sidekick rusty. Thanks for the videos you put up on you tube. have you ever worked on a Gibson GA 45 RVT(not the saturn)but the earlier concertina amp that had 4 8" speakers on it. they call it the holy grail of harp amps. Mine sounds great but I still cannot find the actual schematic floating around for this particular one. looked on several sites to no avail. Any ideas if you have one?
Greetings, Greg. Rusty and I are not familiar with the amp you describe, but it does sound like a real harmonica gem......and perhaps an interesting design for a future homebuild. If we can find a schematic, we'll let you know :)
Thanks Uncle Doug. I can always make a video for you in the future to show you this Gem.
That would be most appreciated, Greg. It sounds like an unusual and desirable amp.
Uncle Doug will do little new to how to post it so may take me a bit I will keep you posted 😁
It can be a little tricky at first, Greg, but if Rusty and I could figure it out, so can you :)
Awesome !............would love to have you as my Uncle !...Haha!.....Subscribed!
Thanks, Lou. Welcome aboard :)
what kind of tool are you using to measure the old capacitors?
It's an Anatek Blue ESR meter. You can order them on the Internet.
I like your discharge tool ,very cool ,next time.could you show it in use,for us novices,and how,you made your sis charge tool ,thanks for sharing .
Thanks, Ron. I have shown how it is made and used in several videos. I'll try to remember to show it all again in the future.
Hi Doug the other capacitor test good ? Thank ….
No, they were both awful.
So Uncle Doug....have the same amp and apparently the same tech worked on it before I got it.....it has the original Jensen speaker in it and I had it reconed but I it was doing the same thing a little tinny and crazy load volume on 1. Could I possibly send it to you to repair. ....I'm not sure of your contact info or where you are located but please let me know.
Brian Peters Brian, I have retired from doing outside work, particularly when it involves shipping. Hopefully you can find someone in your area to repair the amp. The methods I used are explained in the video and may be helpful. Good luck.
@11:26 when you talk about extra parts and the extra money, the unknowingly poor guy had to pay. This really upsets me. It truly seems to be, to me that rip-off artist are just rampant everywhere. This is also the reason why I started learning how to "try" and do a lot of my own fix-it type deals myself. Sometimes you need a pro, but if I can go on UA-cam and find videos, like yours, and many others. There will always be hope for the common man. So, thank you.
You're welcome, Bill, and I agree completely. Rusty and I have developed our skills in electronics, automobile repair, plumbing, carpentry, metal fabrication, etc. largely to avoid being at the mercy of unprincipled weasels whose skills are limited to misrepresentation and fraud. Also, there is always a great sense of accomplishment whenever you can personally resolve your own problems. Hopefully, our videos will help others to achieve this enhanced degree of self-reliance.
+Uncle Doug Amen to that.
Do ,,audiophile capacitors' make any sense regarding the sound quality? I mean if we use oil capacitors instead of electrolyt one's?
+Zacsi Auchan In my personal opinion, "boutique" capacitors are a colossal waste of money. You are far better off buying good quality, reasonably priced "regular" capacitors, and investing your savings (if you wish) in a good speaker and/or good quality tubes.
+Uncle Doug Thank you for your reply. My father has just fallen into this pit... He is building solid state amps out of schematics and he just invested into Janzen, Mundorf etc. caps at horrendous price... I cannot hear any difference between the cheap electrolyt and these "boutique" ones but he keeps telling me that there are "differences" and earns after the swap. Anyway, I love watching your tube, thank you for all your efforts you put into these videos and tutorials.
You're welcome, Zacsi. I guess people can spend their money as they see fit, but as you noticed.....there just isn't much (if any) return for the investment.
The problem with the volume control is due to the strange and unfortunate circuit design. An audio-taper pot is designed to be connected so that the input goes across the resistance element, and the output is taken off the wiper. Gibson chose to connect the input to the wiper, and then take the output across the resistance element. Here you can see the problem.
Instead of having the normal curve, the signal rises much too quickly as the pot it turned up. Further, the tone-control is poor, providing not much adjustment, and mostly just turning down the treble. All in all, not a good design effort. One poster claims that Gibson used linear rather than audio pots for the volume control, which would further exacerbate the problem. This amp shows all the hallmarks of having been designed to be as cheap as possible. Given the tube compliment, this amp could be rewired to have a single-channel input and a real tonestack, following Fender or perhaps Ampeg guidelines. It could be easily turned into a Princeton-like amp. Unfortunately, the public thinks it's worth something just because it was built in the 50s, so that really can't happen. A quick Google search showed that these amps are selling for over $1000, which is ridiculous, given a brand new Princeton can be bought for about $800. Maybe one of these days, people will start to realize that there was no magic being sold in the 50s--some good amps and some bad ones, but no magic.
Well said, Roid. You make several very good points. Thanks for your input.
I watch your videos in complete awe, Doug. Not to pry into your life, but were you a designer of amplifiers for a large company at one time? Just wondering.
Greetings, TF. No, I have no formal training or prior experience in amp design. I restored vintage jukeboxes for many years and developed some amp-repair skills during that period. Most of what I know now was self-taught over the past 4 or 5 years, using the Internet for about 75% of the information and personal experimentation for the rest. I now distill what may have taken me days or weeks to learn into each of my videos.
Uncle Doug
No formal training? You're one of those exceptional people who manage to learn complex subjects quickly, and easily. Your videos are excellent as well. I'm gradually learning the little things by just watching your uploads. Thanks again!
You're welcome, TF. It's my experience that Independent home study is even more effective than classroom study, if you really focus and apply yourself to mastering the subject. I say this despite having been a math and science teacher for many years (now retired).
I noticed that when you first began to look in the back of the cab you were poking your fingers in and around all the capacitors as if you had no fear of shock, but then afterwords you discussed how you were going to discharge the unit before examination. Sorry for the long winded comment, but I don't understand how and when there is a hazard, and last but not least should you discharge the threat before you even start to touch a single component in the chassis? It might be helpful if you were to do a video of what components to be cautious of when examining any unit, and what steps are needed when first approaching a examination of any amplified unit, thanks in advance.
Josh, you make an excellent point. The video scenes were shot and assembled out of sequence. I had already discharged all the electrolytics prior to the scenes in which I pointed to components with my fingers.....BUT, this was not made clear in the video. I just added two notes to the video explaining this and emphasizing the rules of safety. Thank you for bringing this to my attention.
I know it's a little late for this, but those output tubes were re-labeled Russian 6P3S which is equivalent to 6L6 types except GC's. These are commonly sold as a "6L6GT", and relabeled as others as well.
Thanks for the input, TA. Hopefully, the amp's owner will read this comment.
Im new to your channel....just wondering do you make your own rigs? Sounds like you got chops on the guitar and you know your craft when it comes to amplification....you remind me alot of a WW2 vet that fixed my digital modeling amp....that a shop shrugged and said just buy a new amp....told him about it and he said bring it to him 3 hrs later he had it up and running! He never charged me a thing but I cooked up him and his wife a couple of good meals and talked about the good old days!
Ron Gay I do make my own amps and do all my own guitar repairs, Ron. I started out buying all the junked guitars and amps I could find, then repaired/restored them and sold them for a profit. The best part, however, was the experience I gained.....which made me a more competent tech.
Sorry I am late to the party, but have you found the culprit for the excessive voltage? Could it be the OT?
Greetings, AR. It was more a matter of excess volume, and I got it under control by altering the resistance of the volume pots, installing a 12AT7 instead of a 12AX7, and increasing the bias resistance on the Phase Inverter. It turned out great !! Thanks for asking :)
Uncle Doug But didn't those originally come with a 12AX7? You shouldn't have had to change the style of tube....was there maybe some other reason causing the excessive volume?
theshyguitarist
According to the owner, it was always way too loud, which led to the demise of the original P12R. Every component checked out OK and the circuit was untouched when it came to me, other than the shoddy electrolytic cap change. I don't have another early GA-6 to compare it to, so it seems like it was simply designed to have a lot of gain. Recall that this circuit differed considerably from the more modern GA-6 Lancer circuit. Perhaps the changes were necessary to eliminate problems in the earlier circuit.
At the owner's request, I did what I could to reduce the volume in order to protect his new (expensive) Jensen P12R, and the result was still-strong-but-tolerable volume and vastly improved tone. He's quite happy, which is really what matters.
Also, the 12AT7 was a big part of the improvement in tone. In a direct comparison, it dramatically outperformed the 12AX7.....to the point that I would recommend at least trying this change to anyone with an early GA-6. Thanks for your very valid inquiry.
dude, you can use a square d wiggy to discharge capacitors alot more safely. just a thought. i love your videos.
Thanks, Casey.
The previous tech saw the cruddy 10 + 10 Astron caps and had a 50uf Sprague cap on hand, so he was like, "Duh, I'll crudely bypass these and put in my Sprague cap which is (duh) pretty much within +/- 10% spec of the Astron caps -- because if there's two of them, you're allowed to multiply +10% by two. Then he went, "Duh, double caps are unusual," and forgot to bypass one of them.
Gavin McNett Judging from the quality of the work the previous tech did, your scenario is entirely plausible. Thanks, Gavin :)
GO, RUSTY!
Great video, Doug!
As usual i got - my very own ;) - two cents for you as well:
Please, don´t blame "Astron" too much for the electrolytic Caps. As nearly all 50-60 years old Caps will work good by now....It´s all about the cap´s contruction. As the little yellow ones are complete different, they are made with aluminium foil and a insulating foil between it to form the Cap, while electrolytic Caps are made of aluminium foil and some gelish hold in paper. May this leaks out, dissapears or simply just drains out over time.
I had done much more measurements in the "non working" condition, as i´m curious why the amp does behave in the state where it was as you get it....
My own thoughts would be that the screen voltage was waaay to high, leeding the output tubes to a very high gain hence to a loud tone....
Next up, i would have opened the old electrolytic Caps and see in what condition they are, But when you do so, wear some gloves, as the electrolytic is not very healthy for your hands/fingers.
Next is, WTF the repairman - do *not* use the word "technican" here, by reason;) - had done? Replacing a Cap with another with different parameters? Put a 50muF where a 2x muF was, is no *no* good idea since one may risk the rectifier tube....RTFM (in this case the tube datasheet) first ;)
What´s about this changed components bag sh*t...When I repair some old gear, i take a first look inside while the owner is present, and tell him roughly what´s going on and let him decide wether is worth to repair or not....I never do this "bag" thing! i take photos instead and dump bad components, or put them on my shelf if they look impressive in some way...As i got a "beautiful" 15 inch "GAUSS" speaker awfully "repaired" with some hot gun glue...acually someone glued the original voicecoil back on a "half-way" recone kit. Pure bullshit! The speaker was(!!) rated 1500W/8ohms original, if you let it have this 1500W now, the voicecoil simply will "went for a walk" ;)
Thanks for all the excellent insights, TC. The plate voltages were quite reasonable at 332.5 V, and I don't see how the screens could be higher than this, but I will check them tomorrow. Re the Astron electrolytics, I have found the duplex cardboard-bodied ones to be rather short-lived. Whenever I find one in an amp, it is generally way overdue for replacement. Thanks again :)
Hi Doug, no need to check this, because the amp now works fine, and so will the voltages ;)
As i do not know the schematic of this amp, my thought of the too high screen voltage was a "wild guess". Just because i´ve seen this had happened on other amps...
Have a great one!
TubiCal
Yes, but I will keep your recommendation in mind for future troubleshooting. Thanks again, TC.
Albeit not related to the electronics, but wasn't this amplifier used more for harmonica and vocals as opposed to guitars? Just thinking that some of the poorer mods to this amp might have been the result of trying to voice a guitar as opposed to the mouth-harp?
I can't recall any Gibson amps built specifically for harmonica, CB. I think just about any small, portable tube amp can be used for the "harp", but they generally were not originally intended for that purpose. The famous Gibson tweed GA-79RVT was built for accordion use (ugh!), but the vast majority of Gibson amps, including the GA-6, were probably built for use with their guitars. That said, here is a YT video of a guy blowing his brains out through a GA-6: Harmonica Amps Vol.17 Gibson GA-6 (1954)
The volume issue is also a problem with the Vox AC30. Just off zero on the volume control and it drowns out a Boeing 747 at full takeoff power!
Wow.....then, I'll bet the volume increases at a relatively modest rate.
@@UncleDoug Nope, at least not with the AC30 amps belonging to guitarists I have worked with!
I know you probably cant do this for various reasons. But you need to name the shop, that ripped this owner off. So that no one else gets
Taken to the cleaners. Thanks! Curtis
I agree that the crooked tech should be outed, but the amp owner couldn't (or wouldn't) remember his name.
@@UncleDoug the repair bill was in
The amp. Uncle Doug said
@@curtismuma1931 I can't remember if the name of the tech or shop was on the bill. It may have simply been a generic list of repairs (supposedly) done.
The "Gibson Tone" is a super bright amp caused by the "bass sucking" circuit in there. It sounds right with Gibson Humbuckers... I swear. It hates a single coil guitar.
I shudder to think how this amp would sound set to max treble with a Telecaster bridge pickup. It would probably be much more tolerable with Gibson humbuckers. This was, no doubt, a ploy by Gibson to sell more amps to owners of their guitars.
It'd cut you more than a blunt razor.
I've heard of Telecasters being used in "Punish the Audience" mode.....maybe this is what they meant :)
Indeed. I have a 63 GA-5 Brown Skylark... it has a very treble response..... and NO TONE KNOBS... BUT try taking your guitar's tone knob back to 50%... it's surprisingly balanced. I've used it with Gibson, Gretsch, and Rickenbacker... and if you roll the Guitar's treble back, it sounds very good. The guitar's tone knobs rarely get used with most amps.
This sounds like a good suggestion, DK. Thanks for your input.
Fusible resistors from the early electronics age ;-)
akkudakkupl .....also good fog (smoke) generators :)
It still sounds thin to me but then who knows what your mic and my iPad did to it. You shouldn't have to pad the signal down and use lower-gain tubes like that. I would first have been looking for a fault in the NFB chain, but this amp doesn't have one. So I think there must be a shorted turn on the output transformer primary. I would raise the coupling caps to .047uF.
QuadMaestro Thanks for the input, QM. It was a customer amp and is long gone, but I'll keep your advice in mind should it ever be returned for additional work.
No Rusty!? I'm all one for keeping gear as original as possible, but what on earth were they thing with thous electros?
I am too, Pigeon, but (like you), I am mystified by what some supposedly qualified people do to these poor circuits. I thought you all might get a kick out of such an extreme example. Re Rusty, he was happily burying dog biscuits in the back yard (it's his retirement fund) and I hated to disturb him :)
There's no way on God's green earth those were the original caps.I've done TV repair in the past and I'm now into restoring old radios.Those caps date anywhere from the '30s to about 1955.I always replace those old wax caps and the filter caps.You can get a good price on the FP type alone.
Thanks for your input, Dan :)
Lol Your Good!
Thanks, David :)
Wow, they saw him coming. They handed him a mighty midget electrolytic cap, a Sprague electrolytic cap and and a universal can ? Added to that insult I thought everybody knew vintage Gibson's used Astron yellow jackets.
Apparently not everyone knows this, DB, which makes them an easy mark for an unscrupulous "tech" :)
I worked in a TV repair shop as a teenager and the owner had a 5 gallon bucket of old tubes and parts under the bench. When a customer complained about the high cost of repair, he would grab a handful and give them to the customer telling him that was why it cost so much. So the reputation of TV repairmen being crooks was often justified. There's a special place in hell for TV repairmen and used car dealers!
Agreed, George, and I think some people would add certain "politicians" to your list.
You must know what your talking about. I watched the last video about getting shocked,, on amp repaire. My cutter Shocked me when i grabbed it.. I have no knowlege of this,, There seems to be no schematic for my record cutter,, So I need an amp built from scratch,, Something that wont shock me,, Cause i dont see as how radio shack will not carry 600 Volt Capaciters.. Im thinking something more cicuitry,, Myself..But i did take pics before dissasemble.. DO U HAVE A WEBSITE ???
I don't have a website and have retired, Gerry, so I can't help you with your record cutter. Hopefully, you will be able to find someone locally who can. Best of luck.
Would a P90 equipped guitar be as outrageously loud as a humbucker? Maybe Gibson was just trying to get as much volume as possible out of a low output pickup?
Greetings, Steven. Yes, the output signal from single coil pickups is definitely less than from humbuckers. I guess your theory is possible, but recall that Seth Lover at Gibson invented the ("PAF") humbucker pickup in 1955 and it rapidly became the premier pickup on their best guitars. It seems unlikely that they would produce an amp that was not suited for this "trademark" pickup.....but not impossible, I guess.
Uncle Doug
The humbucker was only used on the high priced model. I figure that this amp would be paired with one of the lowered priced guitars. I have a late tweed GA-20T and, even though it was made around 1961 or so, it doesn't really do 'buckers and clean at the same time. Gibson sure released some stinkers into the wild, though. A buddy of mine had a GA30 RVT that he struggled with for a long time, until he rewired a lot of it... It doesn't surprise me that you ended up changing a lot of the circuit.
Judging from the response I got to my video showing how to improve the tone of the GA-35RVT (which sounded awful in original form.....shrill, weak, like nails on a blackboard), I think you're right. Most mid-60's Gibson amps are fixable, but really need a lot of help.
I am appalled by one of the worst botched jobs by a so called professional that should be put out of business. The worst part was the sloppy line cord repair that was a potential shock hazard. Finally presenting bunch of parts to try and justify the high cost of repair that did not exist. Unfortunately I have seen this kind of practice all to often in my own repair shop, it is people like these that give the rest of us a bad rap. I can't count the many times that I have had to clean up somebody else's mess, makes our job tougher. "Great presentation"
+John Cunningham Thanks, John. It's sad but true that a few bad apples can taint the entire amp-tech profession. Part of the video's intent is to make people more aware of some of the deceptive practices they may encounter.
You did a fine job demonstrating that point Doug; it is important that people be aware of some of these bad apples even though they make up a small percentage of the population. "Great awareness wake up call"
😄😎
First Mistake all Astron filter capacitors should have not be used only replaced
Here's the graph showing the volume-control response of the GA-6 vs. a standard volume control response.
geekpic.net/pm-3I2FV6.html
Very interesting and illustrative, Roid. Thanks for posting this :)
The owner of that amp was damned lucky those bad electrolytics didn't take out the power transformer!!!
He was, indeed, Jerry.
Shoddy repair work...i would be pretty upset....Love your videos. Question I have built a couple of amps but i would like to know more about tube amps testing values and repair...is there a book you would recommend..
Greetings, James. There is a book by Gerald Weber that's quite good. Some of it will not make much sense until you have developed some foundation and experience, but it would be a good place to start. I personally read Internet articles about each portion of the amp circuit, to increase my knowledge and understanding.
Uncle Doug Thank you Sir...Cheers.
Gibson never should have dabbled in the amp arena. I have never heard a good thing about any Gibson amp. A few models were decent enough, but with Fender, Supro and Gretsch amps out there that were proven great tone and reliability, why bother? Gibson is guitars. Go Rusty, Go!
Rusty says that from about 1963 to the present, he agrees with you, 6S.....but from the late 30's through the early 60's, they made some pretty snazzy amps, like the EH series, the BR series, a bunch of the GA series, including the GA-40 and GA-79, and some others. Granted, not up to the fame or current value of Fender amps from that period, but very good amps nonetheless. Their guitars, however, were definitely their strong suit.
you need to replace all capacitors that are in it, they become leaky over time, it will ruin tubes because they leak DC through them, witch is what they are supposed to stop from happining
Thank you for your helpful input, TD.
i sorta learned it the hard way
That 50uf cap had a 1971 date code on it! Total rip-off repair. Isn't 50uf too much for a 5Y3 anyway? Also, I believe 12AY7 is the correct tube for this amp.
Thanks for your insightful comments, HS.
@@UncleDoug Nice job on the amp despite this, I should point out. I've seen my share of incompetent repairs over the years, and nothing really surprises me. What caught my eye about this amp however was the obvious deliberate attempt to deceive, the phony "original" caps that looked like they were out of some 40s radio. Then he tacks in a 40 year old Mallory cap to the 5Y3 and it's too high a value, and leaves the rest of the old dud caps in circuit. Just... wow...
Radiotvphonut his channel on UA-cam he will test filter line voltage filter caps capacitors and replace them with new and any old filter capacitors filter caps should be replaced they should not be left in the circuit they get leaky and fail
Scotty likes beens
I know you are not inclined to divulge who it was as you are a good man and teacher but i think you should have accidentally opened a portion of the invoice to help unsuspecting customers who may come across this buffoon who is blatantly ripping people off 😉👌
As I recall, there was no name on the invoice......or I may well have disclosed it.