Biasing Used RCA 6L6GC Blackplates
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- Опубліковано 6 жов 2024
- Don't attempt to do this unless you understand the safety precautions you must take to prevent yourself from being electrocuted. These amps have upwards of 500 volts DC running through them and if you put your hands in the wrong place, it can kill you. The number one rule is to never touch the chassis with both hands at one time. That completes a circuit through your heart. I do it several times in the video but I am very careful where I touch the chassis. Never touch the wires inside. This is my process of biasing my amps. I had to learn on my own so there are probably better ways of doing it but this works and I have used this method on all of my amps for years. You only need a decent Digital Multi-Meter and either a single or dual bias probe. The dual is much faster and convenient. You can make your own probes but I have never looked into it. There are other products out there like the Weber Bias Rite but I have never used it. Some people bias by ear. I prefer to have a number range that I can work with. I tend to bias on the cold side which preserves tube life. The tube plates can glow red if you bias them too high. That will damage them. It turns out it is not that easy to find two tubes that bias within 5% tolerance and it can be a real pain and expensive trying to find two tubes that are matched. There are differing opinions on if matched tubes actually sound better. I'm torn on this one but in general, I think it is best if you can match them. They work together better that way. They still sound good if they are not matched exactly. I don't think Fender ever bothered to match tubes at the factory. They just popped them in.
Link to Weber Bias Calculator: www.tedweber.co...
Link to most Fender amp circuit schematics: www.thevintages...
Very few people are this intelligent and mindful of utlizing vintage tubes , and can practically explain this , my new best friend, thanks bro.
Thank you. I just happen to be re-biasing some RCA's today and 5881's yesterday. I have a friend who has some new RCA's and he is thinking of selling them for at least $200 per tube. That's too much IMO. I even saw some Chinese 6L6's selling for $100 a piece.
So i have a bunch of 6l6gc rca, ge, & sylvania. Im noticing that most of these vintage tubes pull way more current (ie 52 to 77 mA) than my bias circuits are set up for with the bias pot turned as low as it can go. Im using a 64 super reverb and 69 bandmaster reverb black faced. My question is : is my understanding correct to change the bias range 27k resistor on the outside bias pot lug to ground to some higher value to bring the current down to 38mA? Plate on the BM reverb is 425vdc. Any advice? JP
@@58flyingvjp Exactly, yes, raise the bias resistor value. I had that problem but in the opposite direction. On one amp, I didn't have the next resistor value down but I found an adjustable resistor that I use for adding a bias pot on my Tweed Bassman amps. So, I ended up with bias pot in place of the resistor, a bias pot on a bias pot. It worked perfectly. Try something in the 30K-39K range or get a small adjustable resistor, 0-50K. Get it to where the main bias pot is in the center range of its sweep. I recently had a problem where I turned the pot all the way full in one direction trying to raise the bias voltage as high as possible and the pot was bad in that area and caused some serious motor boating. The bias was jumping all over the place.
Hi, just want to say this video was a great help. I rebiased the tubes in my 64 Super Reverb and the difference was amazing. I had some old Fender labeled Sylvania's in the amp and it turns out they weren't matched at all plus they were a bit noisy. So I went through my stash, unfortunately no RCA'S, but found a pair of new testing GE's, which luckily turned out to be matched to within a millivolt of each other. So followed your advice and biased them around 33mv.They sound fantastic and as a bonus are not noisy. The amp sounds so much better. The bass in particular is much punchier and cleaner.Before I had to turn the bass right down because it was too mushy. Thanks again and keep up the videos. Cheers.
That's great to hear. There is a lot of debate on if matching tubes sound better but I think they work together better when they are balanced. It is very difficult to find two that are exactly the same. Some people say they like to bias let's say 32 mV and 35 mV to have one break up earlier and the other one add sustain. That's good too. I like to have them at least within 10% and preferably 5%. That probably has something to do with the amps when they were new where some sounded better than others. I don't think they bothered to match the tubes at the factory so some amps had tubes by chance that were better matched than others. I read were Fender biased most of the amps on the cool side probably so they didn't have any early tube failures and for tube life. It is nice to be able to do it yourself so you can experiment with different tubes and settings. Otherwise you are at the amp techs mercy.
So many varitables had a peavey put new matched in sound bad put in my fender great, set volt, bias same on both amps. Problem? Out trany to far mismatched on primary. Put one old tube back in pv sounded great, looked great on scope. Marked bais on old for reference, played amp at least ten years before I sold it. This was matching one tube to transformer
Thanks for this. Love the RCA for the clean sound. Full dynamic response. But I prefer GE for playing with breakup as the sound *to my ears* is a rad smoother.
Yes, I am going for the cleanest tone I can possibly get.
ótimo vídeo congratulações
Lonnie Brooks passed last week. His sons played some blues at his funeral.I just love Tung Sol 5881's NOS in my 66 Vibrolux. More grit more natural overdrive. No pedals. Love your videos especially these.
Yes, I have been watching his UA-cam videos the past few days. We are all getting older. I had a friend that almost died from a heart attack last week. It sucks when the people you know start having serious problems. Are they the old Tung-Sols or new Russian ones? I love the 5881's but mainly in my Tweed amps because with the blackface amps, I'm going for that clean SRV tone and even the 6L6GC's break up a little too quickly. Back to the Lonnie videos.
The old new stock in those military boxes. I've heard the Russian ones are good also. Lonnie was buried at the Lincoln Cemetery about a mile from my house. He along with Dinah Washington, from what I hear. I'm sure that his sons will carry his legacy for a long time.
Yes they're the N.O.S. in the military boxes i've been collecting for a few years. Some are stubbies. They have less wattage then the 6L6 hence they breakup a little earlier.
Your video just had a Tektronics oscilloscope ad. Like I have that kind of money lying around!
The TAD 6L6's are really good tubes too. They have blackplate GE and RCA copies. Ive been very impressed with the life and tone of both of them. By far the best new production tubes with impeccable matching and a 6 month warranty, not bad for around $50 a pair. Mojotone is probably the closest dealer to you.
It's unreal how close TAD got to the appearance and tone of our old favorites. Ruby has the same tubes too, for a few bucks less a set from Dougs tubes. Buy'em while you can, God knows what's going to happen with the current situation in the worlds sand pit.
I tell you, I wouldn't be anywhere near as calm as you were about those RCA's being that far off. Good luck with that, I hope they rectify....er beam pentode that for you.(bad tube puns)
They quickly shipped out the other pair with copper rods so I could matched them myself. Hopefully something comes close. I can return all of them if I am not satisfied. I have at least five other spare RCA tubes and getting any of them to match is darn near impossible. I should have done a video but I sat there last night going through every combination to find two that were close. The good thing is they seem to sound good if they are close. I know the matching seems to make a bigger difference with my 5881's. I have a mental block on new tubes. I need to try the TAD's. One of my subscribers in Sweden told me they just put the same TAD tubes in his Super Reverb so they must have a good reputation. I just can't believe how long the RCA's last. I've played them a lot for over five years and they seem to be good as new. With new tubes, they always suggest it's best to change them every year. That can be just as expensive.
For fixing tube amps, just buy yourself a second hand oscilloscope. 10-20MHz. They're like under $100 here. So they'll be cheaper where you are.
I just use two bias probes , one for plate volts and the other for the Milliamps and i use the online weber bias calculator for class A/B amplifiers and set the Ma slightly below 70% because house voltage varies !
The calculator tells you the exact 70% Ma so i just turn the trim pot for two tubes at once !
All power tube brands have different plate voltage due to resistance differences in design and materials !
I set mine the same with the Weber calculator but recently switched to the Robinette calculator. You need one of these, vintagesoundworkbench.com/amp-maniac. I keep all of my voltages consistent with the circuit voltages they were designed on.
New production tubes may not last as long as new old stock but they sound pretty good. And they don't cost as much. Also you can ask the vendor to sell you tubes that bias hotter if that's what your amp requires. And they'll match them sometimes free of charge.
Good point. I have yet to experiment with new tubes to find the best ones. I hear TAD is good and the new Russian 5881 reissue is good. But if they only last a year then that is a problem. I have had power tubes in all of my amps for at least 5 years and they still sound great and old preamp tubes seem to last almost forever. I wish they could make them like those old ones that last 5X as long.
5% matched output tubes? are the sides of your output transformer the same resistance? Probably not if it's a vintage amp. I have a great sounding vintage amp from the sixties with 207ohm on one side and 230 ohm on the other. To perfectly match the output I need two different tubes to make up for the difference.
Also there is no great clean tone to be gotten from biasing that cold. I see what you mean if you would say that Chinese tubes sound harsh -- then buy Russians/Easteuropean tubes. They are not as sturdy, sound more vintage, lower gain. Check my video on EL84 where I buy the cheapest from a big local electronics supplier and end up with 1976 Russian tubes rebranded in Germany. In the test you see them hold up pretty well against some vintage Mullards.
You cannot get killed buy touching the chassis with both hands, if it's got the three prong cord. Chassis is ground. Don't touch B+ and ground at the same time. Keep one hand in back pocket at all time. Use alligator leads to attach black lead to chassis for ground. Only manipulate the red to measure hv.
Here's how to measure bias:
1. measure the resistance on both sides of OT with power off
2. measure voltage drop across that side of the OT in DC Volts
3. now divide Vdrop by the resistance, that give you result in Amperes for each tube
4. multiply that by plate voltage to give you dissipation in Watts
I have no idea how that Weber thing works, but are you sure your DMM is in the right setting? I have exactly the same FLUKE 115 and think the dial should be all the way down. But as I said -- don't understand that Weber thing. But you might well be measuring some voltage drop across the tube. Like grid to cathode..?
When you first plug in the tubes at 30:30 the balance out ok. Then you adjust the bias pot a bit and the bias is off... this should mean that the problem is not in the tubes.
Sorry about the long letter, really really dig your playing
I love tube amp tech, but I’ve never brought myself to buy any NOS tubes. Seems like it’s a bit of cork sniffing, but I won’t lie, I’m curious. I’d love to get some RCA 6L6 blackplates.
They really are the best but jeez, $400 a pair the last time I looked. Maybe more. I can only afford used ones which are pretty good and they last a long time so they pay off in the long run. I'm still playing these from 5 years ago every night in this Super Reverb.
I've been reading that the Winged C Svetlanas are very similar to the black plates in tone. Regular Svetlanas are just Sovteks though.
I think I have a set of the Winged C. I know I tried to buy some years ago because I had heard good things about them too. Since I hardly play my blackface amps right now, I haven't messed with the tubes much. It would certainly be a good thing to find a replacement for these expensive RCA's.
Instead of doing a manual bias on the fixed bias pot, could you use a voltage attenuator like the carl hartman as an external bias to bias the amp cooler?(assuming the current bias on the amp is on the hotter end)
Have you thought about sticking some brand new tung sol tubes in there and see what they measure before you start pulling resistors off and then you’re dealing with old worn out tubes Also if you took that amp and changed the filter Capacitors to a higher voltage it’s going to read different also but I would try some brand new tubes
No, but I do have a supposed NOS/NIB Tung-Sol set I'm saving for my Bassman. I haven't played these amps in a year or longer. I'm strictly a Tweed man now. I really should do some work on these. Just yesterday I thought about maybe trying a matched pair of TAD 6L6GC STR's in these.
Please buy a set of alligator clips so you don't scare yourself, and also avoid having both hands in your chassis.
I finally did get a nice set of Fluke alligator leads.
@@millstap great ! maybe you can cut that part where you have you 2 hands on the chassis... especialy when it's not so stable hanging like this on the cabinet... I know you know your stuff but some viewers don't ! great vid anyway :)
@@piamp4012 Hopefully they read the video description because UA-cam doesn't have an editing option that can do that.
My question is this, what gz34 rectifier tube are you using ?
Russian and Chinese 5ar4/gz34 will not feed your amp what it needs.
I've found that the made in Japan tubes are in spec and a lot cheaper than British rectifiers
I always used the expensive Mullard GZ34's. But, I finally came to my senses and have been slowing converting all of my amps to a solid state rectifier. They sound better so I was shocked. I've only done one so far, my 1964 Super Reverb and I used the Yellow Jacket. No need to rebias or anything, just plug and play. They are $15. Try one out. You won't go back. I've blown one too many Mullards. Usually it will start arcing or something inside and blowing fuses. That is why I replaced the one on the SR and it hasn't blown a fuse since I put in the YJR.
You're measuring milliVOLTS with your socket probes, not milliAMPS from the table. Do your probes convert voltage to current?
Yes, it does convert. There is a 1M resistor in the socket. I’m not sure of the value and how it converts.
On the contrary, with the amps that go thru my bench, I've found more complexity and woody tone when the tubes are NOT perfectly matched. A good example is when I use the TAD 6L6GC-STR RCA style "matched" pairs, ones I'll even source from TAD directly, others through a distributor. Some of these advertised "matched" pairs are not always perfectly matched, and they'll be off up to 4-5mA as measured with a TAD BiasMaster. I'll compare the tone with a more matched pair (within. 0.5mA) at the same average wattage, and the tone is less interesting and sounds more bland/boring. In detail, the 1966 Tremolux piggyback I am restoring right now ended up working best with the least matched pair out of several TAD pairs in my inventory. It had more bite, bark, whistle, and woody punch. This particular tube pair sounded best biased like the following:
419.3v DC @ 47.8mA (20w, 66.8% of max)
419.1v DC @ 51.2mA (21.5w, 71.5% of max)
The other TAD pairs which are closely balanced sound more "Hi-Fi" with their tubes biased in this same range (20w-21.5w). Those pairs cannot do as many tonal textures as the imbalanced one.
I spent the last several hours auditioning vintage tube pairs which were mismatched as much as 10mA with no additional hum, and the tone was really nice, even more complex than the TAD sets. With mismatched pairs it's as if one tube can get a different character than the other and still be close enough to play well as a pair. There are other details in the restoration process that matter more for tone, but it is possible most amps, cables, guitars aren't at the level that would allow some people to detect the mismatch being more musical like I do. You'll come across forum threads where it is often said "I don't hear a difference between 30mA and 48mA, so I chose the lower end of the range so my tubes last longer." But we are about the way amps sound, feel, and inspire/captivate, right?
Thanks for the info. Recently, I realized that I had been biasing my RCA blackplates too cool in the low 30's mA. I have since jacked them up. What I found was that all of my blackface amps would only go up to a certain point and then I had to change the resistor to get above the high 30's. I also had problems with my vintage 1964 bias pot when it was turned fully up. It would short out and put about 220mA on the tubes which would make them oscillate. Once I changed the resistor and backed off the bias pot, everything worked fine again. I'm not sure right now if I am at 70% on all of my blackfaces but that is what I will shoot for. They sound better than ever in that range. Played them cold for so many years.
On tubes, do you like the TAD RCA tubes? I have yet to find a good replacement for the NOS RCA's. I'm not real concerned about matching pairs except when you pay extra for them and they really don't match in your amp. In my Tweed Bassman amps, I am going to experiment (the next time I try to match 5881's in one) with the 470 ohm screen resistors. I figure that the difference in those resistor values is the difference I am seeing in the bias when you install supposedly matched pairs. I think I will try to match those resistors to see how much difference it makes in the bias of the pair. The good thing is the power tubes really don't need to be perfectly matched but I like to get close just to see or hear the difference. Now that I am biasing them all at 70%, I need to basically start over and double check them. I don't play the blackface amps much anymore, just the Tweed Bassmans. One amp, my 1964 6G6-B Blonde Bassman, is one amp that has never really sounded great and I suspect it is a power tube bias issue now. That is the one amp that I have done the least of work on. I bet I could get it sounding great now.
@@millstap I too was biasing more cool in the past, and it sometimes does help retain woody punch with open back cabs, but it seems the closed back cabs prefer a warmer bias. Lately, I suppose I am biasing more hot either way. :)
Out of all the vintage tubes I have auditioned, the taller RCA black plate 6L6GC are my personal favorites, and the TAD modern reproductions of those are quite close in tone. When I compare tubes, I'll keep the wattage the same during auditioning, and the characteristics I'm mostly listening and feeling for are the ways the tone "pops" like a rubber band when I fingerpick guitar strings. The vintage tall RCA tubes do it best IMO, and they also don't compress the image into a two-dimensional space as much as, say, a pair of JJ 6L6GC. The TAD tall tubes have that big sound space which gives more room for different tones. I would say they come within 80% of the tonal spectrum which RCA's offer.
Then there's the shorter TAD tubes - they're worth checking out too, but they have a smaller sound space. It has made some of my restored Blackface amps feel like a smaller version of itself, but they have an interesting cohesive tone. More things are in phase... think point-source. If one needed more compression on a bluesy steel track, that might be cool to use.
The taller TAD tubes have a better lower midbass-centered punch, and the shorter TAD's are more compressed with more midrange pop. It would be great to see extremely large 6L6GC glass just for fun. :)
On your 1964 6G6-B Blonde Bassman, what type of resistors are you using on the output tube sockets?
@@Rezonances I'm pretty sure they are still the original 470 ohm carbon comp resistors.
@@millstap Good. Maybe switch the PT primary around and see if the other orientation sounds better. I've had several amps that were just shrill until I flipped the 2 prong plug around. There's some other details that make significant differences, but I cannot go into those aspects here for fear of being shunned by other techs.
If your amp is all original the caps and resisters might be out of spec and causing your bias on each tube to be out. If you've done no work on this amp the electrolytic caps should all be replaced (except possibly the ones on the tone stack) Also the grid and plate resisters have likely drifted high and should be swapped out. Some older fender amps didn't actually have a bias pot the pot was called a hum balance pot and what it did was actually balance the current on each tube so if tubes were out a bit you could "balance them to match" Do a bit more research.
I always change all of the electrolytics as soon as I get the amp. Both the 1964 and 1966 still had regular bias pots. I don't focus much on these amps and spend all of my attention on my Tweeds even though I do play this one everyday. I've noticed its noise floor has increased quite a bit so it's probably time to switch out a bunch of the resistors that are out of spec. I just did that with my 1963 Fender reverb and it is sounding great. It had some noise too but it could have been a funky tube which I also switched.
If you don't know what the plastic spacer between the pins are for then you have no business trying to bias a valuable amplifier like a vintage Fender Deluxe Reverb! Not my amp!
I’m not sure what you are referring to. I don’t own a Deluxe Reverb.
Why not stick the neg probe into a hole on the top and just use the pos to test U are going to slip one day and fly across the room Like Gerald Webbers video LOL
Yeah, that was pretty hilarious. I thought Gerald was faking it but probably not.
They were Never Worth the Extra...I think honestly...and just bullshit/hype....the tall bottle Sylvania's/Phillip's collaboration were much more mainstreamed and sent out in all kinda shit...I had found some of those still NIB's those were BEAUTIFUL and just great smoothness and compression biased up warm. but they went south and ran away not long after...the short bottles were microphonic...
What is not worth it, the RCA's or the Sylvanias? I've been happy with these RCA's. They are still going strong in the blackface amps. The NIB tubes you found were the Sylvanias? I think those are what SRV preferred.
@@millstap yeah they were NIB...and don't remember too much what the RCA's were like once. It was the Pair of Tall Bottles and a pair of later I believe Short bottles..one of the short bottles was really microphonic...and the Tall bottles didn't last long and one shorted/wanted to run away etc...But yeah I did like them..I still gotta couple GE's dual side getter ones-Honestly their kinda bland in comparison...
Then idk few years back China tried some attempt on a cross between Blackplates and the tall bottles dual getter like. They were ok had some harmonic character...but they wanted to strip a little early not great with dissipation...
@@millstap Actually had really liked the JJ 6v6 those were cool..I nuked them had them cooking and charred them Plates...but that's some weirder shit from them like a 7591 or something.
@@jspinks2388 I only like the blackplates with the two side getters. I have never tried any new production tubes because I see that so many of them are bad from the start. It's ridiculous that they can't build a good tube anymore.
@@millstap yeah..its been pretty baddd..had quite a few bad batches of JJ E34Ls...didn't have them biased that hot either...a few of them let go at only 440-450v etc