Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on December 04, 2022, 04:59:36 pm
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I'm trying to provide some long distance help to a Friend in Cleveland Ohio that I build a 6L6 Champ for back in 2010. Please read the description of this ailment and see if it rings a bell in your memory. I had him clean all the tube pins, sockets and jack with contact cleaner plus try different known good tubes but the problem persist. So this is his description of the ailment:
Cleaned sockets and switched tubes but it didn't help. When the sound thins I turn the standby on, the low end comes back, sometimes the first time and sometimes only after flipping it on and off a few times. Also, sometimes turning standby on the low end comes back as the volume fades and then stays when I turn the standby off and sometimes it thins again. Also, there is static when I switch the standby off as the volume fades back in.
Hoping I can help him clean it up without and major actions but from my knowledge, it could be a lot of things and without the amp not being in my hands, hard to tell. So I hope maybe you can point me in the right direction so I can point him in the right direction. Here is a link to the old 2010 thread on it;
https://el34world.com/Forum/index.php?topic=9444.msg85720#msg85720 (https://el34world.com/Forum/index.php?topic=9444.msg85720#msg85720)
I don't have a schematic but here is a pix of a gut shot. Thanks in advance!!! Platefire
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Well - the next thing I would try would be to jumper out that standby switch. Does your friend have a meter and know his way around amps a bit?
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Tubes first.
Filter caps second.
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Thanks guys. I heard back from him this afternoon again. I had ask him did he try another 5Y3, because I was suspecting the rectifier. So he went back and tried another 5Y3 and said it ran good for about 10 minuets before he turned it off. He said he would try it again tomorrow, so I hope that was it. Platefire
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Since we are already discussing this old build, I would request your opinion and suggestions. I did this build with transformers provided by a customer out of a Voice of Music Reel to Reel that he said used a 6L6 power tube. He wanted a 6L6 Champ based on Fender AA764 Circuit. Well I built the amp best I could at the time but I was disappointed at its performance and in my opinion sounded rather puny.
I know my customer would be wanting that 6L6 putting out close to maximum headroom(loudest) as reasonable response. So I'm wondering at this point what might I do to get there?
Now re-looking at it I see some things I did I don't know why? such as all three filter caps are all 20uf/600V Filter caps. On hind site it seems to me using a 6L6 and regular 12AX7 pre-amp tubes, I would have been better amp response using 10uf/20uf/30uf.
Also I wondering maybe by using a 5R4/5U4/5AR4 to get the plate voltage up a bit???? If I did that, that first filter cap may need to be bumped up to 30 or 40uf?
So my file folder for this project showed the 6L6 plate voltage at 361VDC---rather low, more in line with 6V6V? the plate voltage for the 12AX7 plates was 245VDC---little high? AA764 is 200VDC pre-amps
The bias on the 6L6 was left at 470/5 watt with 25/50 bypass cap.
So should I tell him to slip a 6V6 in there and be happy or is there anything that could be done to improve 6L6 performance with a few adjustments? Thanks, Platefire
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I will offer one thought that may or may not be useful in this quest. I would suggest trying a 100uf cathode cap instead of 25uf with the 6L6. I found it can give a "tighter" bass tone and can make the amp sound somewhat more robust.
2nd thing is I'd switch out the speaker to a 10" or even a 12" IF it has the 8" speaker in it. I like the Emminence 1058 and the 10" Ragin Cajun speakers as well as the Emminence Texas Heat with is a 12" speaker.
If the 8" speaker is in the amp, I'd change that before doing anything else to the topology of the circuit.
Just thoughts to consider .........................
With respect, Tubenit
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Hay Tubenit!! Good to hear from you. Hope you been doing well.
The customer, "Bear" I call him, used a old Dr. Z cab that I think takes a 12" speaker. I'm not sure what speaker he has in there. Attached is a picture of the cab with the amp chassis installed. I will pass the cap recommendation on to him. He does have an amp tech locally that he uses--so it will be up to him to act on any suggestions. He hasn't been using the amp and it's been sitting up, so I would like help him getting running right and see him get some use out of it. Platefire
BTW-I'm with you on the Eminence speakers got the 10" Ragin Cajun, the 12" Texas Heat, Cannibus Rex and Private Jack---all great speakers
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+1 on swapping out the filter caps to fix the issue (since you already did tubes)
Without a schematic I'm guessin, but it looks like you have a TMB tone stack (with fixed mid value - like AB763, etc.)This might explain why you're 5F1 is louder.The 5F1 has no tone stack, so you're pushing the 6V6 way harder than you are with the lossy tone stack into the 6L6.Have him lift the stack and see if he can live with the much different tone...or he could keep the TMB and add a "raw" control. Both simple "fixes"
Either way, I think the TONE and volume are gonna be found in how hard you can push the 6L6.
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Hello SILVERGUN
This stock Fender AA764 Schematic (attached) is pretty much is identical to what I built except only one input jack. Added a standby switch. I see now why I used three 20uf filter caps, because that's how they did it on the Fender schematic/layout.Even though the intent for this one was for 6L6, the plate voltage for my build is 361 and and Fenders is 350. Not much difference. With things as they are, it might be a good idea to try a 6V6 in it to see what that sounds like. Platefire
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Plate, the values for your filter caps are perfectly fine. When they fail they can create the scenario you mentioned above - crackling, lack of bass, intermittent loss of volume, etc., - so changing them out would be a good idea for maintenance that may well solve those other problems.If your customer wants the most output, I'd leave the 6L6 in there and just figure how how to feed it more signal because it will take a bigger input signal than the 6V6 will before distorting. And the more input signal you can provide, the more output signal you will get.
Even a failing bypass cap on the output tube is something I would look for. (Reduced bass, lowered volume)
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Might be worth a shot to use diode rectification to up the 6L6 plate voltage. Although you will probably need to adjust the dropping resistor/s for the latter nodes also. If the rectifier "sag" is what he's after, a "sag resistor" may fit the bill.
/Max
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Check the cathode bypass cap on the preamp tube?
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I second the notion on the tone stack. But before jumping into various mods, I would take Sluckey's advice regarding tubes first, filter caps second. At first I think you said he had swapped in known good tubes - but then it appeared that he had not. The amp has been sitting - not great for electrolytic caps - although they seem to last a lot longer than web lore suggests. Anytime someone says an amp is not as loud as they expected, I suspect a problem - find it, fix it, no mods needed (except for maybe that tone stack.) I have built two SE 6L6 amps from old small PAs - they are louder than I expected.
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I heard from Bear this afternoon. He tried it out with the different 5Y3 rectifier and it did the same thing---thinning out and dropping in volume.
I told him we have done everything externally we can do and now it's time to go inside. So it's a question ifhe wants me to advise him what to check and he do it himself or get a amp tech. So like bmvcowan said,we just going to concentrate on getting it working right first. I'm beginning to suspect those filter caps more now.However I have had a loose ground do similar things also. We'll have to check it all until we get it. I wish I had it in my hands because I enjoy trouble shooting. I guess with ya'lls help we'll all trouble shoot it long distance wise:>)Platefire
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But did he change out the 6L6?
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it's time to go inside
Assuming he has actually swapped in known good tubes, does he know enough to be safe? Does he have a multi-meter? I have visions of the movie Airplane - Lloyd Bridges talking Stryker down. If you enjoy trouble shooting, why not ask him to send it to you?
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Hello
I already heard from Bear this morning. Yes he did try a 6V6 along with another 6L6 and he said he can't tell the difference in them. Of course with this issue reoccurring, how can you tell what you really got?
Regarding trying known good tubes, I think he tried that on the pre-amp and power tube but not on the rectifier. I reiterated about trying another rectifier tube and he finally did with no change.
I'm pretty sure that Bear has a multi meter but is limited on what he can do. I just made him an offer to work on his amp labor free if he returned it to me. I don't feel good about directing him to do test and checks to a live amp that may be required to get to the bottom of this issue.
So if he does ship it back, with his input I would also work to get it to sound like he wanted it to so he would actually use it and it wouldn't be sitting up any more:>) Platefire
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You know, a lot of people, including me, don't care for the AA764 Champ sound. May be the case with your friend. Not a lot to work with. If you disconnect that 15K resistor from the BASS control, you'll have something closer to the sound of the older 5F1 champ. But if he wants a cleaner, louder amp, I don't have any ideas to offer.
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All I can say sluckey is hind sight is 20/20. If I had it to do over, I would of done a whole lot more discussion with Bear before I took it on, to have a better idea of what his expected outcome was to be. As it stands, it is
what it is. So yes I agree, it isn't much can be done to improve it without doing a total re-work. On the other hand he has never offered one word of criticism of his displeasure of the amp. Just reading between the lines, its dis-use says it all. I never created a schematic because it is a AA764 exactly with the only difference
being, it only had one input jack. He provided the recipe, the transformers and I built it without question, so my fault on the outcome. I will do what I can to help him get it running normal again. Once we get it fixed, might look at some helps we might add:>) Platefire
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I have had guitar cables cause the exact same symptoms before...or a bad ground on the guitar's switch or output jack or something. I would imagine he's already swapped guitars/cables? Sometimes pots do funny things when they're dirty as well.
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Hi Al
Yeah, last time I had that kind of symptom on one of my amps it was a bad ground or bad connection on a tube sockets. I always try all the simple stuff first like guitar cords, input jacks, speaker connections, plug the amp in a different outlet or kick it a couple of times---just kidding. Seriously I have bought old amps that has been sitting up a long time, that would turn on but wouldn't make any sound and with some good contact cleaner and elbow grease, bring it back to life just by cleaning all the connections. As it has been said, divide and conquer. Platefire
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This suggestion probably won’t fix the issue on hand but once you solve the problem look at the OT.
I couldn’t see any info mentioned about the OT but my Vibro Champ’s original OT was useless. I’m running a 6L6, the 5 watt champ OT was over saturated even with a 6V6. Once l replace it with a 10 watt, big difference in output. Like Sluckey, l’m not that hot on the Champ sound. I made some small changes but never got the tone l was hoping for but the bigger OT did make a fair amount of difference on the plus side, more volume.
Wishing you the best.
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Hay dude
Thanks and yow, we'll eventually get that amp going. Sometimes it takes a lot of patience.
I got a DIY 5F1 Champ made to take either a 6L6 or 6V6. My first new build(Kit) ever in 2001. I have really enjoyed that amp. Platefire
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Bear at the moment has some other things to do.
Will get back to this a little later. :icon_biggrin: