Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jukelemon on July 23, 2025, 06:15:31 pm
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Hi all.
Well..the amp chassis finally arrived to the other house and now I am really baffled.
So, let me start at the beginning..
1- At an out of town show on Saturday, the 77 starts going in and out like it has always done when the bypass cap fails on the 6l6s. this happened within 15 minutes of it being on i.e. around the 3 or 4th song. So..while it was being shipped to my next location, I went ahead and ordered a new bypass cap.
2- Amp comes in today and the darn cap is perfectly fine and the amp plays fine.
3- So.. I play for 30 minutes...all good. I take out the cap...all good. I measure the cap on my meter and ESR meter...all good. I leave the amp on for 45 more minutes...all good.
4 Chopstick around and nothing loose. Test voltages...all are in good shape. Look for broken wires... all good. No physical signs of failure.
5 - I considered an AC loss at the venue BUT we had other things plugged into the same quad receptacle and those were fine.
6 - I considered the speaker but I have never had a speaker fail like that and the speaker is a relatively new recone. I also don't play past 3-4 on the 77 so it is not taxed at all.
I have a three nighter close to Canada next week so I am now a bit worried as I was expecting to find the typical smoking gun.
6l6s are a very nice pair of late 50s RCA. Rectifier is a Groove Tube which I am not crazy about BUT it has been solid for a while now and I would have expected to see a failure within the hour of playing today.
Any thoughts?
Attaching another copy of the schem.
Thank you.
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I don't recall you saying you had a backup amp. If I was playing as much as you I would surely have a backup amp.
Question? How do you know those vintage 6L6's are good? It sure sounds like a power tube getting hot a breaking down. Have you looked in the back of the amp while it's acting up and check for red Plating.
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did you have anything plugged into the "monitor" jack?? if so was that going in n out??
the "going in n out" sounds like the PI circuit could be suspect also, if you strummed hard and it faded out does it fade back in??
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I don't recall you saying you had a backup amp. If I was playing as much as you I would surely have a backup amp.
Question? How do you know those vintage 6L6's are good? It sure sounds like a power tube getting hot a breaking down. Have you looked in the back of the amp while it's acting up and check for red Plating.
Thanks for the reply.
A backup is not an issue. I have that covered always given the past/all of sudden failure of this amp.
To your question..I guess I don't know other than they have been good for 10 years and each and every time this happens it has been the same bypass cap. Certainly is something else now but i would have thought the 6l6s would have again taken a dive after playing into it for an hour? It was like 15 minutes into the set when the failure happened.
BUT..fair point. It could very well be the tubes.
I did not look at them at the time. I just needed to keep playing so I had to switch things up.
i was monitoring them during the shop play for an hour and nothing unusual happened.
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did you have anything plugged into the "monitor" jack?? if so was that going in n out??
the "going in n out" sounds like the PI circuit could be suspect also, if you strummed hard and it faded out does it fade back in??
No, nothing in the Monitor out. I have only used that when playing harp into a large PA. Works well.
No, it was not a matter of hitting it harder than not. It just lost volume a few times and then stopped working altogether i.e. 0 sound at all and exactly like the cap had failed.
Appreciate the thoughts.
Played it again today for a while. 0 issues. Added and removed the cap. No change at all in terms of a failure indicator.
it is odd for sure.
I have never had a tube fail and then play fine. When they went..they just stop working and that was that.
So...very puzzled at the moment and cannot take this on the road until I figure out what is going on.
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Well those intermittent problems are sometimes the hardest to find and the most aggravating. They say divide and conquer. Have you heard of a signal listening amp? With that, when the amp cuts out again, you could connect the leads of the listening amp to where the preamp out grid goes into the phase inverter. If you could hear your guitar signal up to that point, then you would know it wasn't the preamp and then concentrate on the power amp or if you got no guitar sound to PI, you would know it is in the preamp and concentrate on that part. BTW, Doug has plans for a listening amp on this site.
Also one thing I always do on an older amp especially, is to pull the tubes one at a time, spray deoxit on the tube socket and on the tube pins. Then quickly before it drys, place the tube partially in the socket and work it up and down a few times before pushing it fully home. A tube pin not making solid contact with the socket can cause the connection to go in and out. Sometimes a cleaning will correct that, if that is what is happening. Just something to try!
With an intermittent problem, you just have to keep trying things until you finally get it.
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With a tube amp ‘fault’ easy things to try (in order):
1) tube swap
2) cable swap
3) guitar swap
4) speaker swap
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Appreciate the replies/ideas gentlemen.
New cap is in as of last night. checked all the voltages again. Checked bias again. Played for another 30 minutes or so.
Nothing unusual at all. So...
I will play it locally for a bit and use a few other amps for the road gigs.
I did remove the Standby function altogether however.
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I went back and read the original thread on questioning the original design of the standby arrangement. A lot of discussion on possible remedies! The one that stood out was doing away with with the standby arrangement all together. So since this amp seems to be the one you favor very much, I hope this last mod makes your bypass cap very happy :icon_biggrin: ====for years to come.
If you find you can't live without standby, I'm sure with a little study, a B+ standby mod could be done if you can find the room if you would need to add reservoir caps
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I went back and read the original thread on questioning the original design of the standby arrangement. A lot of discussion on possible remedies! The one that stood out was doing away with with the standby arrangement all together. So since this amp seems to be the one you favor very much, I hope this last mod makes your bypass cap very happy :icon_biggrin: ====for years to come.
If you find you can't live without standby, I'm sure with a little study, a B+ standby mod could be done if you can find the room if you would need to add reservoir caps
Hey Plate!
Yeah..I just got rid of the SB altogether. However...It would be nice to know why this design fails :) I will say though..they quickly changed it away from this in the later 77s BUT that may have been only because the 77 went to a fixed bias lol. Who knows.
No other Gibson amp (at least what I have seen/worked on) did this i.e. switch on/off the cathodes.
Hope you're well.