AB763ish build problem

[Rockeraaj]

Hi everyone. I recently built an AB763ish amp. It is basically a one input AB763 without the reverb and vibrato. I went with the layout for the vibrato channel with its three stages of gain. I used all ClassicTone transformers:
ClassicTone # 40-18028 PT, ClassicTone # 40-18087 OT, and standard Fender choke by ClassicTone. I use the 16ohm tap on the OT because I happen to own a Celestion Gold in 15ohm and like it. My voltages appear to be good. I get 402vdc off pin 8 of the GZ34 for instance. Here is the issue: When I power it up, I get some hum which I assume is a grounding issue I'll need to chase out. It's not terrible. When I play the amp, it is incredibly loud. It breaks up early, which is fine since it cleans up dialing the volume knob of the guitar. I think it sounds reasonable, but figure on tweaking. But after playing it for a few minutes, the volume seems to fade slightly and the tone seems to change, then it quits. No hiss, no hum. If I crank it up and slam a power chord I might get a gritty snappy sound. Basically it has died. The fuse doesn't blow. If I wait an hour it fires back up like nothing happened. Then it dies again. If I keep the volume down, it works longer. It might, in fact, work indefinitely at lower volume. I've swapped the OT with no help. I've swapped every tube with known tubes and no luck. I've replaced (one at a time) the five 16uf big caps and no luck. If I remove the third preamp stage, it's a nice, tame Fendery amp that seems to not die. Any ideas? Ask me any questions or if you need more info. And thank you!!

[ToneMerc]

Hi everyone. I recently built an AB763ish amp. It is basically a one input AB763 without the reverb and vibrato. I went with the layout for the vibrato channel with its three stages of gain.

I'm not real clear on exactly what you built, these "xxxish" builds really should be in the technical section. Nevertheless; a layout, rough schematic or pictures would help, otherwise it's like trying to diagnose Cancer over the phone. Now off the top of my head, it could be something as simple as a poor solder connection and I would be checking for voltages at strategic locations.

TM

[Rockeraaj]

Hi TM,

I will post where you suggest. Meanwhile, I will scan my layout and take voltage measurements. And while I'm in the chassis, I will double-check my solder connections. And so you know, I'm no electrician. I have successfully built a JTM45 clone, 50watt Plexi clone, Trainwreck Express clone (that totally kills!) and a few homebrews. I've been pretty successful, but I'm really a hack. Having said that, I follow protocol in keeping myself alive! Draining caps and such.

Thanks!
Aaron

[Gibsonman63]

Have you tried chopsticking? It sounds like a bad (or forgotten) solder joint that opens up with heat if I were to guess.

[Firestorm]

Have you tried chopsticking? It sounds like a bad (or forgotten) solder joint that opens up with heat if I were to guess.

That would be my first call. Maybe a partial B+ short to ground. If it's through a resistor, it would heat and cause those symptoms. Definitely something heat related.

[Rockeraaj]

Well, I started to jot down my voltages and check my solder joints and take a general look around. I found a resister that I hadn't added to my layout. It is the 1M coming off of the third stage .022 uf coupling cap and going to ground. I think it stabilized things a bit. Anyway, I can crank it and it works fine indefinitely. Somehow that resistor has kept the neighbors from calling me too. Someone explain that!!

Anyway, thanks for the help!!! All is well.
Aaron

[ToneMerc]

Well, I started to jot down my voltages and check my solder joints and take a general look around. I found a resister that I hadn't added to my layout. It is the 1M coming off of the third stage .022 uf coupling cap and going to ground. I think it stabilized things a bit. Anyway, I can crank it and it works fine indefinitely. Somehow that resistor has kept the neighbors from calling me too. Someone explain that!!

Anyway, thanks for the help!!! All is well.
Aaron

10-4, glad you got it squared away.

TM