Volume Knob Thump

[JKRamsey1991]

Just finished a build that is basically a single channel ab763 without verb or trem. Amp sounds and plays fine however there is a large singular thump as you turn the volume from 0 to about 2.5. It thumps and output stops momentarily. Rotate the control a few degrees each way and it’s fine.

I’ve checked everything I can think of and can’t seem to find a fault. I have replaced the volume pot as well (twice actually) and nothing seems to fix it. Anyone have any ideas?

[sluckey]

That's odd. It will be very helpful if you post your actual schematic and some hi-rez pics.

[pdf64]

Oscillation is a possibility.
Can you scope the amp output?

[galaxiex]

Probably not this, but have you checked for DC on the pot?

[Ritchie200]

Yes, replace coupling cap and see what you get. That will give you a good thump if leaking.

Jim

[Ritchie200]

Oops should have looked at the schematic first... Darn fenders!


Look at replacing your tone stack caps. Any DC getting through will give you a thump. Are you by chance using NOS caps in this area? Not saying a new one couldn’t be bad but the old film caps would leak.


Jim

[JKRamsey1991]

I’m reading maybe 1mv on the volume pot. Caps are all new no old parts in the amp. Maybe I’ll shotgun the tone stack caps and see if that fixes it.

[Jeremy_FK]

That's odd. It will be very helpful if you post your actual schematic and some hi-rez pics.

Hi JKRamsey1991,

as sluckey says, schematic + pics could really help us... I had a similar issue on my 6G3 adaptation (no trem and no normal channel).

thanks,

Jérémy

[JKRamsey1991]

Well I have slightly disassembled it. Going to replace all coupling caps in a shotgun approach. Found one that may be slightly leaky. Going to replace all of them. If that doesn’t work I may just turn it in to something else, because it’s kind of a redundant amp in my arsenal anyway.

[pdf64]

As you've eliminated Vdc at the grid, then the thump is probably due to oscillation. If you work to systematically investigate, analyse and resolve this, then you will be best positioned to learn from this type of issue; I know I certainly did (and do).
Whereas shotgunning parts then rebuilding isn't going to help you the next time you encounter oscillation, which is very likely as most guitar amps have little or no margin of stability.