Dr Z Jetta schematic

[ampnewbie]

I drew this out in case anyone is interested (didn't include power supply section or the effects loop). It used 12AX7 preamp/inverter, and 7591 power tubes. It's a little noisy if you go past noon on either volume or master volume. If anyone has any ideas on how to make it quieter, I can take a crack at the changes.

[vampwizzard]

the noise would likely come from heaters, layout, and solder dress. Probably not a function of the circuit schematically. Cool piece here though thank you for sharing it!

[pdf64]

If by noise you mean hiss, the 68k grid stopper is likely to be the most significant source in the amp.
Try 33k or down to 10k or even lower.
The input stage's plate to cathode HF rolloff cap should ensure stability is maintained.

[ampnewbie]

Noise is pretty much gone now, amazing difference. I did 2 things:

-completely removed the effects loop from the circuit (I never used it anyway). It was very noisy even when not in use.
- tried various grid stopper resistor values at the input. Lower values were quieter. But, on a whim, I removed the 68k altogether and the difference was pretty profound. That introduced a pop when I plugged in my cable, so I added a 470pf cap from input to ground(in parallel I guess to the 1M resistor). Worked great, and I’m not picking up any RF.

[ampnewbie]

Ok, I ended up installing a simple switch that gives me the option of going through original 68k or with no grid stopper. I am getting a loud pop when I engage the switch though. Any suggestions on how to avoid that? (And lmk if I should move this question to a different forum.)

[sluckey]

How is the switch connected?

[ampnewbie]

like this.

[pdf64]

There wasn’t an fx loop in the schematic you posted?

The bypass switching arrangement you’ve used breaks the grid circuit, leaves it open momentarily. That’s asking for nasty noises.
Just use a simple spst switch to short out the 68k. That way, the grid circuit is never interrupted.

[ampnewbie]

I never drew in the loop...it is the Metro Zero Loss loop.

Not sure I get how I would ground the 68K with a spst switch...if i ground it at either end, all signal will just go to ground before making it to the grid, no?

[sluckey]

...

[ampnewbie]

ug. so obvious. thanks.