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Huh. Got the amp back and fired it up last night. The issue remains. I guess there's no way around this. No fuzz in the loop. Jimmy.

Damn! That sucks

Aww... I was really rooting for this to be worked out for you too.

Apparently this is a result of capacitance, cable+fuzz together working like a condenser mic.

Try a different cable, then? George L is not my favorite in terms of how the connectors attach, but is very low capacitance and mechanically stiff. If you have one laying around, might be worth a shot

It's a fair distance from my board to the loops, and I don't have any George L's on hand...but you've got me wondering about "low capacitance" instrument cables. Like these:
https://gothamcable.com/en/gothamcables ... c1ultrapro

even more poking around shows me that this capacitance issue is NOT isolated to the effects loop.
If I turn the volume way up and tap an unconnected cable with a pen where it enters the amp, i get a similar "microphonic" sound. No squeal, of course, but it's definitely "live".

What if you unplug the cable and tap the front panel near the input jack?

it only happens when a cable is in the input jack.

Sorry for the seemingly annoying set of tasks, but…

Do you have a quarter inch connector with no cable attached you could try? Or failing that, perhaps a short pedal cable?

Would be interested to know if the symptom manifests with the shorting jack’s switch open (not grounding the input to V1), but no cable inserted.

traveling for work this week, but will give this a go.
thoughts from the tech workbench (i'll try to run it down next week):
>> So nothing microphonic with the volume turned up, but no cable
>> plugged into the input rules out microphonic tubes and lets us know
>> the problem is before the first tubes.
>>
>> Since there’s only four components between the cable and the first
>> tubes it’s either the cable or one of those components. If I had to
>> guess which component I’d say it was the input capacitor.

and Brad.
NOT ANNOYING AT ALL. I appreciate you and your input immensely.

echobaseone wrote: ↑

Sun Oct 23, 2022 5:03 pm

I couldn't get the tubes to oscillate via pencil tap, but holy hell, the whole thing is "alive".
You can touch the knobs and hear it through the speaker. Touching any of the cables with fingernails is transmitted through the speaker. Tapping on the faceplate, etc.
As far as tapping the tubes goes, the PI/Reverb/FX loop are the loudest when tapped, but the preamps ones are quite live too.

Looking at this schematic…if I’m reading it right, the switched input jack seems to ground not only the input to V1 (via the capacitor) but also the input to the FX loop when nothing is plugged in?

Do try that dummy plug or short pedalboard cable! Will be curious to see the results.

Got home yesterday. I did the dummy plug experiment. Tapping the faceplate, or the dummy plug, gets some sound transmission through the speaker cab on both clean/dirty channels.

So that would seem to definitively rule out the cable as the problem, which aligns with expectations.

If I were a betting person:

I’d wager this amp has a few tube locations that are *extremely* susceptible to even slight microphonic behavior.

You could probably go to a lot of trouble to select (or a lot of expense to buy pre-selected) tubes with very low microphony (once figuring out the critical spots), or you could just avoid running a fuzz in the loop.

If you want to go the former route, see if you can find selected “microphone grade” or “phono preamp grade” 12AX7s from some retailer.

Not many mics use 12AX7s but plenty of phono preamps do.