So, I have guitar direct in to a single-ended class A (12AX7 and 6V6) Champ-style amp. The amp is from the 60s and unbranded, it's likely a Pepco (Pine Electronics) Silvertone style circuit. It's been re-capped and upgraded with a safety ground.
When I shut it off while playing and keep strumming, I get a really nice compressed distortion that slowly fades off as the unit powers down. It also removes the hum (which makes sense, since AC power is disconnected).
I've tried using a VARIAC with the amp to recreate this effect but it actually cleans up the tone (and lowers volume).
My non-EE's guess at what's happening-- A voltage spike from AC shut down is causing a higher-than-normal B+ (DC) voltage (causing distortion) which is stored in the power tube (?) at a lower-than-normal amperage (causing compression and a lower output volume) which then rapidly depletes (using up stored B+ from power tube) as it shuts down.
If I wanted to replicate this tone, should I use a multimeter to find the B+ line and what DC voltage/amperage it's at, then supply the same voltage/amps myself to that line (with the AC power switched off)? That is, if there isn't some kind of inductance happening or stored AC/DC somewhere besides the B+ causing my "sound".
Of course, my own guess could be totally off. I realize you all might be limited without posting the schematic (which I don't have). Any ideas?