It may be worth it to post on AudioKarma.org. (not trying to hijack DA) Just saying.
There is a large contingent of folks there that are fluent with the Dynaco stuff.
I'm far from an expert, but it looks like some sort of IBBA modification with LED indication of output tube target bias.
Whoever did the work was good at soldering but maybe not so good at structural and spacing [safety] considerations.
That sort of mod is not a bad thing. The intention is to bias the tubes for best performance while protecting them against fault (thus the fuses).
k
There is a large contingent of folks there that are fluent with the Dynaco stuff.
I'm far from an expert, but it looks like some sort of IBBA modification with LED indication of output tube target bias.
Whoever did the work was good at soldering but maybe not so good at structural and spacing [safety] considerations.
That sort of mod is not a bad thing. The intention is to bias the tubes for best performance while protecting them against fault (thus the fuses).
k
Given the Dyna resistor quality and tolerance, having AC balance controls is not a bad idea. But I would rather use better parts instead.
Very close DC balance in the OT is quite critical to a tube amplifier's sound quality. Most matched pairs sold are not good enough.
And what about after the "matched" tubes drift with use?
Possibly those LEDs are bias high/low or balance indicators. The "Stereo 70 series II" circuit had something like that. Maybe that random little electrolytic cap is part of that? I guess those extra small chokes are extra high frequency filtering on the high voltage supply, but it's hard to see where they're connected.
I looked up the series II schematic (here: https://www.diyaudio.com/community/...version-2-1992-panor-corp.395111/post-7251123) and I don't see any electrolytic. It looks like there's some loose ends in there, I wouldn't power it up without first drawing up a full schematic and a little clean up. Not sure what that cap on the bottom of the 7199s is for either. It looks like there is one extra resistor on each output tube socket, maybe a screen stopper? If that's appropriately sized, that's a mod I like to do.
Looking again, that small electrolytic cap is tied in with the front panel switch, which has wires running to the bias supply. Kinda strange. The extra resistors done differently in each channel on top of the board also give me the impression this might have some incomplete experiments.