This is a cathode bias amp. The original bias resistor was 200 ohms, which at 412 VDC on pin 3 had the 6L6 tubes idling at 28 watts apiece. This was a little warm for my tastes. I replaced it with a 250 ohm for starters (still on there at the moment) and currently it is at about 420 VDC and dissipating about 24 watts at idle. Still warm AB1 as I understand it, but we'll see. I may eventually go to 270 or 300 ohms.
I took off the original OT and popped on a brand spankin' new replacement I happened to have (not identical but perfectly suited to this amp) - you guessed it, NO DIFFERENCE!!!!!!
Something very funky is going on here. I almost feel like I am overlooking something very simple, but if so, I can't see what it is. I feel like I have checked everything, even things which technically should have no bearing on the issue.
A little while ago, just to see if something wrong in one channel could be somehow bleeding into the other channel and causing both to act in the same manner, I lifted - one at a time - the signal from each preamp stage feeding the mix resistors. Did not make any difference.
BTW. the tremolo is typical Supro trem as it was somewhat standardized by the early 60s - the only connection to the rest of the amp is via the shared cathode on channel two (one triode of trem tube shares cathode resistor with channel two triode) - I have yanked the trem tube and checked the trem circuit - no change at all.
While having the amp on and signal going in, volumes full up, I chopsticked the thing to death both preamp and power amp - could not get the sound to vary one iota.
I took off the original OT and popped on a brand spankin' new replacement I happened to have (not identical but perfectly suited to this amp) - you guessed it, NO DIFFERENCE!!!!!!
Something very funky is going on here. I almost feel like I am overlooking something very simple, but if so, I can't see what it is. I feel like I have checked everything, even things which technically should have no bearing on the issue.
A little while ago, just to see if something wrong in one channel could be somehow bleeding into the other channel and causing both to act in the same manner, I lifted - one at a time - the signal from each preamp stage feeding the mix resistors. Did not make any difference.
BTW. the tremolo is typical Supro trem as it was somewhat standardized by the early 60s - the only connection to the rest of the amp is via the shared cathode on channel two (one triode of trem tube shares cathode resistor with channel two triode) - I have yanked the trem tube and checked the trem circuit - no change at all.
While having the amp on and signal going in, volumes full up, I chopsticked the thing to death both preamp and power amp - could not get the sound to vary one iota.
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