'76 distortion V4 rebuild woes

[danb435]

I have found the distortion model V4 to be sensitive to oscillations. I usually add screen resistors and move the grid resistors to the power tube sockets.

But that doesn't mean that is what you are experiencing.

I know you were very meticulous in changing out components, but I would check the values again, and make sure your solder joints are clean.

you can also narrow things down by removing preamp tubes. I would start with V5 (12dw7) and if it continues, pull V201 (6AN8) if it continues then it is the power tubes and you should do the things I listed above. (they should probably be done either way)

ALRIGHTY THEN...
The signal wire from the "vintage" 40-year-old coax cable going to the preamp out jack had broken loose at the solder joint on the board. Repaired and the oscillation is now gone, looks like that pesky old power section does not like the input floating around out there... Anyway, this cleared things up enough for me to hear a lot of AC background hum. I decided to replace all of the PS caps just for good measure and this improved the problem a bit but not significantly. Everything else seems to look good, volume and tone controls have all tested good. I can plug in a guitar and the amp is finally moving air again. As for the hum, I can plug a shorted cable into the power amp in jack and nearly all of the hum backs down. I've got about 7.4 volts AC noise on the 565VDC supply directly off the filtered bridge rectifier (point "A" in the schematic), is there an acceptable percentage of ripple in the power supply? More weirdness: it turns out there is an AC residential power distribution transformer within about 15 feet of my humble workbench where the amp resides. Could this thing be inducing AC noise into the amp? Before I rule this out by hefting the amp into the house (I'm getting a little long in the tooth, aka retired, this amp is lot heavier than I remember!) do you have any suggestions where else I might look for the hum issue? If I can get my hands on an oscilloscope, at what test points would you suggest checking?

Another question, I have tried 3 different 6AN8A tubes including a NOS Sylvania JAN version, all 3 are extremely microphonic. Is this normal? Inasmuch as I have yet to accidentally defib myself not being a real tube guy, once again any help would be appreciated. Oh yeah, what value screen resistors do you suggest? Thanks Steve, danb435

The 6AN8s are almost always a little microphonic. most pentode preamp tubes are at least a little microphonic.

The 7.4VAC on power supply point A is about right. its not a huge deal because of the push pull power amp. the hum cancels out... or should as long as the power tubes are really matched.. which never happens, so there is going to be a little hum. one way to reduce it would be to boost the cap values to 220ufs but that really changes the feel of the amp... makes it feel more stiff. One of the really nice things about the distortion model V4 is the really reactive feel it has. this is partially due to the reservoir cap values and part due to the lack of negative feedback in the power amp.

the big power transformer could be inducing some hum... but it would be 60hz, not 120. what happens when you adjust the hum balance pot?

I would add 470ohm screen resistors if you are using 6L6gcs or 7027s. add 1K if you throw some 6550s or KT88s in there.

Very good, thanks much for sharing your knowledge. Dan B.