Gibson GA-35RVT PS oscillation?

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kdawg

Well-known member
Joined
Jun 9, 2004
Messages
129
Location
California
I wonder if someone could help me out. I've got a Gibson GA-35RVT amp that I've recapped, but has pretty damn low volume output. The only thing I can find is that there is weird, jumpy AC oscillation after the choke. The signal path looks strong all the way up till the tube before the output transformer, V5A.

Schematic: http://www.gibson.com/Files/schematics/GA-35RVT.jpg

Once thrown into ON from Standby, I see the oscillation on the left side of the choke, L1. Also on both sides of R39 (3.5K 7W) and R38 (5K 5W). Also on pin 1 of V5A feeding the output transformer. Both those ceramic resistors read OK, but they do look a little abused.

The choke read .022 ohms or so. Could this be a bad choke or should I replace those ceramic power resistors first? I don't have either, so I have to order them.

thanks
-kdawg
 
Pull V3, or turn the vib speed or intensity to see if that affects the jumpy voltage. Don't let the vib circuit send you on a wild goose chase. Can you bypass v5a with a .1 cap from c9 to the primary of t3 just to check for good output?
 
I'll try that next time I get to the amp. Vibrato depth does affect the oscillation, but only in a vertical movement. I'm seeing horizontal weirdness at any depth. I'll see if the gain increases with the bypass cap.

What does the R53 pot in the vibrato circuit set anyhow? What should it be set to?

thanks
 
Tried pulling the tube, no change so it doesn't seem vibrato related. I bypassed v5a and there was no change. I definitely see oscillation still. There is clean output audio-wise, but feels about half the volume it should and I can see oscillation on both sides of the center tap on the secondary of T3.

Should I just bite the bullet and order a new choke and power resistors?

-kdawg
 
Do those power resistors drift in value when they get hot? Maybe a funky output tube? Try just one power tube at a time. maybe move wires around with a chopstick. The caps you put in, are they grounded in the same place as the original? Check for good grounds, aligator clip to ground, neg.
 
I tried the old and new power tubes, no difference. Caps connected/grounded same as previous. I'm just going to order some resistors first.
 

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