GA-20 low power

[Enzo]

You have 35v of signal on the PI plates? I assume it is also at the grids of the power tubes? And only 3-4v signal on the output plates?


I'd be thinking you either have little screen voltage or your output transformer is shot. Are the screens up to full B+?

I don;t know why the power transformer would be involved.

Disconnect the output transformer from the power tube sockets and B+. Take just about ANY output transformer and clip it into place. DOn't worry about wiring the secondary back to the amp, just connect it to a speaker directly. Does that restore the sound. We don;t care if ther is distortion, we don;t care about impedanced, all we want to know is if a different OT is louder.

If it is cathode biased ther shoul;d be zero DC on the power tube grids. The leaky caps were trying to bias the amp very hot. But that wouldn't reduce your output severely.

You can also go to RG's Geofex site and look up his transformer tester. A couple shorted turns could damp out most of your signal.

[drewl]

No, 3 to 4 volts AC out of the output tranny on the load, sorry for the confusion.
Each stage amplifies, just not much.
And I did remove the OT leads to rework them, and tried two different output tranny's with the same low output.

I tried injecting a signal to the PI to rule out the preamps, and the results are the same.

I want to see if I can inject a larger signal into each output tube to see if they will put out any more.

[MWJB]

Let's recap...

"I want to see if I can inject a larger signal into each output tube to see if they will put out any more." I thought you had eliminated tubes & OT with substitution tests?

You have <30v at the PI plates which would seem respectable enough.

So the main problem (still concerned that B+ is a little low) would appear to fall after the PI tube.

Enzo asked after your screen grid voltages, what are you getting?

How did that 4.7K check out?

"Each stage amplifies, just not much." Well they're low voltage, grid leak input, unbypassed PI cathode....

[drewl]

After putting the GA20 aside I went back to it and still have come up with nothing.
here's the good news, I got another GA20 with 9pin preamp tubes to compare it to, and the DC voltages are about the same, even the low plate voltages
The second one works, seems very loud but the power tranny is humming like crazy, buzzing loudly. But it works and is much much louder than the original.
The second one will just need a power trans and I have to rewire the butchereing it got from someone trying to "fix" it.

Sooo....any other ideas why GA20 #1 is barely putting out?
I'll try the OT again, and double check things, it's filament voltages are good I know that.

Note: With the output tubes removed yes the voltages go up alot, but the signal on the grids of the output tubes is much higher.

Also, the screen voltage is about 100volts lower than the plate voltage of the 6V6's, jumpering in another 10k voltage feed does nothing to increase output.

[drewl]

Well in case anyone was wondering, the output tranny is the problem.
I finally got a chance to jumper in the tranny from the 2nd GA-20 in for repair and it was cranking out about 14w, with some tweaking I'm sure I can get a little more out of it.

So either I did something wrong when I tried a couple other OT's or they're bad, which considering the guy that gave them to me is always giving me bad stuff, I wouldn't be surprised.