GA-20 low power

[drewl]

Okay, what am I missing

Got an old GA20 that sounds great, but it's only putting out a few watts.
It's got the octal 6SJ7 preamp tubes and 6SL7 PI, I subbed out all the tubes and even jumpered in a couple different 18w output trannys with no change.
The filter caps have been replaced, it's dead quiet and each stage appears to amplify.
The phase inverter is putting out about 35vac on it's plates, which seems kinda low, so I was going to start checking the coupling caps, it still has all the original big sticky caps!

Thanks for any ideas, I'll report back what I find.

[MWJB]

You don't say what VAC your getting at the OT secondaries?

What DC voltages do you have?

Speaker is known good?

[TD_Madden]

I had an old tweed GA-19RVT....swapping back its original Jensen P12R REALLY woke it up.

[drewl]

You don't say what VAC your getting at the OT secondaries?

What DC voltages do you have?

Speaker is known good?

3.5-4vac output!
6V6 Plate voltage is about 300v
100v on the PI plates

I disconnected the speaker and ran it into a load also with the same output.
I don't think the output stage is being driven enough, but we'll see.

[MWJB]

300v on the 6V6s seems a little low? What are heater voltages like, also cathodes at PI & 6V6s.

What are you putting on the input, 100mV?

Check for drift in the 4.7K resistor from where the signal to the PI 2nd triode is tapped off? These PIs can be quite sensitive to balance.

[drewl]

Check for drift in the 4.7K resistor from where the signal to the PI 2nd triode is tapped off? These PIs can be quite sensitive to balance.

That's where I was going to resume when I get home.
cathodes are good, 14vdc on the 6V6 cathodes.
Filaments are good.

[pdf64]

Check the old sticky caps for DC leakage, assume they're bad unless you can prove otherwise. Don't test in circuit; lift the 'cold' leg, power up and test for DC.
300V B+ sounds a bit low, maybe a new filter cap is bad. Try measuring the AC ripple on the B+. Peter.

[drewl]

Well when I pull the 6V6's there is about 1.5vdc leaking from the PI coupling caps.
I jumpered across each with a good cap but no increase in signal, will leaky ones kill the AC?

And actually the plate voltage is more like 280v on the 6V6's, 80 on the PI plates, 50v on one 6SJ7, 16(!) on the other 6SJ7.
I just assumed they were good as they both amplify, and pulling either one yields no dramatic increase in gain of the other.

With about 100mv input, there's about 2.5vac out of each 6SJ7, and around 30vac out of the 6SL7.
I'll have to look up the gain of those to see if that's right.

[pdf64]

'Well when I pull the 6V6's there is about 1.5vdc leaking from the PI coupling caps.
I jumpered across each with a good cap but no increase in signal, will leaky ones kill the AC?'
They'll be screwing up the bias, they need replacing. Jumpering across with a good cap won't stop the leak. To see if any are dropping signal, just measure the AC either side of them.
However, I've never come across a cap that both leaks DC AND loses AC signal. Peter.

[drewl]

It's a Cathode biased amp, but anyway I replaced the PI coupling caps to the output tubes but it's still the same.
I wouldn't mind replacing all the caps etc, but these things are a pain to work on the way they're laid out, all the parts on top of each other.

This is annoying, there's not a whole lot in these amps, think the power trans is dying and can't supply enough current to drive the thing?
Thanks for the ideas, keep em coming.

[imaradiostar]

Do you have a spare power transformer around that could be clipped in to temporarily test? It would allow you to eliminate that as the problem, even if it wasn't the right one for the amp.

The more experienced people here would know but I assume if the PT was failing it'd be getting pretty hot.

jamie.

[drewl]

yeah, I'm sure i got a few power tranny's around to try.
I'm beginning to hate working on these 40's and 50's amps.
You want to try and keep them as original as possible, but I think it'd be easier to rebuild alot of them.

[Sweetfinger]

FWIW I got an old GA-20 years ago after reading a Ry Cooder interview where he touted the 20 as the greatest sounding amp, ever, other than Elmore James' "unknown" amp. Mine seemed like it had less power than would have expected, and while it sounded pretty good, it felt mushy and got distorted/compressed quite quickly. I certainly didn't think it was the "best". I had it for a few months, then sold it. A few months after that, I found a tape of an old rehearsal that I had used that amp at.......WOW. Serious life altering moment of the realization that what sounds good live can sound just ok on tape, or (in this case) what just sounds ok live can sound FANTASTIC on tape. IIRC mine had the transformer PI, but it has been well over 10 years ago so....

[MWJB]

"Filaments are good. "

"cathodes are good"

I was hoping for a specific answer, your B+ seems low, so I wanted to know whether heaters were low too (under 6.5VAC) in case something is loading down the PT.

Voltages on the plates of the 6SJs should be the same, within reason.

[pdf64]

'It's a Cathode biased amp, but anyway I replaced the PI coupling caps to the output tubes but it's still the same.'
When there was 1.5VDC on to the grid, it would be grid and cathode biased. You won't know what it might have been letting through under dynamic conditions. A tube responds to the voltage between the grid and cathode, it doesn't know what the circuit topology is.
+1 with Sweetfinger (and Ry Cooder) these really are fantastic sounding amps, which don't cut it with a band (unless miked up), so it's definitely worth persevering with. Mine has the noval pre amp tubes rather than octal, so you've got a real find, but it means I can't provide any comparitive voltages for the pre amp.
Mine puts out 9-10VAC into 8ohms with an 18watt OT at onset of clipping ie 10 watts. B+ around 320.
Your PI plate voltage seems too low, but I've no experience of those tube types.
Peter.