Gibson GA20 voltages

[tubesinside]

Need some help from my US friends.  I have a Gibson GA20 (Octal preamp tubes) in for restoration/rebuild and I am trying to find the expected voltages throughout.
The amp was in very bad condition when it arrived and I had to replace almost everything.....output transformer, multi-section power supply cap, all tubes, all coupling caps and some resistors.
The amp finally works but I can only squeeze 6 watts before breakup.
I'm a pro amp-tech but not very many of these over here in Ireland to compare....
I have only 278V AC into my 5Y3GT rectifier tube and 293V DC on the first filter cap.  Input voltage is 115 V AC via a variac to give me 6.3 V AC on the heater circuit.
Can anyone let me know what voltage I should expect out of the transformer?
Thanks in advance......
Schematic attached

http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-20.pdf

[PRR]

Welcome.

More Data!

If the 6.3V is right, then the HV winding is right, unless it is broke.

GE's curves suggest 320V @ 75mA for your 278VAC, but their 50 Ohms may be much less than Gibson put in there. I am not too concerned.

6W measured with dummy-load and meter? Or whacking guitar into speaker? 6W clean or pretty-bent?

What is the voltage at 6V6 cathodes? What is the actual value of the cathode resistor?

What is the voltage at Screens?

How do these voltages change if you pull one tube, then the other? (Don't linger with one tube, it will be mis-biased hot.)

If 6SL7 voltages are WAY off, it won't be able to spank the 6V6es properly.

[tubesinside]

Thanks for the reply.
6 watts clean into dummy load (8 k transformer).  Cathode voltage is 12v dc. Each tube is drawing 29mA (8 watts dissipation)  Cathode resistor is 200 Ohms.  Screen resistor is 22k  V screen is 288V DC.
6SL7 is driving the 6V6 tubes into cutoff easily.....

(Tried 6k6 output transformer also but no better)

C

[chocopower]

22k screen resistor, isn´t too much?

[PRR]

> 22k screen resistor, isn´t too much?

Yes.

The idle voltages are nearly the book values for the 8K 14W condition. But the cathode current should be 74mA, not  60mA; and this should happen with 257 Rk, not 200. So I wonder about the tubes. (Also change the battery in your meter- sometimes they go low without a LoBatt icon.)

The book assumes a "solid" screen supply. 22K is quite huge. Also bigger than shown on the plan. We do KNOW that Gibson put different parts in every amp (it seems). But taking book-value of say 14mA at full output, times 22K, is 300V drop!! That can't happen, but Vg2 (and Vpreamp) must drop very low at FULL output, choking power.

This may be deliberate. It gives power on Pluck without long-term power which would blow up a cheap speaker. If you run the original speaker you might just leave it as is. If you got a new speaker and really need all dozen Watts, you could make the G2 dropper 5K. Of course now you are on the slippery road to 1990s "reissues" with everything over-volted and 23 Watts out of a pair of poor 6V6.