Can only hear it when volume maxed out (Ampeg SB-12 build)

[MickeyB]

Tried different OT, tubes, grid resistors, but can only get a faint distorted sound at full volume. Have 480v at pin 3 and 468v at pin 4, -51v at pin 5 of both 6L6GB power tubes. Bias is maxed out at 26mv, wont go any higher. I ran a power amp and probed a good signal right up to pin 5 of power tubes. With known good tubes, OT and speaker, what causes the power section to not put out?

[loudthud]

I must assume the 26mV is measured across 1 ohm resistors between pin 8 of the 6L6s and ground. That reading should not cause the symptoms you describe.

Possible short to ground somewhere between the output transformer and the speaker. Did you follow the schematic exactly or improvise? Did you isolate the sleeve connection on the extention speaker jack from ground?

If that doesn't fix it, another possible cause is oscillation. Disconnect the 6.8K that goes from the output transformer back to the phase inverter. Any improvement?

[MickeyB]

The NFB resistor made no change so I pulled the jacks from the chassis and its still the same. It does sound like the OT is shorting out somewhere. Is there another way than just the speaker jacks?
I followed it exact except 40uf in place of 30uf filter caps and .022uf in place of .02 off V2.

[MickeyB]

Found that the shorting jack didnt open and kept on shorting. Thats what I get for buying the cheap ones..