6L6: PINS 5 - -58.9
Those output tubes could well be biased OFF, or nearly off. I see there is no bias "adjust" pot in that amp. (But I am looking at a schematic of a 6G4 - "A".) Depending upon the brand of 6L6, that's kind of a highish negative bias voltage measured right on the grid pins.
I would measure the negative bias at the junction of the 2 220K resistors just for fun. In other words, what is the raw negative bias delivered from the bias supply?
I would replace the 56K fixed resistor across the bias supply output, with a 25K linear pot in series with a 33K resistor. Or, you could use other pot values if you happen to have them laying around.
If you have no such part lying around, get another 220K resistor and jumper or tack-solder it across that 56K. That will reduce the 56K effective value to about 43K (56K x 220K /(56K + 220K) = product/sum. That will reduce the bias voltage by about 25%, 75% of 58.9 volts = about 44 volts. That may be just a tad on the hot side, but if the amp comes to life with a big gain improvement, that is your issue. I think it's biased too cold = too much negative volts on the 6L6 grids, those tubes are mostly shut off. This is possibly even more true because your B+ to the output tubes is a little low. Should be no problem, but this will make this situation with too-cold bias WORSE.
When I first power up a new build, it is my habit to bias the output tubes way too cold, in other words, with way too much negative volts on the output tubes. That closes them down and runs them very cold in case there is some problem with my wiring. But I usually build blackface style AB763 amps, my preference. Then, I LOWER the amount of neg voltage and thus bring the outputs into proper range. You don't have such a pot. My reco would be to adapt the circuit such that you can adjust the bias voltage. All it takes is a simple 2-resistor (one of them variable) chain across the bias output.
Topic: Brown Super 6G4 build (Read 4674 times)