AB763 6L6 bias range?

[zfiles1701]

Getting ready to do the pre fire up tests from paul ruby.  Been looking for a range to bias my 6L6's.  Should be getting 440 to 460 VDC at the plates.  After some testing, plan on installing the power tubes with the pot biased way down to prevent cooking tubes.  Then slowly raising to desired voltage.  My guess would be -40 to -50 volts. Any advice or reference would be appreciated.

[FYL]

Fender used -44V to -55V in various AB763s. Try different values an pick the one you prefer.

[PRR]

Add small cathode resistor. Measure actual cathode current. That's what matters, NOT grid voltage.

For 30W plate at 460V you must NOT exceed 65mA. Since this is push-pull it will play OK at much lower current.

For initial smoke-test, figure B+/Mu where Mu is the tube's G2 amplification factor. For 6V6 6L6 6550 EL34, Mu is about 10. On an ideal tube at 460V, -46V would turn it OFF. Real tubes don't turn-off so easy, so it is good to have 20%-50% more, 55V to 70V available.

For fat class A, the final bias will be near one-half of  B+/Mu. About -26V. However in this case we know that will be WAY too much dissipation. And that part of the sound of these big amps is the AB operation. Your bias network should NOT trim as low as this.

We will arrive somewhere far above cut-off but generously below maximum dissipation. And since these amps really push the limits of 6L6, we idle way down the curve where it is difficult for tube-makers to control the flow. (Think of a big water valve. Wide-open it passes 400GPM, but try to get 40GPM and it gets tricky, twitchy, and a little different for every same-type water valve on the shelf.)

> My guess would be -40 to -50 volts

I think that's what it should come out to. But I'd smoke-test with more, and monitor cathode current while bring up to a happy idle.

[zfiles1701]

Add small cathode resistor. Measure actual cathode current. That's what matters, NOT grid voltage.

For 30W plate at 460V you must NOT exceed 65mA. Since this is push-pull it will play OK at much lower current.

For initial smoke-test, figure B+/Mu where Mu is the tube's G2 amplification factor. For 6V6 6L6 6550 EL34, Mu is about 10. On an ideal tube at 460V, -46V would turn it OFF. Real tubes don't turn-off so easy, so it is good to have 20%-50% more, 55V to 70V available.

> My guess would be -40 to -50 volts

When you say 55V to 70V, do you mean grid voltage? 
Also, have a 1 ohm resistor from power tube cathode to ground and got the cathode current at -.049mV.  Which should be approx. -.049mA.  Guess I meant to ask the cathode current range for bias.