I think I'm over biasing my EL84s; but how should I fix it.

[cjlectronics]

I recently completed building a Ceriatone 20W British amp kit... basically, a Marshall 20W 2061X circuit. I bought the kit with everything but NO Power or Output transformer and NO tubes. I used the power and output transformers from a Hammond AO-43 power amp. The AO-43 is a dual EL84 circuit similar to the Marshall 20W (Ceriatone 20W). I am proud to say that for my first amp kit, I only made two wiring mistakes and the amp powered up with no issues and plays GREAT!! However, being a responsible engineer, I know I must check the bias of the EL84's. This is where I think I'm running them a little hard, but I am still under the learning curve for vacuum tube biasing. Below are my measured and calculated values:

Rcath = 150.4 Ω
Rplate = 982 Ω
Vcath = 13.1vdc voltage measured across Rcath
Vplate = 5.77vdc voltage measured across Rplate
Vb+ = 370vdc

Ik = 13.1/150.4 = .087 amps
Is = 5.77/982 = .0058 amps
Ib+ =.087 - .0058 = .0812 amps

Vb+ * Ib+ = 370 * .0812 = 30 watts

I think my Vb+ voltage (370vdc) is too high for the ceriatone circuit, is this true? If so, do I adjust the Rcath resistor or reduce the Vb+ voltage? Maybe I'm out in left field???

BTW, the amp sounds great, the tubes look fine, no red plating, the amp is quiet. It cleans up and grinds like I would expect.

I've included the schematics I'm using to figure all this out.

[eschertron]

Nice work!

From skimming through the post, a couple things.
1. tube dissipation needs to use B+ (or HT at the plate, the OT will drop some DC) minus the voltage at the cathode. So (370 - 13) * .0812 = ~29W, a slight difference. I see you accounted for screen current. 1k screen resistors will definitely help with the tube life.
2. the AO-43 schem looks like it had a tube rectifier? You may want to drop the B+ because of this.
3. changing Rk from 150R to 180R, 200R is a pretty easy trial. Probably easier than dropping B+. Yes, it will change the sound. How much, and will you like it more/less? Try it to find out.

edit: Chuck H runs EL84s at up to 400v. He may have some pointer for you.

[cjenrick]

running pretty dang hot for cathode bias,

6BQ5 Max Plate Disp. = 12 watts, run that at 70% = 8.4 watts per tube,

8.4/357= 23.5 ma per tube and you are running about 40 ma.

357 is better suited towards fixed bias, either knock down B+ (use tube rectifier as mentioned above) or reduce screen voltage by lowering screen supply, using bigger screen resistors will do this but neg feedback may ruin tone, best to do it in the pw

cathode resistor uppage can be a losing battle and sound might get funky, but easy to find out so try that first.

[Dave H]

Marshall overran the EL84s so they could say it was a 20W amp but there's no need to do that. I wouldn't increase the cathode resistor above 150R. I'd reduce the B+ first. On the 20W schematic there's a 30R 6W resistor between the rectifier and first filter cap. I'd replace that with a 220R 10W. It should reduce the B+ from 370V to about 330V and EL84 dissipation to below 12W.

Edit:
On my amp I can switch out the 220R resistor to run it at 20W output (it has a fan to cool the EL84s) but it sounds better with the 220R at 15W output and there's hardly any volume difference.

[pdf64]

Bear in mind that fixed bias can be implemented at the cathode by using a zener diode rather than resistor.
And the HT screen node can be similarly lowered without causing sag.
So there are various options to make the excessive HT usable.
Another is to move to 6V6 / 6AQ5