6v6 2204 Bias

[pmurph1x]

I am just putting the finishing touches to my 6v6 2204. I'm a bit confused with the biasing though. The circuit is the stock dreamtone 2204 circuit but it is designed for EL34s.

Anyway I have 430v on the plates (a little high ). With the Bias adjust resistor maxed. I have 13.9 mA on one cathode and 12mA on the other. Is my dissipation calculation correct:

Power = Plate Voltage X Cathode Current = (430 x 0.0139) +
(430 X 0.012)

= 5.9W + 5.16W = 11.06W

This seems wrong. I was expecting 20W.

Am I calculating it wrong or is there a problem with the circuit?

Thanks
Paul

[phsyconoodler]

My 6v6 2204 is biased at 28ma and sounds fantastic.The voltage is not too high,just keep the bias below meltdown at less than 35ma.
The screen resistors and grid resistors for the EL34's will keep the tubes alive.
Remember,you will likely not play the amp with both the master and gain cranked to the pin,so you can get away with the bias a little hot.
6v6's will live at a high bias condition if the amp is not maxed out all the time.If they aren't redplating at 28ma you will likely not have a problem.
The safe bias for sissies is 21ma at 430v.

[s2]

What he said.

[Bluesmechanic]

The safe bias for sissies is 21ma at 430v.

Huh? Real men run 6550's or KT88's in their 2204's.

[Phil_S]

The circuit is the stock ... designed for EL34s.

I think this is your problem. If you did no redesign of the bias supply, even "maxed" it probably won't deliver voltage in the desired range.

When you say "maxed", I assume you mean the least negative voltage it will supply and maybe that's in the high -20's to low -30's. (too lazy to back in from your info). You should put a volt meter on the grid to verify it's voltage.

The EL34 call for bias voltage of maybe -38v (give or take). The 6V6 calls for bias voltage of around -19v. So you probably need to raise the bias voltage, as you've got the tube operating in a range that's way too cold. To do this, you should increase the dropping resistor that is in series with your bias pot. The good news here is that you can experiment by tacking in another resistor in parallel until you get into the right range.

[pmurph1x]

Thanks for the replies guys. I'll try increasing that resistor.

Anyway I fired it up last night and have some other issues. It sounds good but way too clean. I'm not getting much preamp distortion so I need to figure out whats up with that. I'm just stoked that I'm getting sound out of it.

By the way - Is my bias calculation method correct? I think I screwed it up somehow.

Thanks

[Alexo]

Your calculations look fine.

Remember that the 1 ohm cathode resistor method will give you plate current + screen current.

You want to find plate dissipation, so you need to subtract your screen current and then multiply what remains by your plate voltage.

You can measure your screen current by measuring the voltage drop over your screen resistor and then dividing this number by your screen resistor value, IME it's usually 2 milliamps, +/- 50%. You're probably fine just subtracting a milliamp or three from your measured current.

Things won't really sound right at that low of a bias, I think, so fix your bias supply first and then tweak would be my suggestion, with a cold bias, your rail voltage is probably sitting pretty high, which will alter the bias of your preamp tubes, etc..

...be sure to keep rechecking your plate voltage as you bias your tubes, because it will decrease as you run more current through them tubes!

Have fun!