output 6l6 pair cathode resistor question

[9 volts]

Hey there, basic question, why do some schems using 6l6 output pair not have a cathode resistor. When looking at 6l6 specs, it mentions a 220r cathode resistor? I built up an amp similar to an early fender and am wondering what effect no cathode resistor has?(eg higher plate voltage) Thanks. I'm looking at building a dumble 40 next and noticed it has a 1r cathode which caters for el34 and 6l6....

[dave g]

You don't see a cathode resistor because those amps have a fixed bias. Rather than referencing the grid to ground and then using a cathode resistor to raise the cathode to a positive voltage and therefore give the grid a negative bias voltage (from the tube's point of view), a fixed bias amp actually has a negative bias voltage supply. The grids of the power tubes are referenced to this voltage, so that the cathodes can simply be tied to ground.

Yanking out the cathode resistor from a cathode biased amp and tying the cathodes to ground is a really bad idea. No bias voltage means lots of current draw. You will annihilate the tubes and possibly damage the rest of the amp.

Also, those 1 ohm resistors aren't cathode bias resistors; they're there as a way to measure the bias current through each tube (measuring the voltage drop across a 1 ohm resistor will obviously tell you the current flowing through it)

If I were you, I think I would do some more reading up on tube electronics before jumping into another build, especially a Dumble. The tech articles at Aiken Amps are a good place to start.

[9 volts]

Thanks for that. Research time. It's been a year since my last build and it seems I haven't retained too much...last time I opted for the cathode bias due to simplicity.Thats cleared things up and yep I'll head back to some reading before going further. Thanks again