cathode bias conversion???

[grumpy]

I have a silverface Vibrolux reverb donor D type amp. I chose to use diodes instead of the 5u4 that it originally had.
With a 390ohm resistor now in place of the choke I'm getting over 490v on the plates of my 6L6's.
I have to say it's a pretty harsh sounding animal as it is at present and seeing as the O/T is only rated at 30w or so I am thinking of converting it to cathode bias.
Could anyone help enlighten me as to how I should go about deriving the resistor size for the cathodes of the 6L6's?Do you still aim for 70% dissipation?

Cheers John

[skyboltone]

Decide on a grid voltage, say -37 volts, multiply by 1000 then devide that by the zero signal combination of plate and screen currents in milliamps. At least that's what the RCA 1965 receiving tube manual says. That's the resistance for each tube. Everything I've read says separate resistors for each tube. 300 ohms? Beats me

Don't know about the 70%. A typical set of 6L6GCs in AB1 draw 116ma at 0 signal. At 450 volts that gets us 52 watts in. Each tube then has 26 watts on the plates. That's 87% of max for 0 output. Those numbers are for a fixed bias circuit. At full output and 210ma at 450 volts we get 94.5 watts in and 55 out, or about 20 watts plate disapation per tube. About 66%. A couple of watts may go away in the screens. Anyway, all of this is at fixed bias of about -37 volts, and it's book values. May not have anything too do with guitar amps.

Would you aim for the same voltage in cathode bias. I dunno.

The examples of cathode bias for KT-66s show a 500 ohm cathode resistor at 425 VDC plate voltage.

Remember, I'm frequently wrong so YMMV

Dan

[rfgordon]

In my amp I run a pair of 6L6s cathode biased with one 250 ohm/10 watt resistor for both. I put 430 volts on the plates. My middle-aged brain can't remember what the cathode voltage actually is, though. I'd say try the cathode resistor and keep your present voltage to see how it sounds. The tube only "feels" the difference in voltage between the plate and the cathode, so if your resistor puts 35 volts or so on the cathode, you can subtract that from the actual plate voltage value to get the functional plate voltage. {This is the voltage that matters for impedance calculations, if I'm not mistaken.} If you bring the voltage down a wee bit and tie pins 1 & 8 on the sockets you'll be able to pluck-n-chuck all manner of tubes from the 6L6 and EL34 families and even try a 6V6 if ya get the urge. Just double the load impedance when you switch to 6V6s. (But you probably already know that, and I'm rambling uselessly...)

Since plate voltage values on the tube data sheets vary so much, the only "wrong" voltage is one that red-plates the tubes and/or sounds yucky.