Bias based on valve or amp power?

[leroy]

So I have a cathode biased amp I built. 340v plate to cathode and a 4k OT. According to this the Max power output is (340x340)/4000 = approx 30watts.
If I run 6L6gc (60watt Max power) in it do I bias 90% at 60watt or 30watt??

Thanks
Leroy

[sluckey]

I would just plagiarize. 250Ω is a common and widely used value for a shared cathode for two PP 6L6s. A single 6L6 would want a 500Ω cathode resistor. I would start there then measure the actual dissipation to decide if I wanted to increase or decrease the resistor value.

[shooter]

while you're messing, I always add a 1ohm R below the bias R, (saves 1 math mistake :) .
use the Max plate value from tube data sheet, then have a handful of bias R's, gator clip til you find your happy R. ( I typically do; plate V - cathode V * meter VDC value across 1 ohm(=ImA)).  PP I stop around 60-65% max and play for awhile, SE ~90% and play.

[PRR]

> If I run 6L6gc (60watt Max power)

One 6L6GC is rated 30W plate dissipation (and that may be bold).

You have not said if this is a singe-ended or a push-pull amp.

Sluckey's plagiarized/rule-o-thumb suggestions ARE a good start.

[leroy]

Sorry it's push pull.
I'm trying understand the theory. I could just use a standard fender value but i'm trying to make sense of this.
Every bias chart or calculator I've seen doesn't take the OT into consideration.
If the HT and primary OT limit the max power to 30 watts what happens if I bias the valves based on their combined ratings?
Will it possibly burn out the OT? With it limit the current draw and only provide 30 watts?

Thanks
Leroy

[sluckey]

You must use an OT with a high enough power rating to deal with the power the amp will provide. Here's an extreme example. If you use a 10W OT in a 100W amp and actually pump 100W to the speakers, the OT will overheat and eventually burn the primary winding open.

Not much different than dissipating 100 watts in a 10W resistor.

[PRR]

> Every bias chart or calculator I've seen doesn't take the OT into consideration.

The Designer picks the tube, the voltage, and the load so they all work well together.

The Field Service Tech sometimes has to check bias without knowing the Designer's intentions. Those Bias Tables give probably near enough values for well-designed amplifiers.

You can certainly think-up some aberrant operating conditions. In fact most later hi-power guitar amps are "under-biased", they distort small signals. "Fixing" that would mean red-plating the tubes at idle.

It's not real easy to burn-up an audio transformer. The "Power rating" is more about bass distortion than thermal stress. Oh, I'm sure a 10W OT worked at 100W voltages and flogged hard may break-down from voltage puncture.

[shooter]

what happens if I bias the valves based on their combined ratings

In self biased (cathode) class A(1) usually we can get away with it since both are conducting most or all the time it's.  the tube(s) "power" still gets dissipated somewhere.  a good "rule", make sure those parts can handle the dissipation.

wire a 5W speaker to a 100W amp, it works, for awhile