bias current with parellel output tubes

[Tage]

So I was trying to search for this info, but Google keeps interrupting my search with a malware warning message.

So I'll post and take whatever flak I get. I was working on a 6505+ the other day and just replacing tubes noted that the bias adjustment only allowed a bias current of around 20-24mA. Now I know these amps a rebiased cold and that is just the way they are, but I was checking bias on an orange AD200B and noticed that the bias current was only around 21mA, this seemed way too cold for the amp to actualy put out the rated 200 watts. I'm sure that I'm missing something and I know it's bias point is not the only deciding factor in output power. Is the bias half the "normal" calculation of 70% because the tubes are in parallel?

Understanding output stages eludes me a lot more than preamp design. If anyone can point me in a good direction I really need to work on my understanding of output stages, and Merlin says he's gonna take a while to write a power amp book... Maybe a TUT book?

[Steve Conner]

In a push-pull amp, output power is more or less independent of bias. You would get your 200W even with an idle current of zero, it would just sound terrible at low volume.

[pdf64]

Not applicable for the amps mentioned, but with cathode bias, if a 'higher than normal value' bias resistor is used to cut back the static dissipation (ie a low bias condition) then the output power will also be restricted; this is due to larger currents at high signal levels increasing the voltage across the cathode resistor, which thereby reduces the plate to cathode voltage and so the available signal swing.
Sorry for the convoluted sentence - Pete.

[Steve Conner]

That's right. A cathode biased amp can never get far away from Class A for long. If it tries to get out, the cathode bypass capacitor just charges up and hauls it back. And in Class-A the available power is directly related to the idle current.

However, neither of those amps is cathode biased, so I rest my case...

[drewl]

What you need to do is tweak the bias circuit so you can get enough range to set the bias properly.
The amp will sound alot better too.