Measuring Output Class

[ETR]

Good info guys and thanks. So constant current/voltage drop across Rk is result of Class A operation. So how much change in current/voltage drop across Rk is to be expected when deciding operation is no longer in Class A? Are we talking couple ma?

It depends on the amp/tubes but it shouldn't change more than 10% or so. In reality most "class A" amps aren't truly class A but are usually very close to it. Close enough for rock n roll anyway.

[Merlinb]

Can someone please tell me how to see if a power amp is operating in Class A push-pull? I modded my 5E3X2. The plate voltages are at 300v and there is roughly 32ma on each 6V6.

You can predict if it's class A graphically, but you also need to quote the screen voltage and OT impedance.
If you can measure the output power across a dummy load (true RMS meter say) you can estimate class, as long you think you're not going into grid current limiting. A Class A amp will be around 40% efficient or less, where as Class AB will be more than this.
The only way to be really sure is to use scope though.