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Do what? First of all, there will be many conditions where the 20 watt
amplifier will be well into gross clipping while the 250 watt amp is
loafing along. Next, even assuming 50% final stage efficiency, which
is a stretch for most tube implementations, there's no way in hell the
tube amp will draw merely 50 watts at full output.



** Just for the record:

The final stage efficiency of a *class B* amp is NOT a number but a curve
on a graph.

The *maximum* efficiency, with sine wave drive and a resistive load, is
78.5 % and occurs at full output swing ( rail to rail).

At 1/2 power, efficiency falls to 55%.

At 1/10th power it is 24 %.

At 1/100th power it is only 8% !

So, a 100 watt class B amp dissipates over 12 watts when delivering only 1
watt to the load ( aside form any idle dissipation due to the bias setting).


OTOH:

A class A amplifier has a maximum efficiency of 50% at full power.

So, a 100 watt class A amp will dissipate 100 watts when delivering 100
watts.

At 1/100th power, efficiency falls to a mere 0.5% !!

It will dissipate 199 watts when delivering 1 watt !!!!





.......... Phil