nicoch58,

I always find these figures for output power to be optimistic for what someone would accept for good sound in a music system.

A guitar amp can be different, though.

From the table:

350V, 63mA, 2000 Ohm load, 10.6 Watts, -21dB 2nd harmonic.

-21 dB 2nd harmonic is 8.9% distortion, but it does not include the 3rd harmonic distortion.

If the 300B current swings from 63mA to 126mA, (and in the other direction to 0 mA), the power is:

(Isquared * R)/2

The 2 comes from the fact that you want the rms power, not the peak power.

63mA change is the peak current change.

(0.063 squared * 2000)/2 = 3.97 Watts rms, or 7.94 Watts peak.

Of course, 0mA is clipping, and the peak current will be more than 2X 63mA.

But those current level extremes are all distortion, not the original signal.

So yes, you can have 10.6 Watts, but at more than 8.9 % THD (Total Harmonic Distortion).

For a real amplifier, the power will be less than that, because of the insertion loss of the output transformer.

Example, a transformer with 0.5 dB insertion loss will only have 0.89 times the calculated power from the tube.

3.97 Watts * 0.89 = 3.54 Watts.

Accept lower power, more distortion, or change the circuit and/or parts.