For pentodesor beam power tubes - R

For pentodesor beam power tubes - RL = (0.1) RP
For pentodes, tetrodes or beam power tubes the impedance is criticalfor best performance - higher power and lower distortion.
The output power is about 1/2 of PP- plate dissipation.Pout= 0.5 PP
WHERE:RL- load resistance /impedance of transformer primary/; RP- plate resistance of chosen valve.
Also determine loudspeaker impedance Zto match with the transformer- that is usually 4, 6, 8 or 16 Ohm. That will be reflected on the primary /transformed/ with the transformer.