The RCA tube manual publishes some typical operating specs for 6F6 tubes
operating class A in push-pull. With 315 volts on the plate and 285 volts
on the screen, and with a cathode bias resistor of 320 ohms, zero signal
plate current is 62 ma, and zero signal screen current is 12 ma. That would
result in 74 ma cathode current, giving a cathode voltage reading of about
24 ma. Assuming no output, power dissipated by each tube would be 11.5
watts (total of plate and screen power). The 6F6 is rated at 11 watts plate
dissipation and 3.75 watts screen power dissipation, for a total of 14.75
watts.

The SX-25 circuit has 280 volts on the plate and 270 volts on the screen.
The cathode resistor is 350 ohms and it specifies 18 volts on the cathode.
That would mean 51 ma of cathode current. Estimating 10 ma of screen
current and 41 ma of plate current, that results in 7.14 watts dissipated by
each 6F6 plate, 1.4 watts by each screen, and each filament consuming 4.41
watts for a total power dissipation by each tube of 12.95 watts. That is 26
watts of heat in a confined area. Put a 25 watt light bulb in the sme area
and after 10 minutes, try to grab ahold.

73, Colin K7FM