thanks Paul
How do I
1. Measure to derive the 25 Watt
2. What component do I change to ensure that my amp put out say 22 or 24 watt to be safer
You can measure the DC resistance of the cathode resistors (approximately 1000 ohms for the two in parallel), but they are 5% parts so it's worth checking. It will take some time for the bypass cap to charge up - use clipleads and be patient. Then in operation, measure the voltage at the hum pot wiper and calculate the current (voltage over resistance). Then you can measure the voltage across the tube - hum pot wiper to the plate, A2. Multiply current times voltage to get dissipation. The actual value will vary from one tube to another due to individual variations in parameters (gain and transconductance), and with the power line voltage. The nominal values are 70mA and 350v.
To reduce the dissipation without changing the circuit, you need a lower-voltage power transformer. As it happens, I have two power transformers with lower voltage - they were prototypes from when we were developing the transformer.
To power the 1.4 amp filaments, you would need a filament choke that can take that much current without saturating the core. We could have our supplier produce a special run of the filament choke with a larger air gap.
An alternative would be to increase the cathode resistor value (to reduce current) and replace the output transformers with a higher-impedance transformer (to avoid increasing the distortion). You could replace the 2K cathode resistors with 3K for a parallel 1500 ohms, giving about 50mA instead of 70mA; this would call for a 5K output transformer. Magnequest makes two, the DS-050 and the Robin Hood; both would require some new chassis holes to mount them - I think the RH would fit, but the DS-050 would have to go on top and would extend up to or even past the edge of the existing chassis plate.
I believe these changes would work, but of course they have not been built and tested. Power output would be reduced, probably to about 6 watts.
