Hi Paul,
Here is a copy of the post that you made on Audio Asylum's SET forum. I have left none of it out and I hope that the context remains intact.
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(Topic) What KIND of 2A3, ...or 45 ....for that matter?
A special note for Jeff :^) ---
The amp Astroimage has is convertible by changing the cathode resistor and a resistor in the power supply, to get the current and voltage right for the respective tubes. (There is a tap change on the plate choke, too.) Both tubes are run at the maximum spec, 300 volts and 15 watts plate dissipation for the 2A3 and 275 volts/10 watts for the 45. However, it is possible to set the voltage and current for the 45 and simply plug in a 2A3. The 2A3 will then run at 10 watts plate dissipation, which I know Jeff prefers for longevity - especially important when using expensive tubes (!). The load impedance is then a little low by my preferred operating point calculations, but still well within the range that Jeff has advocated before. Power is intermediate, probably 2.5 to 3 watts.
Note for Astroimage ---
The high power available from the 2A3-S only comes when using it as if it were a 300B. That particular 2A3, like the JJ 2A3-40 and a few others, is able to withstand higher voltages and currents than the classic 2A3, and can be used in a few 300B amps - those with switchable filament voltage. Otherwise, to get the higher power you would have to design an amp specifically for that tube.
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If I am reading what you have stated correctly the Stereomour is able to run with 2a3 OR 45
