[maxhifi]

Ok this sounds about right - haven't thought about it for a while. The number is also affected by B+ being added to the peak voltage, since in the amplifier we are talking about the RC network is connected to ground, right?

So say for a push-pull output stage which from the RCA tube manual puts out 26.5W, with a 6600 ohm load on the output transformer, and 360V of B+, the following applies. From ohm's law the RMS voltage across 6600 ohms to get 26.5W is 418.21V, this is from plate to plate. From plate to centre tap, this will be 209.1 RMS, or 295.7 peak volts. Add this to the 360V B+, and the sum is 655.67V. So in this case, a 1kV capacitor should do the trick. I suppose that if the amplifier does not have a load, or has a load of a higher impedance than rated, the maximum output voltage can also rise a little bit, so the capacitor should have a decent safety margin. I haven't designed any tube equipment in a while, feel free to correct me if I am wrong!

So I will assume that the RCA amp in question probably contains voltages similar to above, add a safety margin and a contingency for the fact that the voltages and power level may be higher than in my example above, and I am quite sure a 1kV or 1.5kV capacitor is perfect for the job.

-Max