Odd problem with amp (OT gives funny readings)

[ElectronAvalanche]

Again many thanks to all the kind help and suggestions!

I have the amp up and running. The new PT works as it should and the amp sounds fine.

Measruing the power output of the amp, I get around 72Ws which still puzzles me a bit. The B+ is 450V on the plates, bias is about 28mA per 5881. With 4 5881 I would have expected about 100W output.

Power was measured using a scope with either a 1kHz or 500Hz signal into the amp and measuring at the onset of distortion of the sine-wave. I measure about 26V AC (RMS) with the scope or an DMM at 8 Ohms (into dummy load).

So the amp works but puts out less power than expected, but maybe my expectation of output power is totally wrong. Another amp I have here with a 1k7 Marshall OT with 4xEL34 and about 460V B+puts out a nice 112W.

Addendum: I replaced the PI tube and now get 85Ws.

Thank you again for your help.

Electron

[pompeiisneaks]

The 100W is a theoretical value that isn't super easy to get unless everything is optimal. They give those output ratings in lab tests with everything optimal. All kinds of losses happen in amps that make them 'not quite' that hot. I think 85W sounds like it's in the ballpark of a good range. I can't recall but somewhere in my past reading someone said this is pretty common. Maybe someone will come along and set me straight

~Phil