Fender Showman Plate Dissipation is 0mv

[twilightofthedogs]

Hey y'all, I am working on a 1966 blackface fender showman. I have it powered on and all the DC voltages seem pretty good. 450 at the plates and voltages making sense all the way down the line. The screen grids at the power tubes are reading around -53 which is about where they should be. Adjusting the bias pot I can achieve a range of about -42 through -57, so that seems to be working correctly. However, when I check the plate dissipation I am getting a reading of 0.00mv. Any idea what might cause this? Thanks in advance! Amp doesn't produce much sound, but if I strum it will occasionally send a super compressed decayed signal through, which is about what I would expect from an out of wack bias.

[nickb]

The screen grid should be near 440V than -53. Can you please tell us how you are measuring the dissipation?

[twilightofthedogs]

The screen grid should be near 440V than -53. Can you please tell tell us how you are measuring the dissipation?

Apologies. i didn't mean screen grid resistor. I meant the grid at pin 5. Using eurotube bias probe.

[pdf64]

My guess is that somehow, the tube isn't getting screen grid or plate voltage, eg perhaps the bias probe / tube socket has a bad connection.

[nickb]

Apologies. i didn't mean screen grid resistor. I meant the grid at pin 5. Using eurotube bias probe.

OK, so what is the voltage at the screen grids?

[Enzo]

Let's tighten up the terminology a bit.

Pin 5 is the control grid, not the screen grid, and that is where -50v belongs. The screen grids are pin 4, and they should have voltage similar to the plates. Plates are pin 3. So both pins 3 and 4 need about 450 or whatever you have there.

Plate dissipation is in watts, and is the product of voltage across the tube times current through the tube.

If your bias probe is measuring in millivolts, then we assume it has a 1 ohm resistor inside in series with the cathode, so the millivolt reading corresponds to the same number of milliamps through the tube. so that reading is tube current rather than dissipation.