Measuring Plate Voltage Trouble

[rhaigh]

I have a simple 6V6 single ended amp that works fine. When I measuring the plate voltage, the multimeter makes a buzzing sound and the meter screen shuts off. I can turn the meter off and on again and the meter is fine. But when I try to measure the plate voltage, the same thing happens. I can measure the screen voltage fine. Also, when the 6V6 is removed, it lets me measure the voltage on the plate pin 3 just fine (290V). Any idea whats going on here? I tried it with a second multimeter and the same thing happens, so its not the meter. I can measure the other voltages in the amp just fine. Why does measuring the plate voltage sound like its trying to blow my meters? Thanks.

[mozz]

Maybe it's oscillating?

[Enzo]

Reacting to the inductance of the transformer? Try measuring voltage to ground at teh B+ end of the transformer, then measure voltage across the transformer. SUbtract the voltage across it from the B+ voltage, that is the voltage at the plate.

Or try reversing your leads. it will read negative, but so what.

Instead of grounding the meter to chassis, measure from plate pin to cathode pin.

Is your meter an autoranging one? If so, set it to measure the B+ supply, then click the range hold button, now measure the plate voltage.

[Mick Bailey]

I sometimes get peculiar readings with certain amps when using one particular DMM due to oscillation. It's autoranging and goes nuts.

Some amps are particularly sensitive to having a probe attached to the plate or screen terminal. Often the oscillation is above audio frequency but the sign is excessive current draw and maybe some PSU growl. I've had a few Fender Blues Jrs where the tube will red-plate if the probe is held on long enough. Sometimes a sign of marginal stability, though.

[pdf64]

I think it's better practice to measure the HT voltage at the appropriate node, rather than trying to probe tube socket terminals; it will be a volt or 2 higher than the plate voltage, but so what? With normal mains voltage fluctuations, all the HT derived voltages tend to move around more than that anyway.
The likelihood of measurement error then becomes minimal, and the risk of a blunt probe tip slipping off the terminal, possibly shorting the plate to adjacent terminals or the chassis, seems lower.

[Dave H]

If attaching the meter lead directly to the plate causes it to oscillate try using a "stopper" resistor between meter and plate. I clip a resistor (47k?) to the end of the red meter lead and then probe the plate with the other end of the resistor.

[rhaigh]

Thanks for the suggestions. I'll try it all out when I get a chance this week.