[AMPS] RE: filament voltage

[Peter Chadwick]

> I assume the Fluke measures P-P voltage, and is this an issue?  

Most DVMs measure RMS of sine waves. Complex waveforms give errors, but
that's not an issue here.

More to the point: when was it last calibrated? Even DVM calibrations drift
a bit. If you were using an analogue voltmeter of say 2% accuracy of full
scale, and it was a 10v full scale meter, you'd be within its error band.
DVMs are similar in percentage accuracy, as well as the last digit being
+/-1. So a reading of 5.17 means a reading between 5.16 and 5.18, probably
between 5.165 and 5.175. Then there's the accuracy on top of that.

Using a scope isn't likely to be much better. Most scopes are 2% at best,
and even the fancy Tektronix ones with moveable cursors and digital readouts
still have the amplifier accuracy to contend with.

Unless you are really certain about the DVM accuracy, I'd be inclined to
suggest checking by borrowing another DVM (one DVM might lie, but if 2 of
them lie the same, they're probably somewhere near the truth). Otherwise,
use resistance in series with the xfmr primary until the PEP drops, and then
push it back up one or two tenths of a volt. The DVM is probably OK to give
you a differential of 200mV, as it isn't that critical.

One point is that you may find that the DVM goes mad with RF. I have a
regulated screen supply which according to the moving coil meter drops by 2
volts in 300: according to the DVM, the 300 volts goes up to 850!

73


Peter G3RZP

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