> Given the relative low accuracy of normal AC panel meters, how can you be
> sure that you are actually within the filament voltage tolerance specified
> by the manufacturer? A high accuracy AC meter is a rather large and
> somewhat expensive animal.
One method I've used is to add high-quality test point jacks to the amp's
front or rear panel. Low-wattage metal-film resistors of 1K-ohm or so can
be used to isolate the filament connection from the test jacks in the event
a fault occurs during measurement. This allows the use of a high-quality
DVM (e.g., true RMS) meter to be used only when needed.
Since a DVM is a high-Z sampling device, the isolation resistors essentially
play no part in measurement accuracy. I have a Fluke Model 189 DVM and it's
used as my general purpose shop instrument. This eliminates the need to
tie-up an expensive meter just for filament measuring purposes.
Paul, W9AC
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