[Amps] Getting Filament Voltage on an AL-82

[Paul Christensen]

> Actually, the resistors will affect the reading, but if you have a very 
> high impedance DVM the effect will be small.  I prefer to not use 
> resistors so that I could use any voltmeter I have and not have to 
> calculate how far off it would be.

Some form of isolation and current limiting is required as part of the 
filament voltage sample.  I agree that using a Simpson 260 or other low 
"ohms-per-volt" VOMs will cause errors in the reading when using high-value 
isolation resistors.   But when using a DVM, especially one with true RMS 
capabilities, the measured sample voltage will be identical to that as 
measured at the tube.

Even when trying your best to avoid a test lead short, it seems that Murphy 
is always present.   At a minimum, one could use a lower resistance value to 
solve the dilemma you present, but use very low-wattage resistors, or even 
small micro fuses in line with the sample leads and close to the socket and 
bifilar choke.

-Paul, W9AC