|[AMRadio] Metering circuits|
k4kyv at hotmail.com
Sun Jan 2 19:03:34 EST 2005
>What your speaking of Don is shown in the following article.
> >If you measure current from the filament transformer center tap to
> >you'll have not only cathode current, but plate current, as well. Just
> >over at the grid current meter, subtract that from the plate meter, and
> >your plate current, AND it's in a nearly ground potential position in
> >series circuit.
>Return the grid leak resistor and/or PA bias supply to filament xfmr ct.
>instead of to ground, and your meter will read true plate current. If
>a screen grid tube, return the negative lead of the screen supply to
>filament ct. as well.
One word of caution. If you use the above circuit with directly heated
cathode tubes, be very careful to avoid shorting either side of the filament
line to ground. I lost a plate meter when I accidentally clip-leaded one
side of the filament (I had intended it to be the xfmr ct) to ground, while
running some tests. Half the filament voltage, in my case 5.75 volts,
appeared directly across the DC millammeter terminals. It didn't actually
burn out the coil in the meter before I realised the mistake and
disconnected it, but by then it had literally shaken apart the movement.
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