I would use small wires that can easily blow open (should you
accidentally have a short on them later) or have small fuses with
them. Filament voltage is typically measured at the socket pins
unless there is NO socket, in which case the flanges on the tube are
even better. If the pins are loose, then you should fix it. Using 50
mV or so drop as Rich says is probably good. In our systems, we try
to measure the filament with RF off, due to the susceptability of
many 'true rms' digital meters to interference. Iron Vane meters are
real nice. You can also use a lot of RF bypassing and choking on the
wires, but its up to you. Even having a couple of pin jacks on the
panel is enough, to stick a good meter on occasionally.
Now for GG operation, it will be tricky. I would suggest sticking a
meter inside the deck somewhere where you can observe it through a
screen. If that sounds tacky, then at least be very careful about
your hookup, use the chokes that you mention, and watch for being a
source of RFI though the meter hole. In one particular cathode
follower amplifier here, we have the meter on the AC primary to the
fil transformer, and made a calibration, checking the actual filament
AC at the tube, with RF and HV off. Then marked the AC meter or put a
scaling factor in the logbook. This is certainly the easiest way for
a GG or CF amplifier.
John
K5PRO
>
>I'm not sure if this has been brought up before, but wouldn't the best way to
>get a truly accurate reading of filament volts on a glass tube like a 4-1000
>would be to solder small wires directly to the filament pins and run them to
>an accurately calibrated AC voltmeter? There can be a small amount of
>resistance where the socket grips the tube pins and in a high-current
>filament circuit even a small resistance can drop the filament voltage some
>fraction of a volt? Is it feasable to measure the filament voltage at the
>pins in an operating GG amplifier where the filament circuit is above ground
>hot with RF? I would think using a pair of small RF chokes at the pins would
>isolate the AC voltmeter from the RF but could they cause problems with
>making the circuit unstable? Has anyone ever done this? Thanks for any
>comments. 73 Todd Roberts WD4NGG .
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