[AMPS] MEASURING FILAMENT VOLTAGE

[Phil Clements]

During annual cleaning, inspection, and blower oiling is a
good time to check the tension on the tube socket on all
the pins. Yes, you will need RF chokes in line to the meter,
and a way to adjust for the loss through them. I use a diode
bridge, a D.C. meter, and a pot to calibrate it against a good
A.C. voltmeter hooked to the tube pins. This is also a part of
my annual ritual. If the socket is good, it is not necessary, IMO,
to solder the meter leads to the pins. If a resistance develops
between the tube pins and the socket, it should be seen on the
filament voltmeter as a change from normal.

(((73)))
Phil, K5PC

Subject: [AMPS] MEASURING FILAMENT VOLTAGE


>
> 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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