[AMPS] MEASURING FILAMENT VOLTAGE

[Tom Rauch]

> 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? 

Normally you can just measure at the socket pins, if the socket 
has noticeable drop you need to change it or repair it anyway.

If you want to temporarily measure at the pins, pull the tube. Use 
teflon wire-wrap wire like from Radio Shack, and wrap a stripped 
end tightly around the pins at the tube base end of the pins. Plug 
the tube back in, measure voltage with an accurate high 
impedance AC voltmeter, so there is almost no current through the 
wire wrapped around each filament pin. 

If you want to measure with drive, add a couple 200 uH or larger 
chokes near the pins and some temporary bypass capacitors on 
the meter side of the chokes.

2.5mH chokes are good for VLF amplifiers. For 1.8-30 MHz 
amplifiers, use a few hundred uH.

73, Tom W8JI
W8JI@contesting.com 

--
FAQ on WWW:               http://www.contesting.com/FAQ/amps
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com