[Amps] Measuring fil V

[Randall, Randy]

Possible stupid question alert.  Why not measure filament current instead of voltage?  Then you do not have to worry about accounting for losses in the filament path or the position of the voltmeter connection?  Or is there too much variability in the current drawn from tube to tube to be useable?

Randy AB9GO

-----Original Message-----
From: Amps <amps-bounces at contesting.com> On Behalf Of jim.thom jim.thom at telus.net
Sent: Tuesday, June 7, 2022 12:59 PM
To: amps at contesting.com
Subject: [Amps] Measuring fil V

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Be careful if using a 10 kw resistor hanging off each cathode lug on a GG triode.
That might work if using a true rms dvm..since the dvm has a high Z input.

But if fed to an iron vane type of panel meter, you will end up with a massive V drop across both resistors.

A pair of  low  dc resistance chokes, bypassed at cold ends to chassis, is dead simple.  Then another pair of bypass caps at panel mounted test jacks.

Years ago, energy onix used RF chokes on one of it's  FM GG triode amps..and the dc resistance across the chokes was too high..and resulted in a vdrop.  When the fil V was tweaked for  'normal' under load, using their front panel iron vane meter, the actual V at the cathode was way too high.
 This resulted in several premature tube failures.   And if a dvm was in
parallel  with the panel meter, it too will read on the low side.

Jim  VE7RF
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