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[Amps] Measuring filament voltage

Subject: [Amps] Measuring filament voltage
From: "jim.thom" <>
Date: Mon, 6 Jun 2022 09:15:19 -0700
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On directly heated GG  metal triodes,  I wired one RF choke from each
cathode connection, then a .01uf cap on  'cold' side of EACH
chassis.  Then 2 x  shielded wires up to the 0-15 vac panel meter.  A pair
of banana jacks installed, just below the panel meter....and wired to the
meter terminals.

Custom test leads made up  (using 5 kv  belden test prod wire)...with
banana plugs on all 4 x ends.   Fluke 87 plugs right into the banana jacks.

Ok, now I can read the exact  true RMS voltage...right at the
cathode....on  RX  and also  TX.   ZERO  drive RF gets into either the
panel meter, or the fluke 87.

On the HB 3x3  GG triode,  I used a sola constant V xfmr, to feed a
constant 240 vac into the input of a small variac.... which feeds the input
of the fil xfmr.

My line V from the street is all over the map these days. Used to be  247.2
vac in summer..and  239.9 vac in winter.   Lately it's 228 to 234.  Then it
will shoot up to 238 vac for an hr.   Hence the sola  + variac.

I tweak the variac for exactly 7.5 vac on the fluke 87...then tweak the
zero set screw on the 0-15 vac panel meter for exactly 7.5 vac.   Good
enough.  In normal operation, the fil V is reduced a bit, (after tube has
been run at rated  7.5 vac for 200 hrs).  Fil V at the socket, does not

The same pair of RF chokes + pair of bypass the socket, has been
used on the bigger metal tubes, with far more drive involved. Same deal, NO
rf into the metering.

I re-used the grid chokes I pulled out of my L4B's..when grids were bonded
to the chassis. Forget the inductance, but the DC resistance was extremely
low, like just .9 ohm.

Jim  VE7RF
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