[AMRadio] 100 V filament transformer primary

gkb gkbrown at gwi.net
Tue Feb 20 16:49:15 EST 2007

If a variac is used at 120 volts, you can get 140 volts out of it, guess it
depends on the variac connections. So, that kinda shoots that theory down.
----- Original Message ----- 
From: "John Lyles" <jtml at losalamos.com>
To: <amradio at mailman.qth.net>
Sent: Tuesday, February 20, 2007 2:30 PM
Subject: [AMRadio] 100 V filament transformer primary

> The reason that manufacturers install 100 V primary transformers, then add
a rheostat in front of it, is to allow some lattitude to adjust a filament
+/- around the nominal value. When the tube is old and emission starved, the
filament primary can be jacked up a few % this way, and the tube continues
to play until the next downtime. If a 120 VAC primary were used, there would
be NO headroom to boost the voltage higher, only lower. Makes sense.
> Metering the filament via the primary AC power after the rheostat, is a
bit cheap, but in this case (GE) it gave one meter which would globally be
responsive to the filament setting. Its up to the engineer to correlate the
reading from that AC meter to the individual socket voltages. Don't forget
to use either a true RMS meter or iron vane movement when setting filaments
on tubes to the correct voltage.
> 73
> John
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5:01 PM

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