Dear Nelson,
There are reasons to not use a variac because of
what the effects are on the transformer it controls,
but these concerns take a back seat to the advantages
gained from closely monitoring correct filament voltage
at the tube socket.
If you're running a.c. on the tube filaments, be sure to
use a meter displaying a.c. RMS volts!
Hal Mandel
W4HBM
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Nelson Wittstock
Sent: Sunday, January 28, 2007 2:36 PM
To: Amps@contesting.com
Subject: [Amps] 8877 Longetivity
FWIW I am using an 8877 in my amp that I built back in 1977. It is still the
original tube. My filament transformer is run off a Variac and I have a
panel meter to monitor and see that it is five volts. I also have a timer
interlock so that the HV cannot be turned on until the tube has had three
minutes to warm up. When I shut down I run the Variac back down to zero and
leave the blower on for a few minutes for cool down. The amp was built from
a QST article in the September 1971 issue. It has seen a lot if use. I
added 160 meters to it and it has served me well.
Nelson, K8DJC
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