[Johnson] Warning about high ac line voltage.

[Jerry K]

Yes, this is pretty much common knowledge by now. Notice that almost every
big rig, whether homebrew or commercial, incorporates metered and adjustable
filament voltage for the power/modulator tubes. OTOH, my old faithful Swan
Mark II's pair of Eimac 3-500Z's have run without a hitch for nearly 30
years (actually this is the second set, the originals ran for 22 years and
were replaced just for the hell of it) on whatever line/filament voltage it
was fed at the time from half a dozen different radio shacks. The
difference, I suspect, is that these 3-500Z's get turned on an used maybe 2
hours a week, if that, and they NEVER get run harder than about 1200W
output. In contrast, the 4-400's or 833's or whatever in a commercial rig
will be asked to run full bore for maybe 1500 hours at a time before they
are arbitrarily replaced and become "pulls" for sale to us hams. My T-368C
has metered and adjustable filament voltage, and during a long evening
session on AM the filament voltage will wander around +/- about 0.2 VAC just
from variations in local line voltage, merrily exceeding the 3% limit
mentioned. Doesn't seem to have any effect on tube life, although again this
rig doesn't get used in 24/7 commercial or military service like it was
designed for, and is never run with "red plates". I usually keep it backed
off to around 240W of carrier output on AM, although it will easily run well
in excess of 400W with no strain. I suspect the severity of service in a
given application is the other unspoken half of the equation, at least for
transmitting tubes, and Eimac's handbooks of course deal mostly with
commercial applications. So, in amateur applications, I suspect this isn't a
big deal as long as a reasonably close filament voltage value is maintained.
73 Jerry W5KP


-----Original Message-----
From: johnson-bounces at mailman.qth.net
[mailto:johnson-bounces at mailman.qth.net]On Behalf Of Sherrill Watkins
Sent: Wednesday, October 06, 2004 7:09 AM
To: johnson at mailman.qth.net
Subject: [Johnson] Warning about high ac line voltage.


Gentlemen: I happened to be reading the book entitled "Care and Feeding of
Power Grid Tubes" published by Eimac in 1968 and noticed something
interesting. Eimac states that for power grid tubes an increase in the
filament voltage of just 3 percent over the rated filament voltage can
reduce
the tube life by as much as 50 percent!! (Please don't argue with me about
this. I suggest to obtain a copy of the book and read it.) My measured ac
line voltage is about 5 percent over the U.S. Standard of 120 vac.
Therefore,it may be reasonable to presume that any vacuum tube equipment
operated on a line volatage of 125 vac will have a filament voltage that is
5
percent higher than the rated filament voltage. -73- Sherrill W. k4own
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