[Johnson] Warning about high ac line voltage.

[Glen Zook]

Varying filament/heater voltage is why RCA came out
with the "dark heater" arrangement on their power
tubes.  The tubes (i.e. 6146A, 6146B, etc.) with the
dark heaters can take at least a +/- 15% variance in
heater voltage without any problems.  Of course they
were designed for mobile use where the supply voltage
from the system can vary considerably (from low
battery only to full charging voltage from the
generator/alternator which can vary by 25% or more). 
But, varying AC input voltage can have the same
effect.

Also, for most of the United States the nominal AC
line voltage is now 125 volts +/- 2 volts.  During the
summer most electric companies try to hold on the "+"
side because of loads from airconditioning.

Those tubes, and tubes like them that the other
manufacturers came out with, are not really affected
by the heater/filament voltage.  However, Eimac tubes,
and other manufacturers who made tubes like the
4CX250B, etc., definitely can be affected by over (and
under) heater/filament voltages.

Glen, K9STH


Sherrill.Watkins at dgs.virginia.gov> wrote:

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.

=====
Glen, K9STH

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