Mike asks:
"Can anyone tell me the reliability consequences of running the filament
voltage on a set of 811As a little on the low side? Specification is 6.3VAC.
At Vline = 114.4 VAC, I am measuring Vfil = 5.95 VAC. Filaments are
thoriated
tungsten."
My 1942 Radio Handbook has a couple of answers. Perhaps the most concise is
in a RCA advertising at the back of this handbook. Remember, this was
published during the "Great War".
In an article entitled "How to Make Transmitting Tubes Last Longer", they
say:
"For types using thoriated-tungston filaments and oxide-coated filaments,
the filament may be operated on the low side - as much as 5% below normal
voltage - if the loading is light. The filament voltage should be increased
gradually to maintain output. Toward the end of life, additional service
may be obtained by operating the filament above its rated voltage."
The handbook itself states that: "If the amplifier is to be greatly
overloaded, a filament voltage slightly higher than the rated value will
give greater tube life."
Comments:
While I do not want to question your measurements, it might be useful to use
another meter of known accuracy. Most line voltages are running over 120
volts now. Recently I measured my "high" line voltage and it measured 252
volts. This seemed high. So, I got a second meter to confirm the results
(it measured 252.2 volts).
73, Colin K7FM
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