Hi Bob,
Harmful advice was presented in the Hint&Kinks section of July
2001 QST.
One should NEVER *under any circumstance* reduce the filament
voltage of a indirectly heated tube below the tube or equipment
manufacturer's minimum recommended operating voltage. Any
reduction below that point in a metal-oxide cathode tube can
"poison" the cathode and permanently damage the tube.
Secondly, MOX cathode tubes can require inrush limiting, and are
sometimes as much or more susceptible to damage than directly
heated tubes. The indirectly heated cathode has a long thermal
lag, causing heater areas in closer contact to the cathode to
remain cold for a long time while areas further away from the
cathode instantly heat. This causes hot spots in the heater, where
resistance and heater dissipation is much higher than normal. The
problem is not the same as in directly heated tubes, where the
filament can mechanically distort and short to the grid, but rather
one of reduced heater life from opening of the heater.
Some indirectly heated tubes with larger cathodes, like the
3CPX5000 and its little brother, the 8877, have a tendency to
maintain high starting current for a very long time until the heater
temperature equalizes along it's length.
One of the best guarantees of proper inrush performance is to NOT
use "overkill" filament transformers, chokes, and wiring. Use the
minimum size components necessary, and you will have built in
filament inrush protection. Many amplifiers, the SB series
Heathkits, the Ameritron series with separate transformers, and so
on have absolutely no problem with inrush despite not having a
filament step-start. Transformer and component resistances limit
inrush current without external circuits. It is true that amplifiers
using filament windings on large HV transformers are begging for
problems unless a step-start is added.
Finally, reducing voltage until CW power drops, and then bringing
voltage up until full power is just restored is no guarantee that IMD
performance will be within spec. Normal peak emission is several
times the average emission current, and so the tube must be
*comfortably* above the point where full peak power is reached on
the worse case band. Once again, indirectly heated cathodes
should NEVER be reduced in voltage below manufacturer's
minimum recommended voltage.
It is unwise to randomly modify amplifiers based on folklore and
popular opinion unless we thoroughly understand what we are
really doing. Reducing filament voltage is mostly a spin-off of
commercial applications where NON-linear class C PA's with
overkill tubes (like 4CX5000A's in FM transmitters operating at 20%
of normal rated power) that simply don't need the full emission
capabilities of the tube. Reducing voltage can be harmful to tube
life, as well as the quality of our emissions in linear modes.
In amateur service, very few (if any) failures are due to voltage
correctable emission life of tubes unless the filament or heater is
operated ABOVE or BELOW rated voltage.
73, Tom W8JI
W8JI@contesting.com
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