[R-390] QSK; standby reduced fil. voltage; separate B+ on/off

[Sheldon Daitch]

Henry,

There are two aspects involved with either full or
partial filament voltage operation for high power
broadcast transmitters which come into play.

Very high power tubes, 100kW plate dissipation
and larger, have quite high cold filament inrush
currents, sometimes limited only by the impedance
of the filament power supply.  Those high currents
may cause movement of the filament strands of
large tubes.

The second issue involves the expansion and contraction
differentials of the filament structure in very high power
tubes, differences in the expansion of the filament
structure and the filament support structure.  With
repeated filament on-off cycles this can cause the filament
structure to stretch and then the filament strands can break,
causing grid to filament shorts.

Some newer transmitters do have ramp up filament
voltage control systems, and some also have provisions
for reduced filament voltage during standby periods,
called black heat.  See:

http://www.contelec.com/pdf%5Camfmaint.pdf

Reduced filament voltage operation should be
used for thoriated tungsten emitters only and does not
apply to oxide cathode-type tubes.

With the very large and expensive tubes, it is simply
a tradeoff of power consumption during off times vs
the reduction in tube life.

73
Sheldon

Henry Meils wrote:
> Hi
>
>
> 2. Was thinking of running  always-on reduced standby filament voltage, especially on VFO - ( full voltage when operating)  for frequency stabilty
>     and prolonged tube life. I understand that commercial broadcast stations run never-off full filament voltage on their PA tubes to prolong life time.
>
>
>
> vy 73s, Henry, OZ1UF-Copen.
> ______________________________________________________________
>
>