[Amps] 8877 filament.

[Ron Youvan]

   Randall AB9GO Randy wrote:

> That scheme was tried in TV's and consumer radios in the 60's and 70's and called an instant on feature.  The series string filaments were run through a diode when the set was "off" and the diode shorted when the set was turned "on".  It worked as you described, but tube life was reduced due to cathode poisoning.  I wouldn't try it on such an expensive tube.  If it was a workable solution someone would offer it on an amp today as a "feature".

   Some high power TV transmitting tube heaters are kept warm with
reduced current when beam Voltage is not applied with a feature called
"BLACK HEAT" (primarily during servicing hours) these klystron tubes 
cost $32,000. each.  Full filament Voltage is applied with bean current.
Everything else is maintained at normal levels when there is no beam
power.
-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.