Sirs,
The new amplifier will be running a pair of 3CPX1500A7 triodes
with floating filaments through bifilar chokes, powered by 5 volts
Direct Current through a floating bridge rectifier and filter.
In order to reduce the thermal shock to the filaments in the tubes
a method of reducing the inrush current is sought.
Please note the filament transformer, (5vac @ 22a) also provides the
22vac @ 1a and 44vac @ 1a for the various control, logic and for the
G3SEK triode board. I would prefer not to vary the input voltage to the
above transformer, as in the usual step-start method of injecting
resistance,
but would much prefer to step-start the D.C. 5 volt lines to the tubes
with a
time delay relay and a DPDT 30 amp contactor that would insert resistors
on
both polarities.
So.
Do you think that a series resistance of let's say 10 ohms at 50 watt
dissipation
for some period of time would suffice to lengthen the tube life with
regard to inrush
shock? How long should the tube be in the inrush current-limiting state?
I understand that ceramic tubes have quite different filament and
electrode
geometries and that some say an inrush protection is not necessary.
However,
when new tubes are $1400 I want them to have a sheltered childhood.
Thanks for your advice.
Hal Mandel
KA1XO
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