Tim Thomson wrote:

west wrote:

Gentlemen,
There always seems to be much talk about inrush current being detrimental
to tube filaments, light bulbs, etc. How many of us have turned on a cold
light
bulb and have it blow open the filament? A power amplifier's filaments
can
draw in excess of 15 steady state amps and much more cold. In tube
circuits the inrush filament current can hasten the life of a tube, some
of which can be rather expensive. Of course a thermister in series with
the filament can alleviate this problem, but it poses other problems,
such as increased heat (especially bad in Florida). If you turn the
system on-off-on, the fuse most likely will open because recycling of the
thermistor is slow. So I am searching for alternate ideas to the
thermistor. The HV is no problem because I simply add a 555 timer that
shorts out a power resistor in
series with the B+. The relay activates after the tubes safely warm up.
I've posed this question on my beloved NG, RAT with little satisfaction.
I believe the problem is that this is an electronic, not tube problem.
All suggestions are very welcomed and appreciated. Thank you very much in
advance for you time.
Cordially,
west

I dont have proof but my thoughts on why a cold light bulb goes poof
is...... It was hot when you turned it off and while it was hot some of
the hot argon leaked out. Then when it was turned off and cooled some of
the gas was replaced via the leak with oxygen..........this repeats over
time untill there is enough outside air inside the bulb for the combustion
of the tungsten filament.

On the workbench, whenever we start up an older tube radio rig, we plug it
into a large power reostat. We slowly increase the voltage from zero, up
to the AC operating voltage necessary. The reason for this is:

1. to charge the paper capacitors, slowly, so that the possibly dried-out,
dielectric wax paper is not breached. 'Slowly' means a steady increase
over approximately 5 to 8 seconds, with readiness to immediately stop upon
any strange noise, or smoke!

2. so that the tube 'heater' elements are brought up to full capacity with
'inrush' current limited.

DISCLAIMER !!!
I caution everyone to NOT fool around with any electric / tube type
equipment, AC mains, including monitors, and, television sets, as the
charge built up naturally, in the capacitors, can be of lethal levels
( usually considers as any current above approximately 10 milliamps,
depending upon humidity!) unless experienced, and, you have proper safety
equipment in use!


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