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.