First let me apologize for the pictures not coming out properly. Don't know what I did wrong but I'll probably have Big Red post them for me. Matt, A pole pig is a telephone pole utility transformer used as a plate supply. Hi Def, while a thoriated tungsten filament warms up in seconds as opposed to an oxide coated cathode that takes several minuets to warm up, it's still a bad idea to apply plate voltage simultaneously with filament voltage. It is much easier for a cold power grid tube to have an internal arc occur then one with a hot filament.
OKAY,
Why would a cold tube tend to arc over as opposed to a warm tube?
Examples of successful time tested designs that switch filament and plate on simultaneously using thoriated tungston filament tubes:
Collins 30L-1
Heath SB-200/201 SB220/221
The care and feeding is good reading. It's been updated in the last decade to include MSDC IOTs. Many of the original contributors to that book are SK.
There is no reason whatsoever that this tube type will arc when cold. Internal arcing is a function of element spacing and hard vacuum. Your big blast was going to happen anyway.
Eimac does say not to draw cathode current without filament voltage. That would require keying the amplifier in any sane design. That would still not cause an arc in a good tube though.
Cathode current should not be drawn before the heater or filament has the ability to emit a space charge. The space charge prevents cathode (filament) damage. There is nothing changing from cold to warm that will make an arc happen.
Some damaged tubes will actually test OK when cold but measure grid to cathode shorts when warmup happens. I've seen this on 8877 and 8122s.
The only reason to run the filament with the plate voltage off on an external anode tube would be to getter the tube. Some of these tubes have no other way of gettering. This process would take much longer than a filament warmup timer. That's why you run GS-35Bs and old 4CX1000s for a few hours before use.
Examples of successful time tested designs that switch filament and plate on simultaneously using thoriated tungston filament tubes:
Collins 30L-1
Heath SB-200/201 SB220/221
I'm curious what you think changes from a cold tube to a warm one that would cause or not cause an arc.