[ARC5] [Milsurplus] Cold Filament Inrush Current

[w8au at sssnet.com]

At 03:22 PM 3/15/2018, Jeep K3HVG wrote:
>One of the factors mentioned in the Eimac power 
>tube book is to address in-rush by ensuring that 
>the filament transformer (especially in 
>thoriated tungsten types) is rated for the 
>actual current of the tubes, and no more.  That 
>way the lower, in-rush resistance it initially 
>mitigated by sag in the transformer.  The 
>opposite scenario is that an overrated 
>transformer will sink whatever current necessary 
>during the cold, heat-up period.  I have seen 
>tubes "bloom" when power is applied where the 
>extra current is available.  Not a good situation...

I guess BuShips took the FIL xfmr sag into 
consideration for the big 861 amp in the TBK/TBM 
series since the secondary was rated the same as the
tube...11 volts at 10A.

When helping out at USS Massachusetts in the past 
I noticed that not all their stock of those big 
bottles had good filaments and wondered if the shock
of a cold start contributed to shortening their 
life.  Then I realized that the start button 
energized the Motor/Generator which had it's own time-constant
in reaching full RPM and full voltage.  Those 
guys at BuShips were pretty sharp in their specs 
for the Westinghouse and RCA products back in 1939.

However, with my RCA TBK-12 being fed with a HB 
AC supply I added a Variac ahead of the FIL xfmr 
primary to ease the 861 to full brilliance. ;-) Those
bottles are rarely available and not cheap.

Perry     w8au
   
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