[Amps] tube fils..again

[Jim Thomson]

“The idea of slowly raising the voltage on a tube filament upon "turn on" remains a disputed area (to do or not to do, that is the question).”

Check out what a new 3CX-3000A7 or 6000A7, etc cost.... and you will soon get the picture.   I take fil management real seriously
on expensive tubes used in my hb amps.   Since my line V wanders all over the map, it actually jumps several volts  up... then several hrs later
it drops like a rock, I have to employ a solas constant V xfmr... which regulates it to less than 1%.   I use analog Voltmeters and also AC
ammeters on the sec side of the fil xfmr,s   I also use digital meters  plugged into special jacks... that are wired directly to the socket..via
rf chokes in each leg.   A new  or rebuilt tube is  run with fil only for 48 hrs..and no drive or B+ . Then out into operation.  After a total of
200 hrs, emission hits a peak... at which point the fil V is slowly reduced, till power output drops a few watts...aka knee point. 
Then the fil v is brought back up by .1 vac    PO  vs  fil v is checked once in a while.   last thing I need is fil V  going up-down cuz of the
power co...hence the sola. 

The fil xfmrs I have , one dahl ,and one made by northeast xfmr co.... are both capable of excess current on a cold tube.   the northeast
one is rated at  8.25 vac @ 79 A CCS..and has a 208 vac single phase input.      I gotta use the varaiac anyway... to drop it down to
aprx 179 vac... to run the 3CX-3000A7 fil... which requires  7.5 vac  @ 50A ccs.   In normal operation, the fil V is way less than 7.5 vac
so it has to be turned down even more.   With a 0-75A  Ac ammeter on the fil xfmr... I can see the fil current on the tube at all times. 
Bring it up too fast, and the meter will peg.   The  3CX-6000A7 uses 7.0 vac @ 80A  for  its fil.   I use a 0-100 A  AC ammeter for that
tube.   It uses a dahl fil xfmr, with loads of taps.... but the variac is still used on it too. 

I have no experience with oxide tubes like 8877, etc.... but even then, they are just too expensive to replace  cuz of fil issues.  Leighs 
thermistor idea would fit the bill...and is dirt cheap to implement.   Now if the fil and plate xfmr are all one xfmr...then use step start
resistor in the 240  line. 

later... Jim  VE7RF