“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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|