----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Saturday, June 16, 2012 9:23 AM
Subject: [Amps] tube fils..again
> “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
CL type Thermistors are fine as long as they are run within their MINIMUM
and MAXIMUM current ratings. Choose wisely.
Carl
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|