[R-390] Another ballast question

[Barry]

It occurred to me after I wrote this that different 12V filaments run at
different current ratings.  Using a 12V tube whose filament current rating
is the same of that of the two oscillator tubes combined in series is what
is needed to work correctly.  I typed before I thought it all out.  Sorry.

Just wondering why you say don't measure the DC resistance across the 12V
filaments?  My ohmmeter uses a 9V supply so there shouldn't be a problem
doing this, right?  Of course, 9VDC isn't the same as 9V RMS so some
conversion must be made to ensure 9VDC isn't too much for 12V RMS, but this
shouldn't be an overvoltage situation, should it?

Thanks,

Barry - N4BUQ

----- Original Message ----- 
From: <roy.morgan at nist.gov>
To: "Barry" <N4BUQ at aol.com>
Cc: "R-390-List" <R-390 at mailman.qth.net>
Sent: Tuesday, January 03, 2006 10:51 PM
Subject: Re: [R-390] Another ballast question


> Quoting Barry <N4BUQ at aol.com>:
>
> > Some have suggested using a 12V tube's filament as a "ballast".  I can't
> > seem to locate a 12V tube, but I do have a dual 6V tube (a 6201).
>
> If the rated current for the 6201 (run on 12 volts) is the same as the
tubes in
> the PTO and Crystal Oscillator, then use it.  (Sorry, I did not take time
to
> look it up.)
>
> > Measuring the filaments in series, the resistance is 15 ohms (7.5 ohms
for
> each
> > heater).
>
> Don't DO that!
>
> > If a 12V tube drops the same voltage as a 45-ohm (or approximately that
> > value) resistor, then why does the filament only measure 15 ohms?  Does
the
> > filament resistance increase as it heats?
>
> YESSSS!  From 2 to 5 times, depending on the tube.  Regular incandescent
lamps
> do the same thing. Measure a 100 watt lamp cold and figure the starting
current
> at 120 volts.  Halogen lamps run the filament at higher temperatures than
> normal everyday light bulbs, and likely have a greater increase in
resistance.
> Large transmitting tubes had to be started on low filament current and
ramped
> up very slowly, or the very large inrush current could destroy the
filament.
>
>  It would make sense as I *think*
> > resistance increases with thermal activity, but not sure about that.
>
> You may now be sure.
>
> Roy
>
> Roy Morgan
> 7130 Panorama Dr.
> Derwood MD 20855
>