[R-390] Another ballast question

[roy.morgan at nist.gov]

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