[R-390] Re: BallasTube Question

[Drew Papanek]

On BallasTubes David Wise wrote:

" If it's fully lit, it's not regulating.
   If you don't mind a clumsy adaptor or
   an incompatible mod to the IF deck,
   supplement the 3TF4 with a 22-ohm 5W
   series resistor."

That would allow us to use a good resource we couldn't before.  Another way 
to accomplish the same result (brace yourself) would be to add a diode in 
series with the 3TF4.  With the half wave rectification a voltage of 
17.8VRMS would be presented to the series connection of ballast and 12.6 
volts worth of heaters, leaving 5.2 volts for the 3TF4.  That is certainly 
within the 3TF4's regulation range of 4.3 to 8.3 volts.

If one doesn't mind some loss of regulation headroom (who runs an R-390A at 
100VAC anyway?) Dave's suggestion could be used to extend the life of the 
3TF7. Up to about 13 ohms  could be used.  Such a scheme was recommended by 
National for NC-300 et. al. to extend the life of that troublesome 4H4C 
BallasTube.

David Wise did not mention the most elegant BallaSolution of all: the 3DW7 
tubester he developed.  It plugs right in and fits entirely within the tube 
shield.  It is a true 2 terminal device which will regulate down to about 
18v input.  It dissipates almost no power, relying instead on zero crossing 
phase control via a sophisticated microcontroller/MOSFET implementation.

Drew

"Vicariously modifying and repairing R-390(*) through advice to others"

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