[R-390] Variacs and solas: additional thoughts.

[Roy Morgan]

At 02:00 PM 5/1/2005, ToddRoberts2001 at aol.com wrote:
>... - can using a variac possibly extend the life of the 3TF7 ballast tube?

Todd,

I would think so, depending on how you use the variac.

>... the ballast tube will glow much more brightly for a few seconds due to 
>the current surge of the cold low-resistance BFO and PTO tube filaments in 
>series with the ballast.

Yes. Closely watching the ballast at turn-on may frighten you a bit.  Back 
when they were $8 each, no problem.  Note: receiving tubes of certain 
manufacturers (e.g. 12AX7's from Europe) show a very bright, momentary 
light from the filament upon startup.  These tubes apparently were designed 
to do that repeatedly and not fail, but it can be alarming.

>If one were to bring up the line voltage slowly using a variac, then this 
>might cushion the initial voltage/current surge into the venerable 3TF7.

It may well do that.  But what about a more or less fail safe slow-bring-up 
device you can install and forget: the inrush current limiter?  That sounds 
like an ideal solution to the whole thing.

Roy


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