[R-390] 3TF7 Ballast Tubes

[2002tii]

randy wrote:

>There is, however - a much more elegant solution - a
>capacitor.  By correctly calculating the capacitance
>reactance - a capacitor can be used in place of the ballast
>tube - and while it will *not* provide any regulation (but
>then neither does a resistor - or using 2 12BA6s) - it drops
>the required 12.6 volts with nearly zero heat...

Dunno about elegant, but indeed it will make a voltage divider with 
little power dissipation.  If my calculations are correct, at 60 Hz 
you would need an approximately 36 uF non-polar capacitor (at the 
operating resistance of two 6BA6 heaters in series -- about 42 ohms), 
giving about 74 ohms of capacitive reactance and a 60 degree phase 
angle.  At startup, when the heaters are cold and their resistance is 
low, the capacitor should limit the current to ~ 350 mA.

Do NOT use a "non-polar" electrolytic, or a film cap with only a DC 
voltage rating!!!  A good film cap rated for the AC voltage and 
current is required (the application is very similar to a motor run 
capacitor).  Note that suitable caps will be pretty large -- 
significantly larger than the 3TF7.

Here is one candidate:

http://www.mouser.com/ProductDetail/Cornell-Dubilier/SFP24S35K375A/?qs=JlSiUoO6twkxQypibr45LQ%3d%3d

The voltage rating is overkill, but you are not likely to find 
AC-rated caps with much lower voltage ratings.

Best regards,

Don


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