Moto VT71: a better ballast?

[polyphase]

Here is a home-brew ballast that pin for pin is a direct substitute for the 17A485459 ballast in my VT71. It uses capacitors in the filament strings instead of resistors. The result is 80% less heat dissipated and elimination of the current surge at turn-on. The tubes instead enjoy a slow, gentle life extending warmup. A pic of this contraption, (along side its predecessor) and a schematic are shown below.

The math, in spread sheet format, is as follows:

reactance X of cap =10^6/(2*PI()*F*C)
for F=60Hz and C=10uF, X=265ohms

resistance R of HOT filament string =260ohms.

X in series with R results in impedance Z:
Z=SQRT(X^2+R^2)=370ohms, so current at 110Vac line voltage is 110/370=.3amp

The COLD resistance of the filament string is about 50ohms.
Substituting 50 for R in the above formula, the result is about .4amp, +30% of nominal. This is trivial compared to the .71amp (+150%) surge that occurs at turn-on with the original 105ohm resistor ballast.

note: The caps shown are Panasonic ECQ-E2106KF, avail. from Digikey
ballast17A.post.jpgbalschem.jpg