[R-390] SS replacment for 3TF7 - back-to-back Zeners.

[Kenneth G. Gordon]

The RCA modification to the power supply in the SRR-11/12/13 
receivers did away with the ballast tube which was being used 
to regulate the oscillator filament voltage, and substituted a 
resistor and a pair of back-to-back Zeners. The output waveform 
is a clipped sine-wave and regulation is very good, the amount 
of clipping varying with input voltage to the Zeners.

Ken W7EKB