[Hammarlund] Oscillator filament voltage regulator.

[Kenneth G. Gordon]

I will attempt to describe the simple circuit that regulates filament voltage in 
the SRR-11/12/13 receivers. 

This circuit replaces the ballast tube that was formerly used for this purpose, 
and the same circuit could be easily adapted to any HFO.

The circuit consists of three parts:

1) A dropping resistor, properly sized

2) Two 6.8 V Zener diodes, again properly sized. 

For a single tube with a 150 mA, 6.3 VAC filament, the dropping resistor is 
31 ohms 5 or 10 watts and the Zeners are 1N5342Bs.

The Zeners are connected cathode-to-cathode. The anode of one Zener is 
connected to ground, and the other anode is connected to the junction of the 
dropping resistor and the tube filament.

Input voltage between the 31 ohm resistor and ground is 14 VAC. If the 
voltage wasn't expected to drop very low, one could use 12.6 VAC and re-
size the resistor value. The subject receivers were used on ships, and the 
line voltage could vary quite wildly. At home, our line voltages probably don't 
drop that badly, so we wouldn't need as much "overhead". 

The output of the circuit is a trapezoid wave (sine wave with the peaks 
flattened) and the measured voltage at the tube socket is 6.3 VAC.

Ken W7EKB