example:
The four 6146 tube heaters require 6.3 volts DC at 5 amps total. Heater voltage is an important factor in tube life. The vehicle's electrical system can vary from 10 to 14.5 volts, therefore, a simple dropping resistor would not provide correct heater voltage under all circumstances, and would generate 30 watts of heat.

A self-excited buck-type switching regulator was designed, and is shown in figure 3. A goal was to avoid the necessity of using special IC's and complicated circuitry. The inductor is a surplus 1mh, 8 amp unit. Several inductors were tried, and 500uh to 2mh ones worked well also. An important aspect of the construction of such a regulator is that the wiring shown in bold should be heavy-gauge wire, such as #12. A low impedance path for current from the negative end of the 10,000uF input capacitor to the diode D1 is essential to stability. The regulator works as follows:

1. When power is applied, there is no voltage present at the output, and the reference voltage across the 4 diodes is 2.5V. Because of this, the op-amps, which are operated in open-loop for maximum gain, present a high voltage level at pin 1.

2. This turns on the Darlington-connected pair of Q2 and Q3, which turns on Q1.

3. Voltage from the 12V supply is presented to L1, and current begins to flow through L1, charging C1 and providing power to the load.

4. As soon as the load voltage reaches 6.4 volts, pin 6 of the op-amp exceeds 2.5 volts by a few millivolts, and the op-amp's output at pin1 goes to near zero volts.

5. This turns off Q2, Q3,and Q1. Current has been flowing in L1 and now that Q1 has turned off, the voltage at the collector of Q1 would go very negative due to the imminent collapse of the magnetic field of L1, possibly damaging Q1, if it were not for the diode D1. D1 provides a path for L1's decreasing current flow until Q1 is turned back on. The current through D1 is generated by the energy stored in L1 and is delivered as the magnetic field collapses.

6. Now, with Q1 off, the load voltage drops to 6.2 volts. The voltage presented to the op-amp pin 6 is now a few millivolts less than the 2.5 volt reference, and the op-amp again brings its output on pin 1 high, starting the process over.

The frequency of operation is about 40 KHz and the efficiency is 90%. The key to this efficiency is to saturate Q1 during its ON time, reducing its dissipation. The 3.3K resistor in the low side of the error amplifier divider chain may need trimming in order to allow the 500 ohm pot to adjust the output over the range of 5-7V. In the case examined here, a 22K resistor was placed across the 3.3K resistor. The heater voltage should be 6.3V +/- 5%, per the RCA Transmitting Tube Manual. Note that the measured heater supply waveform is a square wave, rather than a DC voltage. The frequency and duty cycle vary slightly but the averge output voltage remains reasonably constant. Purists may wish to add a filter section comprised of a 1mh inductor in series with the heaters, bypassed by a 10,000uf capacitor to facilitate precise measurement and low noise. In that case, the voltage feedback connection should remain directly at the 'unfiltered' output of the 500uh inductor L1 to assure stable regulator oscillation.