Vanguard Labs

196-23 Jamaica Ave Hollis, N.Y. 11423

Jim Fisk WA6BS0

1560 Glencrest Court

Son Jose, California

Photos by Jim Harvey WA6IAK

The Mi

J low many times have you wished that you had one more source of high voltage? Invariably when breadboardi i ig circuits, one of the problems that arises is the need for an additional source of regulated voltage. This is particularly true in YHF and microwave work when setting up if strips, mixers and other peripheral equipment. Furthermore, this type of work usually requires a certain amount of portability. Unfortunately, most bench type regulated supplies are rather bulky affairs which don't lend themselves too well to moving around. In addition to being simple, inexpensive and lightweight, the truly versatile supply should provide both variable positive and negative voltages. With these requirements in mind, the "mini-supply" was constructed,

Front view of the Mini-Supply.

i Supply

This unit is compact, inexpensive, and provides an adjustable positive voltage from 125 to 3T5 volts and an adjustable negative supply from zero to —180 volts.

Until recently, the realization of a really lightweight, high-voltage regulated power supply was next to impossible; with the advent of inexpensive semiconductor diodes this is no longer true. There is no doubt whatsoever that transistors are here to stay; but for many high voltage applications vacuum tubes are still a better choice at the present state of the art. } iowever, the use of silicon rectifiers and regulator diodes cuts size, weight, and cost. In fact with a little shopping around, semiconductors may be obtained at a lower cost than equivalent vacuum tubes. For example, the total cosl of the diodes in this unit was $3.57. The vacuum tube equivalents (assuming a 5U4G ful I-wave rectifier, a 6X4 half-wave rectifier and three OB2's) would run about $3.75, even on the surplus market, rhe hybrid design of Fig. 1 evolved by using the best characteristics of both silicon diodes and vacuum tubes to the most advantage.

In essence, the positive voltage section of this power supply consists of a simp e series regulator in series with the B+ line. A small portion of the output voltage is picked off at the 25 K pot and fed to the grid of the 6AU6 voltage amplifier, The cathode of this tube is maintained at 90 volts above ground by the 90 volt, 450 milliwatt zener diode in the cathode lead. The voltage fed into the grit i is compared to this 90 volt reference signal and the plate voltage varies accordingly. This voltage controls the grid of the 6AQ5A series regulator and determines the voltage drop across it. In other words, the 6AQ5A series regulator acts like a voltage-controlled resistor in series with the high-voltage line.

To show how the regulation process works, consider the case where the load increases and tends to lower the regulated output voltage.

As the output voltage goes down, so does the voltage on the grid of the 6AU6. As the grid voltage decreases, the plate current flow decreases and the plate voltage goes up because there is a smaller voltage drop across the 220K ohm plate load resistor. Of course, this increasing plate voltage is reflected on the grid of the 6AQ5A regulator tube; the cathode current increases and the cathode voltage goes up. Actually, all this happens instantaneously and any time the output voltage tends to increase or decrease, the voltage amplifier and series regulator immediately compensate just enough su that the output stays at the same level.

When you want to change the output voltage, rotation of the potentiometer changes the voltage at the grid of the voltage amplifier, This voltage change sets the electronics in motion and the circuit immediately tries to compensate; this results in the desired change at the output.

Any compact high-voltage power transformer with about 350 volts each side of center tap at about 100 mils is suitable for this supply, The exact voltage is not too important because there are inherent differences between regulator diodes and slight component adjustments will be required anyway. However, don't let this frighten you; the only component that will, change is the 6800 ohm resistor between the 25K pot and ground and the proper value is easily determined. Simply attach the lower end of the 25K pot to ground and measure the voltage at the catUode of the 6AU6 as the potentiometer is varied from one

Bottom view of the Mini-Supply, Note the mounting of the zener diodes (right center) and the way the components ore stacked up around the edge of the chassis.

end to the other. You will note that the cathode voltage remains constant at about 90 volts over a rather wide range of potentiometer resistance, but abruptly falls oif as the arm of the pot passes a certain point approaching ground. Below this "break-off" point, the cathode current of the 6AU6 is insufficient to maintain proper zener regulation; therefore, a resistor must be placed in series with the pot to limit the resistance between the 6AU6 grid and ground to the required level.

To determine the required series resistance, set the pot slightly above this breakoff point, turn off the power and measure the resistance between the arm of the pot and ground. Assuming that the measured resistance is 5600 ohms, the simplest approach at this point would be to remove the 25K pot from the

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