Here is a fairly simple regulated power supply circuit using a zener reference and a series pass transistor that I built to power a vacuum tube microphone preamp project. It’s not really something that I designed. It’s just based on text book examples, some existing circuit designs, some tinkering on my part, and a lot of help from friends.
The power transformer is chosen so that with no load the full-wave-rectified, filtered, unregulated DC voltage will be 1.4 times one-half the rated AC voltage of the center-tapped high voltage secondary winding. (This is because the filter capacitors charge up to the peak AC voltage value which is about 1.4 times the RMS value.)For example, I chose a 500 volt center-tapped transformer (such as Allied Electronics stock number 227-0113) for my power supply which provides almost exactly 350 volts unregulated DC (1/2 of 500 = 250 times 1.4 = 350). This allows enough input voltage for good regulation over a fairly good range of commonly used vacuum tube plate supply voltages (250 to 300 volts).
The Zener diode is used as the reference that sets the output voltage of the regulated power supply. It can be a single device or it can be made up of 2 or more lower voltage devices to create the necessary zener voltage. For example five 60-volt zeners in series will make a 300 volt reference for the base of the pass regulator transistor. Zener diodes come in various voltages and wattages. I played it safe and used 5-watt zeners (1N5371B which are 60 volts each) in series and about a 10K 5-watt resistor in series with them. The TIP 50 that I used as the pass transistor is a darlington type and has relatively high voltage capability and is a pretty good choice for a plate voltage regulator such as this. Use a heatsink to help cool the TIP 50 which is in a TO-220 case.
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The value of the series resistor between the first and second filter capacitors is not very critical. About 1.5K to 2.2K ohms at 5 watts works fine. This resistor along with the additional filter stage decreases the amount of ripple going to the TIP 50 transistor. The resistor also tends to limit the current going to the regulator transistor and is just a good idea in general.
Originally I had used 22 uf (microfarad) filter capacitors, however Douglas Fearn of D.W. Fearn was nice enough to e-mail me to suggest that these capacitors should actually be much larger which will tend to improve the performance of the audio circuit being powered. He suggested that values as large as 400 uf could be used. The working voltage of all the capacitors should be at least 450 WVDC.
Finally I should mention that this is a low current power supply. The transformer I chose is rated at 40 milliamps. This should be good for most projects like vacuum tube microphone preamps or compressors. It is not heavy-duty enough for a power amplifier however.
My final design is pretty basic, but it works well and provides good regulation with very low ripple and noise on the DC output.
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