Power Supply Schematic

Posted on Feb 7, 2014

The power transformer has three secondaries: 720V at 120mA center-tapped for the plate and screen supplies, 6. 3V at 3. 5A for the tube filament and bias power supply, and 5V at 3A (not used). Since the 5V secondary is not used, a little more current than 120mA can safely be drawn from the high voltage secondary. The high voltage secondary feeds a c

Power Supply Schematic
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onventional full wave bridge rectifier to convert the AC to unfiltered DC, which is then fed to the plate power supply filter. Since the the PIV rating of one diode is not enough, two must be used in series, so the four diodes D1 through D4 are actually two 1N4007 diodes in series. For the purposes of the plate supply, the transformer center tap is not used. However, the center tap is used for the screen supply. The output from the plate rectifier is direct current (DC) but with a large alternating current (AC) component superimposed. The output from the rectifier in fed to a choke input filter. The filter choke allows the DC component to flow through, while offering a very high impedance to the AC component. The filter capacitors are essentially an open circuit to the DC, but they effectively short circuit the AC component to ground. The result is that little of the AC component reaches the output. Since the capacitors have a maximum rating of 450V, two are used in series to form a 900V 50uf capacitor. Two 40k, 10W bleeder resistors in parallel with each of the capacitors serve several important functions: (a) They bleed off the charge on the filter capacitors when the unit is shut off, preventing the possibility of a dangerous electric shock that could occur even though the unit were turned off and unplugged. (b) They provide a minimum load on the power supply to prevent the output voltage from soaring during standby...

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