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100uF 330UDC

Fig 3—This is how my supply is wired for a tetrode amplifier that has its screen grid tied to dc ground. For use with a more conventional grounded-cathode amplifier circuit, see text. C3 came from my junk box. You will probably need to use two lower-voltage capacitors in series. Install a 100 kii, 2 W equalizing resistor across each one. C4 is a bank of 17 salvaged capacitors connected in parallel with a total capacitance of 15 pF. Diodes D1, D2 and D3 are 1N4007S. D4 through D7 are salvaged from various microwave ovens. F1 and F2 are slow-blow 15-A fuses. F4 is made of 5 inches of #40 copper wire supported by terminals on the interior Plexiglas panel. Relay K can be a single DPST relay or two SPST relays. I used two salvaged SPST relays with their +12 V, 160-H coils wired in series. They are similar to the Omron LY1-DC12 (from Digi-Key). R6 consists of five 10 MQ, 2 W carbon-composition resistors in parallel. They were already installed across five of the capacitors comprising C4. Other types of resistors, such as carbon-film, may not withstand the voltage. Z1 is three Radio Shack #276-568 metal-oxide varistors in series.

side of neutral), connect the cathode end of D1 to the neutral wire. (I don't recommend this circuit if you plan to use a 117 V outlet. It is too easy to end up in a situation where the polarity is reversed making the amplifier chassis "hot" and potentially lethal.) The circuit is safe with 230 V outlets because they can't be easily reversed.

Capacitive voltage multipliers have a reputation among some experimenters for poor voltage regulation. This can happen if the reactance of the capacitors is too high (capacitance is too low). Using large value capacitors at C2 and C3 results in good voltage regulation. The voltage at F3 drops less than 5% with a screen current of 50 mA.

Transient suppressor Z1 is included in this circuit because I became tired of replacing D2 when an overload of the high-voltage line occurred. This happens because a low resistance path (short) on the plate line causes the negative high-voltage line to "overpower" the screen supply. This increases the voltage across D2, destroying it and sometimes Dl, as well. This doesn't happen with a "conventional" setup, where the screen floats above ground. (See Ian White's article for more consideration of screen supply issues.3) If an overload occurs now, I need only open the fuse holder and pop in a new F3. (The overload is dissipated in Z1 until F3 blows.) For a conventional screen-grid arrangement, place Z1 on the output side of F3 to help protect the screen grid.

Construction

Each microwave-oven transformer weighs more than 10 pounds. I built a 12xl2-inch frame of steel angle stock (l'^xl'^x'/s-inch) to support the transformers. It rests on an insulated platform inside the enclosure shown in Fig 4. Fiberglass makes a sturdy material for the platform. I used painted wood 2x4s and 3A-inch plywood.

The lower half of the enclosure in the foreground is a light-blue, enamel-coated-steel crisper drawer from a refrigerator. The upper half is a similar white drawer folded back on hinges (in the background). On top of the steel frame is a U-shaped frame made from a microwave-oven cabinet.

Time-delay and screen-supply components are mounted on a perforated phenolic board at the left. On the far side of the U-shaped frame, I bolted a Plexiglas panel that holds the high-voltage rectifiers and output connectors.

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