Fig 2—Transformers for microwave ovens are usually constructed with separate winding areas. Some builders find them useful for rewinding because the highvoltage secondary can be cut away with a sharp wood chisel. This leaves plenty of room to install a low-voltage secondary for filament supplies or other low-voltage applications.

should have a flat end and a cross section almost as large as that of the shunt.

Plate and Screen Supply

Fig 3 shows how I used four transformers and a number of other microwave oven components to build a plate and screen supply for a vacuum-tube amplifier. The output of this supply is arranged somewhat unconventionally because the amplifier uses a grounded screen with the cathode floating at -500 V.

It is easy to modify this design for more conventional grounded-cathode applications. Simply disconnect the meter (M) and ground the terminal labeled CATHODE, where all the transformer secondaries tie together. If your amplifier needs a conventional, positive-output screen supply, reverse C2, C3, D1 and D2, then connect the screen grid to the free end of M. If you have a grounded-grid amplifier that needs no screen supply, remove those components along with R5, Z1 and F3.

I was fortunate enough to find transformers where the cold side of the winding was not internally connected to the transformer core. If your transformers have internally grounded secondaries, you can use them in this (unmodified) design, but you must insulate the core from the chassis. I did so in my supply— just in case I need to use internally grounded replacement transformers some day. The insulation between the primary and core will be more than adequate for 500 V dc.

C4 is actually a bank of parallel connected capacitors that can store a large amount of energy. Attempts to charge C4 by suddenly applying full power results in large currents, which could damage the rectifiers and other components. Therefore, power is first applied through R2 and R4 when S1A and SIB are closed. CI charges through D3 and R3 until the voltage across it is high enough to close the contacts of relay K, which applies full power to the power supply.

When SI is thrown to the OFF position, the coil of K is shorted by Rl, which causes the relay contacts to open. This scheme eliminates the additional high-current relay or switch needed in other stepped-turn-on methods.

The screen supply can be modified to provide a lower screen voltage if needed: If you need a screen voltage of 330 V dc (assuming 117 V ac on each

0 0

Post a comment