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If you want to convert a 147 (or similar) amp to 122 input specs, but you don't want to add an extra tube to handle the switching of the speed control relay, here's a simple circuit which will do the trick. This circuit comes from BobSchleicher by way of MikeFulk.

Of course, another option would be to add one of BobSchleichers (or another type of) SolidStateRelays.

If you use a TIP102 for the darlington, it has internal resistors between the base and the emitter, so the 1 Meg resistor can be omitted. (I also used 68k resistors to the G-Gs rather than the 56k on the schematic. Worked fine.

As for the relay, Omron P/N LY1F-DC24 or Potter & Brumfield P/N K10P-11DT5-24 will work. (As will many, many others.)

Note: These component values vary from the ones shown in the version of this circuit on Bob's website. That version shows the 56k ohm resistors as 2.2M, and the 1M as a 56k. That's a slight different application, though (voltages are different.)

Also note: where the above diagram says "+25v from 6550 cathode", really it should say "+25v from 25v filter capacitor". (I.e. if your amp is wired for 147-style cathode biasing, you want to tap the 25v from the center-tap of the output transformer, not one of the 6550 cathodes.)


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