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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.
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.
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