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EM to PM Conversion

Rewire the Amp

The cleanest way to do this for an M-3 is probably to rewire the AO-29 preamp/amp. Get ahold of the schematics for both the EM and PM versions of the AO-29, and make the appropriate changes in the power supply section.

?EricSimball has written up detailed notes on what's involved. See AO-29 EM To PM Conversion.

WARNING: If you do this without also replacing the power transformer and output transformer with those from a PM version of the AO-29, you will fry something. The EM version uses a 380V transformer, and the PM version a 320V transformer. Use the Hammond-recommended method below instead:

Fake out the Amp

You can just replace the field coil winding with an appropriate power resistor. This is Hammond recommended method. (In some of the older organs, like models M, S and S-1, Hammond recommends changing or adding a power supply filter capacitor as well if this method is used.)

See TSB 27 for details on this method.


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