Playing around with this idea a bit, I think what I'm going to do is get a voltage booster, and put it in a small separate device.? On that device I'll do an RCA PTT jack from the radio, put the 78L05 regulator in that case tapped off the 12v input, and then the +5v output to the amp.? That way the amp can remain 'stock', and I have another small companion device that will stay with it to boost the voltage to 22v, and provide the +5vdc keying signal.
