[AMRadio] AM audio problem

John King k5pgw at yahoo.com
Wed Dec 31 13:44:12 EST 2008

I have a Heath Apache that I acquired with the audio modifications already made which included changing coupling caps and removing the clipper circuit.

Recently, it developed a peculiar problem. That is that when the AF gain control is increased, with no audio being applied to the audio preamp section without a microphone being connected, the modulator plate current rises as the audio gain control is turned clockwise. I suspected microphone problems and maybe hum driving the modulator, but WITH NO MICROPHONE CONNECTED, the situation still occurs.

When I remove the first audio preamp tube, 12AX7, the modulator current does not rise when the AF gain pot is increased with no audio input. The modulator resting current remains the same. Removing the 12AX7 or the 12BY7 audio driver stops the current rise when advancing the audio gain pot. Something is introducing audio to drive the modulators with the microphone removed. Sounds like something is being generated within the audio stages to cause the EL 34s to draw more current with merely and increase in the audio potentiometer.

I have listened to the signal on a receiver and there is noise, but not hum such as ac hum.  There was added a feedback loop consisting of a .1uf cap and 270 k resistor from the 500 ohm tap on the secondary of the modulation transformer to the cathode of the 12BY7 audio driver cathode.

I would suspect a shorted .1uf cap in the feedback loop causing feedback or oscillation except if that was happening, would removal of the 12AX7 audio pre-amp tube clear up the problem since the feedback circuit is from the modulation transformer to the cathode of the audio driver tube?

As you can see, I am puzzled. I have changed the 6AQ5 clamper tube in the final amp section and swapped out the 12AX7 and 12BY7 tubes with no change in symptoms.

Have any of you AM audio experts encountered such a problem. Your input will be greatly appreciated, but for those who always ask "is it plugged in" IT IS PLUGGED IN, but maybe the operator isn't plugged in. Thanks and 73, John, K5PGW


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