I have seen this many times, its ultrasonic feedback.
Most times its in the feedback loop from the secondary of the mod
transformer to the second preamp stage.
You might experiment with a small cap to ground in the feedback loop.
Phase shift in the components (mostly iron) causes the very high freqs to be
positive in feedback.
Brett
N2DTS
----- Original Message -----
From: "John King" <[email protected]>
To: <[email protected]>
Sent: Wednesday, December 31, 2008 1:44 PM
Subject: [AMRadio] AM audio problem
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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