[AMRadio] Ultrasonic feedback

Larry Szendrei ne1s at neandertech.com
Tue Feb 3 08:47:32 EST 2009

> My Apache modulator squeals when the microphone gain is raised
> and the modulator tubes draw a lot of current. As long as the
> microphone gain is kept very LOW the current does not take off
> and there is no squeal coming from the modulator section. I am
> going to go through the section with a screwdriver and a
> nut driver to tighten up the grounds to the chassis and tube
> socket.

In my experience the most common cause of these symptoms isn't feedback
within the modulator itself, but RF rectification somewhere in the audio
chain prior to the gain pot. Generally this is an indication of too much
RF in the shack. I had this problem when I was using balanced line and a
non-optimal tuner for this situation, creating imbalance in the system and
subsequent radiation from the feedline. I have since built myself a proper
balanced tuner at that operating position which has eliminated the

You can try wrapping the microphone cable around a ferrite or powdered
iron core of the type used for RF filtering. Usually 3-4 turns will take
care of it if it's going to at all. Put the core as close to the mike jack
at the transmitter as you can. Trying a different style of microphone can
also help. If you are running an EQ and/or processing ahead of the
microphone input these devices can also be susceptible to RF


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