[AMRadio] Ultrasonic feedback

k0ng at inebraska.com k0ng at inebraska.com
Mon Feb 2 22:16:55 EST 2009

John: If you can hear the squeal, it is not ultrasonic. That would be rare
I think.

Some tubes and their wiring have been known to oscillate at RF frequencies
which may have several "carriers". The difference between these carriers may
land in the audio frequency spectrum and be amplified by the modulator
circuitry. Putting ferrite beads or parasitic chokes in the grids and plates
of the tubes will not harm anything and may cure the problem. I have  
this before.

Loose nuts and bolts on the audio transformers or loose laminations are a more
likely cause of accoustical feedback.

Bad inter-stage bypass capacitors allowing electrical feedback between stages
is the first place I would check. (B+ by-pass caps near the preamps/drivers).

Good Luck.  Charlie,  K0NG

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