[AMRadio] Ultrasonic feedback


John King k5pgw at yahoo.com
Mon Feb 2 22:27:08 EST 2009


Thanks Charlie. Good input. 73, John, K5PGW


--- On Mon, 2/2/09, k0ng at inebraska.com <k0ng at inebraska.com> wrote:

> From: k0ng at inebraska.com <k0ng at inebraska.com>
> Subject: Re: [AMRadio] Ultrasonic feedback
> To: amradio at mailman.qth.net
> Date: Monday, February 2, 2009, 6:16 PM
> 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  
> experienced
> 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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