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Re: [Amps] Amplifier oscillating - any ideas?

To: Sean Williams <m1ecy@m1ecy.me.uk>
Subject: Re: [Amps] Amplifier oscillating - any ideas?
From: Steve Thompson <g8gsq@ic24.net>
Date: Fri, 19 Aug 2005 16:30:37 +0100
List-post: <mailto:amps@contesting.com>
Sean Williams wrote:

>Hi All,
>
>Having just got to testing my pair of Gi7bs on 144 MHz, I notice it oscillates 
>quite happily at 478Megs.....
>
>The amp is built to YU1AW easy build design, with one slight modification - I 
>am using a 100pf doorknob as the HT decoupling cap.
>
>Swinging any of the controls has no effect over frequency or power of the 
>oscillation.
>
>I have relocated the anode RFC so it has shorter leads, the grids are well 
>grounded with brass collets that cover 99% of the grid connection and are well 
>bolted to the aluminium chassis.
>  
>
Without being able to probe with test gear looking for resonances etc., 
it's always difficult to comment. Apart from obvious things like the 
materials and construction of the box, some thoughts - the oscillation 
frequency could be related to the 3/4 wavelength mode of the anode line, 
or to the height of the anode compartment (roughly 1/4 wavelength). 
Whatever it is, changing the cathode wiring/layout to change the 
impedance at UHF might be helpful.

Parallel amps like this can have push-pull oscillation modes. A slot 
down the centre of the anode line is sometimes used - in this case, 
using two separate cathode matching inductors from a common point could 
also disrupt a UHF oscillation.

The FR4 bypass capacitor is likely to be more lossy than your doorknob 
cap at UHF, so you could try adding it in parallel, or swapping.

A final comment, probably not related to the oscillation, personally I 
hate series loop output coupling. You can't rely on getting the right 
loading, or even monotonic adjustment. Capacitive coupling from the 
anode end of the line gives you both.

Steve
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