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Re: [Amps] Really silly question

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Subject: Re: [Amps] Really silly question
From: "Tom W8JI" <>
Date: Wed, 28 Jun 2006 11:28:25 -0400
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> Running into a good 50 ohm load, all is well at 3.5MHz. At 
> 3.75 up to past 4 MHz, tuning the PA to resonance leads to 
> a peak in PA current, and the bias apparently collapsing, 
> leading to the amp tripping off - just as if it was 
> oscillating.
> As I say, I haven't had chance to dig into it. First 
> thoughts are that maybe the the resistors (or one of them, 
> anyway) have gone open, and the input transformer is 
> resonating around 3.8MHz with the input capacity of the 
> tubes. Interestingly, on 1.8, 7, 10, 14, 18, 21 and 24 
> MHz, the amp works OK. I haven't checked with any detail 
> the gain, which if the resistors had gone open, would be 
> expected to go up. Output tuning is big ceramic caps for 
> tuning and loading, a variable inductor and a 500pF 
> variable on the output in shunt with a set of switched 
> capacitors. The same tuning cap is used across the whole 
> 80m band.


Non-inductive is a VERY relative term. The little gold color 
non-inductive resistors are really quite inductive at radio 

The way most of them are made internally is with two 
concentric opposing windings. They depend on the field from 
one winding, through mutual coupling, canceling the field 
from the other to remove inductance. If a winding opens up, 
they really get bad.

That may not be what changed, you could have lost a screen 
bypass or anything, but I'd still get rid of them. I'd never 
use them as a grid swamping resistor.

73 Tom


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