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Re: [Amps] Suppressors

To: amps@contesting.com
Subject: Re: [Amps] Suppressors
From: Steve Thompson <g8gsq@eltac.co.uk>
Date: Wed, 30 Aug 2006 17:56:03 +0100
List-post: <mailto:amps@contesting.com>

Peter Chadwick wrote:
> Rich said:
> 
>  >Higher series R means lower parallel-equivalent R - and that means  
> lower gain.<
> 
> Not so. The gain is determined by the total anode load impedance, not 
> just the resistance in the network. A given length of resistance wire in 
> a coil has a certain reactance and a certain resistance, when 
> measured as series components. As frequency goes up, so does the total 
> impedance when considered as series components, and so does the total 
> impedance when considered as parallel components - at the same frequency.
> 
> An inductor in shunt with a resistor in parallel shows an impedance 
> change as frequency goes up that is asymptotic to the impedance of the 
> resistor. That says the network impedance has a maximum value, and thus 
> the amplifier gain has a maximum value.
Only at frequencies where the rest of the network between the suppressor 
and ground looks close to a short circuit. Any inductance in that path 
means that the anode load is not the Rp of the suppressor network. 
Outside the valve, there's several things in series from the anode to 
ground - you've got to look at the whole lot of them in their series 
equivalent form, get the overall Rs and Xs and only then work out the 
parallel equivalent if you want to know the parallel load that the anode 
is seeing.

Worrying about the vhf Q of the suppressor in isolation isn't quite 
irrelevant, but is misdirected.

Steve

PS I'm sorry if my contributions appear disjointed - my ISP has major 
problems meaning that email is not arriving in anything like 
chronological order.
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