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Re: [Amps] New Guy, Old 813 amp

To: "k7rdx" <k7rdx@charter.net>,"Roger D Johnson" <n1rj@adelphia.net>, <amps@contesting.com>
Subject: Re: [Amps] New Guy, Old 813 amp
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 27 Apr 2007 09:03:09 -0400
List-post: <mailto:amps@contesting.com>
> The LDG amp module is easy to install and use in a HB amp. 
> The purists will
> all agree against using an L network for cathode matching 
> and I tend to
> agree to some extent. I bought an LDG amp module for my 
> 3CPX5000A7 project
> and so far have had much success with it.

This is an important point. Tubes with very low cathode 
impedances and operated with higher conduction angles have 
significantly more stability in input impedance as the drive 
level and output loading are varied. They have more power 
gain also, and are more immune to what goes on at each end 
of the tube.

The 3CPX5000 is an extreme example that way, although I 
would still use a good reasonable Q input that produces a 
low impedance at harmonics. (Like any tube with a cathode 
common to the heater, you have to watch hum from filament 
voltage drop across the cfilament choke.)

On the other hand the extreme in being "input circuit 
critical" comes from tubes with higher cathode drive 
impedances or more prone to harmonic or intermodulation 
distortion. Tube with higher grid current as a ratio of 
anode current fall into this class.

The reason for this is the input circuit is in series with 
the output circuit in a cathode driven PA. When the cathode 
has a very low impedance, like a tube in the order of 20-25 
ohms, and an output impedance that is very high (several 
kilovolts at modest current on the anode) and very low grid 
intercept the input is much more isolated from things going 
on in the tube.

On the other hand a tube with 100 ohms of driving impedance 
and less anode impedance is a different world, especially 
when the grid current is several times higher.

73 Tom



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