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Re: [Amps] Plate Impedance, ARRL

To: craxd1@verizon.net, amps@contesting.com
Subject: Re: [Amps] Plate Impedance, ARRL
From: jkearman@att.net (Jim Kearman)
Date: Wed, 12 Apr 2006 16:38:44 +0000
List-post: <mailto:amps@contesting.com>
Will,

> I read this over again last night, and it didn't mention anything about 
> triodes 
> only. The main reason I posted this was to show the differences, plus find 
> where 
> the illusive 1.8 came from in print. So far, the only book I've seen 1.8 
> listed 
> in was Bill Orrs Handbook. I don't have any newer ARRL handbooks past the 
> 90's 
> as they seemed to be the same old thing, over and over, with not that much 
> new.

Amen, brother! I got my insight into the tetrode situation from Rich Measures' 
website, which I should have mentioned in my OP. Until they published (mangled) 
George Daughters' 4CX1600B amplifier, recent ARRL Handbook legal-limit amps 
have used triodes. The 8877 was king and I think they based their writeup on 
that. Rich wrote a chapter for them that was never published for political 
reasons, but it's on his site and worth a look. 

Frank Carcia gave me the insight I needed to help me pick the values of tuning 
and loading caps when he reminded me of the relationship between Q and 
impedance as you vary them. Assuming you're using a tapped coil, not a variable 
inductor, you can pick a value of L for each band that will let you cover a 
range of Z and Q that are still in the park. Regardless of the actual value of 
Z, the Q will be high enough, but not too high. It takes several iterations of 
the program to figure it out, but it was a real relief to get numbers that 
should work without a lot of reworking. 

> I would like to find out though where the factor of 1.8 was 
> first mentioned.

It would be interesting to see how the rule of thumb came about, for sure. In 
designing my pair of 4CX800As I've planned to make the loading cap large enough 
to cover my butt for a range of load impedances, by padding it on 80 and 160. 
It'll be easy enough to remove unneeded caps, not so easy to install them 
afterward. 

73,

Jim, KR1S
http://kr1s.kearman.com/
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