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[AMPS] 4-1000/ BW 850A TANK

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Subject: [AMPS] 4-1000/ BW 850A TANK
From: (Paul Hellenberg K4JA)
Date: Wed, 17 Feb 1999 08:11:43 -0600

The idea is to lower the Qloaded on 20/15/10, where it is too high. One 
loses too much power in the tank circuit and in the extreme case you can 
melt the plastic supports. I have two of these amps, and I increase the 
Inductance of the 850 tank circuit to lower the loaded Q. If you want the 
details drop me a note and I will email them to you ( I do something a 
little different on each amp)
The other idea that we have seen presented is to add inductance between the 
tube and the 850 tank circuit. This in effect creates a second pi net 
circuit. The tune cap of one and the load cap of the other are in parallel 
so original tune cap looks like in increases in value. I know this added 
inductance may looks like it forms a L net but it is really not. Now it is 
easy to design this to work on a single band. Design the first pi net to 
transform the impedance down to 1/2 value (or what ever). The Q of this pi 
net is improved a little ( over not using this added inductance ) since the 
Cmin of the cap is now missing ( Qloaded = R/Xc ). So you get maybe 30 % 
decrease in loaded Q, but not really since the second pi net also 
contributes to the overall Qloaded, so maybe when you work out the numbers 
the Qloaded goes down by 20 to 25%. I worked out the numbers for 10meters 
with a R of 7K, and transformed the first pi net to 3.5K, with a Ctube + 
Cparis of 25 pf, the added L was 2.2 uh and the C next to the tank was 35 
pf. The problem was it worked only on 10 m , it is a very narrow band 
circuit and its useless on 15m and 20m without changing the L. So if you 
want to use this for multiple bands you end up with multiple switched 
inductors. That my be ok but not what I wanted. W1MK 

Paul J Hellenberg K4JA ex W9JA KS9K
Tru Line Lithographing Inc.
414 554 7300 -125    Fax 414 554 8217    Home 414 554 9170

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