DE:W1MK
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
http://www.contesting.com/tours/W9JA/index.html

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