Take a look at Paul's (WD7S) tuned input board:
<http://home.earthlink.net/%7Ewd7s/TU-6B.htm>
73, Tony W4ZT
Bob Selbrede, K6ZZ wrote:
> Hi Folks,
>
> I would like to build or buy a tuned input network
> (160M-10M) for an amp running a pair of 3CX800A7's. Here
> are my thoughts so far. Comments appreciated.
>
> Buy:
>
> So far, I haven't found anyone on the web building and
> selling tuned input networks suitable for this application.
> I could probably just buy one of those LDG units that was
> mentioned on the AMPS List the other day, but it seems like
> overkill. After the values for the input matching network
> are know and set, they most likely won't change so an
> autotuner seems to provide features that aren't really
> necessary. Then again, for $125 maybe, it may be worthwhile
> in the long run.
>
> Another thought would be to buy a tuned input board for an
> existing commercial amplifier such as the Ameritron AL-800H.
> I'm not sure how much it would cost, but it runs a pair of
> 3CX800A7's so it would most likely work fine.
>
> Are there other "buy" alternatives? I called Omega
> Electronics and they apparently don't have any more tuned
> input boards on hand.
>
> Build:
>
> Building a tuned input network seems pretty straightforward.
> A switchable PI network with tunable slug inductors seems
> like a good approach. I'm assuming that the input impedance
> for the tubes would be approximately 25 Ohms so I would be
> looking at matching 50 Ohms to 25 Ohms from 160M through
> 10M. Shooting for a Q of 2-4 I should be able to use a PI
> network calculator to get the proper component values.
> Sound about right?
>
> Another approach would be to build the network utilizing the
> same component values as used in the AL-800H or other 2x
> 3CX800A7 amplifier input circuit?
>
> Am I overlooking any other build/buy alternatives?
>
> 73, Bob K6ZZ
>
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