[Amps] Building or Buying a Tuned Input Network

[Tony King - W4ZT]

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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