Bob
The biggest issue you have to deal with is how much effort you want to put into
the project versus the available economic resources. The LDG Circuit is a
drop-in arrangement that will give you minimum grief but if you want to roll
your own you can probably get better results at half the costs.
At the low Q values used in input circuits you can copy and design for the tube
you're using with fixed value components and get good results. The only band
that gives anyone problems is 10 Meters.
In your post you didn't mention whether the amp was completely homebrew or a
manufactured design. If you are dropping in a network to a manufactured amp
this in my opinion is the best way to go. If you are building an amp from the
ground up and you have a penchant to get the input circuit dialed-in exactly
what I would do is drop the slug tuned coils in favor of using ceramic trim
caps on both the input and output and then build an access door on the amp to
gain access to the circuit for final adjustments. Using a capacitance meter I
would preset the trim caps to the values in the design or derived from
formulas. The last check after everything is assembled and installed in the amp
is to test the circuit cold. Remove the tubes and connect a carbon resistor and
silver mica cap of values that will simulate the tubes you are using. Connect
a reflectometer to the circuit input and feed an appropriate level of power to
the circuit and measure the input SWR. If it is approximately
1.5 to 1 or less do not not attempt readjust the caps. Only after everything
else is working would I go back and tweak the caps. Remember at these Q values
recognising a peak may be very difficult. Don't forget to remove the simulation
components before you apply drive power.
If you choose to used fixed value components use silver mica caps.
73s
Bob W6AH
-------------- Original message --------------
From: "Bob Selbrede, K6ZZ" <k6zz@ccis.com>
> 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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