[AMPS] update on 13% eff 6M amp

[Steve Thompson]

Jon wrote
>Hi Dick,
>
>There is a trick that some of us talked about a few months
ago when I had
>the same problem with a 4-1K at high voltages on 10M.
>
>The tube does have a 7.6pf output capacitance.  However,
put that puppy
>in socket and add a cabinet around it and your capacitance
probably
>doubles or tripples.  Then you have the problem of matching
a large
>impedance with a somewhat large tune capacitance.  The
answer is a
>super-high Q output tank.  I calculated some possibilities
with my amp to
>be Qs of around 40 or so!  Too much circulating current!
>
>The trick I used and used successfully, was to add an
additional coil on
>the output of the tube before the tune capacitor.  This
coil is in series
>with the anode output line.  Really you can put it between
your blocking
>cap and tune C.  You'll need to experiment with different
coil values,
>but suffice to say, it will be fairly small.  Just a few
turns is all
>you'll need.
>
>What this coil does is to act as an impedance step down
transformer.  You
>can work out the math using series to parallel conversion,
etc. but it
>does work.  Then you'll be able to use your pi-net to match
a lower
>impedance to 50 Ohms which is easier to realize.


You might also consider ditching the Pi matching system and
resonating the tube with a parallel inductor. Adjust the
loading with a variable position coupling loop.

Steve


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