Tony King - W4ZT wrote:
> Assuming no problems using the LDG tuner, it's sure an easy solution. As
> pointed out by others, we still need to determine whether it will hunt
> and click and burn the relay contacts with all the switching under
> power. I would, at the least, put the equivalent of the 10 meter output
> C directly from the cathode to ground.
This idea keeps coming up. In my opinion, the *only* advantage of this
solution (unless you are building a completely automatic amplifier a la
87A) is that it avoids the need to calculate input pi-net values. And
there are problems, such as the unknown Q, the input impedance changing
when the output is tuned, the fact that it will need to clickety clack
whenever you change bands, etc.
What I, a very lazy person, suggest is 1) either steal the component
values from an amplifier design using the same tube(s), or 2) use one of
the pi-network calculators available for free or at a nominal cost
<http://www.ifwtech.co.uk/g3sek/netcalc/netcalc.htm>
<http://www.rfcascade.com/antenna.html>
<http://www.kvindesland.no/yl2qm/software/developer/developer.htm>
etc., etc.
Then, when you have designed or stolen the values, build the networks
with mica compression trimmers for input capacitors, fixed silver mica
capacitors for the outputs, and T68 size toroid cores. Adjust it by
temporarily installing a non-inductive resistor equal to the input
impedance of the tube(s) from the cathode to ground, and with filament
(but not plate) voltage applied, tune the networks for minimum SWR with
an MFJ or similar antenna analyzer by squeezing/spreading the turns on
the toroids and adjusting the trimmers. If you build the networks so
that the capacitors are safely accessible from outside the enclosure,
you can touch up the adjustment under full power.
This is easy and it works. You do need to start with the correct value
of input impedance for the tube(s), but the information is available.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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