[AMPS] Tuned input question

[Phil Clements]

> I recently aquired a nice dentron mla-2500 that has been converted
to a
> single 3cx800a7 (and did it VERY nicely).  Great little amp but when
I
> bought it I forgot these dentrons do not have a tuned input.  Even
without
> any input circuit at all some bands have less the a 1.5 SWR on the
input
> side, the ones that dont are easily tuned by the radio's auto tuner.
Is it
> OK to do this?  I know dentron used to sell a little box that went
inline
> with a switched tuned input and some people have mentioned they seen
a tuned
> input circuit in one of the magazines that doesnt need to be
switched.

Before the days of SS devices in the finals of transceivers, most
manufacturers,
including Collins, relied on the  Pi network in the exciter to provide
the flywheel
action to reduce phase distortion. A specified length of coax between
the exciter
and the amplifier assured that the total electrical length in degrees
between the
plate of the exciter and the cathode of the amp was some multiple of
90 degrees.

To answer your question, if your tuner in your exciter is a pi
network, it should
reduce phase distortion below that of an un-tuned condition, and
provide a good
match for your transistors. Perhaps someone with such a set-up can
make some
measurements for us to see if the flywheel action is generated by the
tuner. The
coax from the tuner to the amp should be kept below 1/20th of a
wave-length.

(((73)))
Phil, K5PC




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