> Have any of you documented this kind of test before? Have
> you published
> the data? Would you care to? If you have, would you share
> it? I have
> searched the web but have found no published comparison
> between running
> with and without a tuned input circuit on a Grounded Grid
> amp. I don't
> have the equipment to run this kind of test but it
> certainly looks like
> it would be worth while doing. We could argue theory all
> day long (and
> we have!) but a practical demonstration would be worth a
> thousand arguments.
I've never compared IM performance related to input
circuits, but I have observed efficiency changes.
A prototype (pair of 3-500Z amp) I built for Heath had
horrible ten meter efficiency. It was in the area of 30 or
40%. The input circuit was a low-pass pi with proper Q, but
it was located away from the tubes and connected through
RG178 coax about 25 inches long. The combination of
reactance at the input network's output and cable length
resulted in the tubes having a high cathode impedance at the
second harmonic of 28MHz. When that system was altered so
the impedance looking from the tubes back to the input was
low at the second (and higher) harmonic efficiency went up
to over 60%.
The same effect to a lesser extent occurred in the Ameritron
AL12 series of amplifiers. By simply altering the cable
impedance between the tuned input and the tubes, efficiency
on ten and 15 meters could be noticeably improved.
Decreasing cable impedance from 50 ohms to 17 or 25 ohms
significantly changed efficiency.
Based on my experience looking at this in several amps since
then the impedance seen at the cathode on **harmonics of the
drive frequency** is critical to efficiency. It has to be
low at even harmonics or the plate efficiency suffers. I
included a second harmonic series-tuned L/C circuit from the
cathode of one amplifier tube to ground to improve
efficiency.
This means if you don't use a tuned input, the cable length
from the exciter to amplifier and the exciter 's output port
impedance at harmonics is critical to amplifier performance.
An unlucky placement or type of input circuit can certainly
affect efficiency.
As for IM, I never tested it before and after that change.
It might be worth looking into.
73 Tom
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