Amplifiers such as the MLA-2500, SB-230, 374, 78, and 76PA have no tuned
input. This is not a problem when such a g-g amplifier is driven by a
tube-output radio such as the TS-830S. The reason is that although the
input Z of a cathode-driven triode fluctuates from roughly 1/3 of the
average driving Z on negative peaks to a virtual open circuit on positive
peaks, the Pi-network tuned circuit in a tube-output radio acts as a
flywheel to smooth out the Z fluctuation at the cathode/s. However, with
modern solid-state radios there is no tuned output, so something is
needed to smooth the Z fluctuation. A possible workaround, short of
adding a bank of tuned input circuits, might be to put a ultra fast
recovery diode in series with the input terminating R. The diode is
oriented so that the terminating R conducts only when the driving
potential is positive -- which is when the tube is not conducting..
Thus, on the negative half of the driving cycle, when the tube has a low
driving Z, no driving power is lost in the terminating R and the driver
should see a lower SWR.
- - Unfortunately, I do not own such an amplifier to test the idea on.
If anyone out there has such an amplifier and is willing to test this
idea, please inform me and I will send you a freebee 2A, 35nS, 300v
switch diode.
thank you.
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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