Once again I would like to thank everyone for your input on my amp
woes. There was a massive pile of private responses, which I have
not fully worked through in terms of replies. Each and every one is
appreciated!
I have discovered I have two apparently separate issues:
1.) LF oscillation with no load on the amp output. It will sometimes
succeed in taking off during the instant the bias and relays are
switching from transmit to receive. I intend to work on LF
bypassing, since I feel this amp should be stable even without a
load. The current bypassing is woefully inadequate at LF. I will
attempt to look at bias vs. relay timing to see if I have a problem
there.
2.) Lack of tuned input. I should have known this bad decision would
eventually come back to haunt me. The sudden, wild jumps in plate
current, power out and even SWR (amp to antenna) while tuning are
related to changes in input impedance driving my exciter into SWR
protect! Sudden reduction of drive explains the plate current and
power output jumps. The amp-to-antenna SWR jump is more perplexing.
Apparently when the exciter goes into protect mode, spurs or
harmonics are being generated by the system. This is not good.
The present input configuration is a 50 ohm RF load resistor with a
.01 cap coupling to the cathode. I am trying (again) to figure out
some practical method of adding tuned input to this amp but I am at
a loss. Lack of space and switching problems are problematic.
73
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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