I remember that it is a 4CX1500 or 4CX1000 but I don't recall the configuration -- was it
cathode-driven or grid-driven?
You certainly do NOT need a tuned input if it is grid-driven class AB1. The 50-ohm load
provided by the resistor should provide a perfect match to the exciter, and the impedance
won't change if there is no grid current (even if there is some, it will be swamped by the
resistor). All you will need is some compensation for the input capacity of the tube on 15
- 10 meters.
In this case, maybe the swamping resistor is intermittently open? If it is a thick-film
type (looks like a transistor except only 2 leads) then it needs to be adequately
heat-sinked or its power rating goes way down.
If it's cathode-driven, I am not sure why you would need a 50 ohm resistor. In that case,
you WOULD need a tuned input.
On 4/16/2013 7:43 AM, N1BUG wrote:
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
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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