[Amps] More instability problems

[Vic K2VCO]

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/