[Amps] More instability problems

[Vic K2VCO]

It does sound like a parasitic of some kind. But why suddenly? Another possibility: 
something in the system is arcing. An arc might have a similar 'jumping' characteristic.
What I would do at this point is hook the amplifier up to a good dummy load with a known 
good hunk of coax and see if you can recreate this without any of the other variables like 
antenna switches, etc.

On 4/12/2013 8:02 AM, N1BUG wrote:
> I just said:
>>
>> What makes me think oscillation is:
>>
>> Suppose I have the amp tuned for maximum output at 28 MHz. Indicated
>> antenna SWR is low, output is 1500 watts.
>>
>> Now assume that for the following procedures I maintain RF drive at
>> all times.
>>
>> Now I vary the amp loading slightly. As I decrease loading
>> capacitance just a bit, I see a slow, gradual drop in output of
>> maybe 50 watts, then all of a sudden a drop of at least 500 watts,
>> and at that same instant, indicated SWR jumps up wildly. This is
>> absolutely repeatable on several bands with different antennas.
>>
>> Once the sudden "jump" has occurred, returning the load cap to the
>> previous setting does not return things to normal. Going well past
>> the proper spot, toward higher capacitance, does at some point
>> produce a sudden decrease in SWR and increase in output power.
>
> FURTHER to this....
>
> If I now leave the load cap at the max output spot and vary the tune cap... I see a 
> sudden decrease in output and increase in indicated SWR when going to slightly less OR 
> slightly more capacitance. Unlike the load cap, returning this one to the previous max 
> output spot does cause things to return to normal (I don't need to continue past the 
> previous sweet spot).
>
> With the load cap, the issue only occurs if I go toward less capacitance. With the tune 
> cap, it occurs going either way.
>
> 73,
> Paul
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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/