[Amps] More instability problems

[N1BUG]

On 04/12/2013 10:28 AM, Jim W7RY wrote:
> Perhaps you have an antenna system problem?

The problem is occurring with several different antennas. However, 
there is on thing I have not yet taken apart for inspection: an 
antenna switch.

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.

73,
Paul