[Amps] More instability problems

[Gary Schafer]

Check the screen bypass capacitor.

Are you driving the grid or the cathode? 
Is there a tuned input or a passive input?

Does the plate current and output jump around with a dummy load connected
directly to the amp?

73
Gary  K4FMX

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of N1BUG
> Sent: Friday, April 12, 2013 11:02 AM
> To: Jim W7RY
> Cc: AMPS
> Subject: Re: [Amps] More instability problems
> 
> 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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