[Amps] instability issue

[Ron Youvan]

   Hi  Paul Kelley, N1BUG:

  <<
I'm trying to understand a problem with one of my amps. It is homebrew, 
160-10 meters, using a 4CX1500B with screen grounded, grid RF grounded, 
cathode drive. It uses a conventional tapped coil pi network with vacuum 
variable capacitors.

The amp was going into oscillation when using it on 12 meters (frequency 
of oscillation unknown, but not in-band). When it did this it was also 
destroying the vacuum relay on the output. The relay would fail with 
contacts stuck in the NO position (contact connected to amp output).

While investigating I discovered the amp will always go into oscillation 
as soon as operating bias is applied IF there is no load on the output 
AND the LOAD capacitor is set to anything MORE than about 700 pF. 
Oscillation will stop if the load cap is reduced to something under 700 
pf. This happens regardless of band switch position or plate tune setting.

On most bands the amp does not break into oscillation during normal 
operation. That was only happening on 12 meters for reasons not 
understood by me. I managed to get it to stop doing that by adjusting 
the coil tap so that more output capacitance is needed. (That coil tap 
needed to be optimized anyway, as it was never in the ideal spot)

Clearly this amp is not as stable as it should be. I am trying to 
understand what is going on here. I am having trouble grasping why the 
load cap seems to be the critical parameter, with coil tap and tune cap 
having essentially no influence. Any suggestions?
  >>


   Your conditions combine to the point of causing confusion.
Any amplifier that "looses it's load" or becomes underloaded will 
develop an extremely high output Voltage.  This could cause the output 
relay destruction, or the output relay failure could be the source of 
instability.  (from the load loss)
   The loading control needs to be normally set for FIVE percent more 
loading (more capacity) than the maximum output power point to keep the 
output Voltage from damaging components. (or causing an arc) {because of 
unknown variations such as a peak of power line Voltage}
If you change to this one band and the normal setting of the loading 
capacitor starts out, from the previous band, with the amplifier too 
lightly loaded for you to correct in time arcs can be the result. 
Changing things (tap) to prevent this unwanted condition is your first 
step in my opinion.
-- 
   Ron  KA4INM