My thanks to all who have responded.
Yesterday I hooked the amp up to a dummy load. If I tune quickly, the
tuning on 160 m behaves normally, even at moderate power-600 watts out. If
I hold the key down, the grid current begins to go up from its initial value
of 20 ma. As the grid current is going up, neither the plate current nor
power out shows any change whatsoever. I unkeyed the amp when the grid
current reached 40 ma.
After the amp cooled for a few minutes, I keyed it again-had the same
output, and plate and grid currents, as I had when I keyed the amp the first
time. This time, when the grid current started to increase, I increased the
loading, power output dropped, plate current didn't change much, if any, and
grid current came back to about 20 ma.
Inspection of the tank circuits, plate choke, and B+ bypass capacitor showed
no visible signs of distress. After a 3 minute period of 300 watts output I
could not detect any heating in any component in the plate tank compartment
. This inspection was done in an appropriate manner.
This problem shows up only when the amp has been producing power for awhile,
the length of time depending on how high the power output. Something
appears to be heating up-but there is no change in the power output.
It would be easy to say the trouble is in the grid current measuring circuit
except for the fact that the problem occurs only on 160 m.
I am missing something.
73 Paul W5DM
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