I have a Command Technologies HF-1250 amplifier (single 3CX800A7 tube). The
amplifier does not have grid over current trip out protection. On all bands
except 160 m It is working fine (700 to 800 watts output, grid current
about 20 ma) with tuning and load capacitor settings very close to the
settings of those on the factory tuning chart specific to this amplifier.
On 160 m, starting with the factory settings, tuning is normal at very low
drive. The tuning and loafing capacitors are both close to the factory
settings. I increase drive slowly (I do not have experience with such a
high gain and fragile grid tube as the 3CX800A7). As I get to 250-300 watts
output the fun starts. If I hold the key down for any time at all (either
into an antenna on a dead band or a dummy load-it makes no difference) the
grid current begins to increase and increases more rapidly with time. This
increase can be stopped by either decreasing the drive or increasing the
loading. By increasing the loading drastically I can get to 500 watts out
with a plate current about 600 ma and grid current 20 ma. At this output the
tuning capacitor setting is very close to the factory tuning sheet number,
but the loading capacitance is only about 60% of the factory setting. If I
start to decrease the loading, the output starts to rise rapidly, but the
grid current starts rising even more rapidly. At no time is the SWR higher
than 1.3:1 (using an LP100A).
Could someone please tell me what is happening?
73 Paul W5DM
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