Running a tube at other than the specified tolerances shown
on the published curves by the manufacturer can and will
Eimac is long QRT on 3-500Z tubes, but they had a rating
system called IVS duty. Contrary to what Rich implies
everything Ameritron sold was run past the people
responsible for the tubes. Eimac had no problem at all with
the manual telling people they could TUNE to 550mA, and then
back drive off until 400mA is reached. They never once
rejected a tube for any sort of operational failure due to
operating instructions or equipment design.
The only person I'm aware of who has a lifelong problem with
the manual suggesting momentairily exceeding 400mA while
tuning was Rich. As far as I know he doesn't work for Eimac
and they didn't call on him for technical or warranty
Once the line starts to deviate, that gets it into the
non-linear regions, or is the way I was taught to read
Unfortunately you were taught incorrectly.
It is impossible to look at one constant current line and
predict transfer characteristics or odd-order IM distortion
levels. If the constant current characteristic curves at one
current value set the distortion level why would negative
feedback reduce distortion? How could the output be linear
when the tube is cut off for more than 140 degrees of the RF
It is a transfer function problem. The level of IM
distortion products is a function of the slope and shape of
the gain change as drive level is varied.
If your read Care and Feeding of Power Grid Tubes (published
by Eimac) all of this information is there. They even say
they intentionally design to create an even-order curvature
slope to avoid creating odd-order IM!
I suggest you look at page 96 section 4.3.2 of Care and
Feeding. It's all there in black and white and worth
reading. It will help sort unfounded opinions from the true
facts. The ideal transfer characteristics for power
amplifier conduction angles less than 360 degrees and more
than 180 degrees (class AB) aren't even a straight line!
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