While not claiming to know the answer to Tom's question, I would like to add
some information using a similar tetrode.
The 4CX1000 was originally not designed for linear amplifier service. Eimac
specifies the 3rd order distortion as -23 db and 5th order distortion is
specified as -26 db. The Collins 30S-1 used that tube, but used a cathode
drive configuration (which has inherent feedback) and applied additional
feedback to arrive at a clean amplifier.
Without feedback, though, a 4CX1000 with a resistive grid and perfectly
stabilized voltages ought to yield no better than a -23 db 3rd order
distortion.
The 4CX1500 tube was designed specifically for linear service. Although hte
physical construction looks similar, Eimac specifies that the performance
is -44 db 3rd order distortion - a whopping 21 db improvement in distortion
figures. I understand the main difference between the tubes is the grid
construction.
The Russian tetrode that is used in the Ten-Tec amplifier tested by QST was
not designed for linear service. The main quality is that it is
considerably less expensive than a 4CX1500 tube. The distortion levels may
indicate that construction is similar to the 4CX1000.
Colin K7FM
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