I agree with the assumption that building large pentodes is more
difficult. Alignment of wire grids for G1 and screen G2 is difficult
with the older technology, to get consistent results from batch to
batch. Add a suppressor grid, for which alignment for optimal
electron trajectory is just as important, and you've got quite the
manufacturing trick. However, Eimac did produce or at least advertise
the 4E27/5-125, 5-500A, 5CX1500A/B, 5CX2500A, 5CX3000A, 8576, 290 and
8295A power pentodes. None of them were large enough to require
handles, although they went as high as 5 kW dissipation. Since the
suppressor's purpose was to reduce secondary electrons at the screen,
it allowed plate V swing to drop as low or lower than screen voltage
without having excess dissipation, and get more output out of a tube.
At least this was stated in "Care and Feeding".
With development of pyrolytic graphite grids in modern tubes with
handles, the premise is gone, since the screen can dissipate higher
power without warping. The work function of the material is quite
different, so that secondaries are not such a problem. There is
little need for a third grid, and the manufacturing is simplier.
Pyrolytic graphite makes very uniform grids, and alignment is
locked-in during the placement of the entire grid 'cup' in the
structure.
>
>>More seriously, is there a reason why there were relatively few pentode high
>>power tubes, as opposed to tetrodes?
>
>Ä Building something large with a suppressor grid is apparently
>difficult. Pentode sockets are pretty complex. There are seemingly no
>pentodes with handles.
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