A little tube history here:
RCA designed and manufactured beam power tetrodes such as the 6L6, 807,
and 829B to increase power output, allowing the plate voltage swing to
go lower without a large increase in screen current during the minimum.
Without using a suppressor grid (as in a pentode), the beam was
'squeezed' (converged by focusing) where it passed through the gaps
between screen grid wires, and so intercept was reduced. In addition, a
space charge region was established out front of the screen between it
and plate, to minimize the return of secondary electrons from the plate.
This was called a virtual cathode, and it returned secondaries to the
plate. A beam power tube would have lower screen current than a pentode.
They did this by adjusting the pitch between grid wires, the alignment
and spacing between grids and the other two electrodes, and the voltages
applied.
Beam power tubes were a revolutionary step from earlier tetrodes with
electrons from cathode that sprayed out through the grids, with no
particular means of alignment. In 1955, RCA extended this concept to
high power with tubes like the 6448 and later the 4616 tetrodes. In
these, unit tetrodes were made with a filament rod or bar, enclosed on
three sides by a copper frame. The edges of this U-shaped frame were the
control grid. Carefully aligned was screen grid on the outer side
following the same principles of the smaller glass tubes. Similarly, in
triodes RCA built electron-optics using beam forming walls around each
cathode bar, and shielded the control grid wires or bars from direct
electrons. Carrying it further, they build shielded-tee structures that
overlapped the grid bar, and reduced capacitance from it to plate. The
shield acted like a zero-voltage screen. Big tubes like the 6949 made
this way could produce 1/2 megawatt of CW power above HF.
RCA also developed radial cathodes that had alternating dead and active
emitter bands, to emit beams in strips. By placing the grid wires in the
regions without beam, it would greatly reduce grid current and emission
due to bombardment from the cathode beam.
So what did Eimac do to improve their product? They developed a line of
focused triodes and tetrodes. Using similar striped cathodes like RCA,
they focused beams to avoid interception, This not only lowered grid
current but also reduced IMD. Examples include the 4CX600J and 4CW800B
beam tetrodes, and a series of focused triodes including the 8874,
3CX800A7 and U7, 3CX1500A7/8877, 3CX4000U7, 8938, 3CX1500U7, 8962, 8963,
3CX5000A7 and U7, and so forth up to the 3CX15,000B7. One of the big
problems with oxide cathode tubes was the deposition of barium on the
close-by grid, leading to loss of beam control as the grid became an
emitter. Coatings were applied to the grids, permitting higher
dissipation without excess grid emission. Sputtered carbon-titanium
coatings called "Pyrogrid" were one example. Gold was a more expensive way.
Both companies, and I'm sure others as well, followed similar lines of
development to produce excellent tubes that allowed power levels to
rapidly increase to hundreds of kilowatts without grid destruction.
73
John K5PRO
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