Tim Shoppa wrote:
On May 9, 3:03 pm, "N9WOS" wrote:
All of them have an electrode structure that upon close inspection that
looks like/is a beam power tetrode. But the sales pages, and even the
datasheets call it a pentode. The datasheets show it with a suppressor grid,
where it actually has beam forming plates. What gives?.

You have a few people that have determined what they actually are by direct
visual inspection, and actually call them what they are, but that is few and
far between.

Is it just that the curves are close to a pentode, so the manufacturers are
trying to avoid confusion to the average end user?

And as far as people calling them beam power pentode, unless they actually
have three grids, and beam forming plates, then I don't consider it to be a
beam power pentode.

Is it just a big international conspiracy to confuse me?

RCA did the right thing: they always called their tubes "beam power
tubes", completely sidestepping the tetrode vs pentode terminology.

I think RCA was careful to not call them "beam power pentodes" because
some European company held European patents on power pentodes with
similar beam-forming plates. I think RCA was trying really really hard
to not have them identified as pentodes. Licensing and intellectual
property rights to tubes' innards was something that companies were
really protective of - look at the wording on the flap of any tube box
up through the 70's to see some legaleze.

Although my gut feeling is that they are actually pentodes (maybe if
you don't run the beam/supressor grid out to a pin it doesn't count as
an electrode?) because those beam forming plates gotta be a tube
element.

The ARRL (almost) always drew beam power tubes like 6146's and 807's
as tetrodes and classified them as tetrodes in their tube charts. You
will note that 6146's and 807's don't run the beam/suppressor grid out
to its own pin but make it share the cathode pin.

Tim.
There are BOTH beam power pentodes and tetrodes.

RCA used to show the beam power tube with the symbol for the beam
deflection plates, but later they got lazy and just drew a third grid.
If you actually broke the tube open you WOULD see a 'cage' instead of a
third grid.

Most beam power tubes used as AF power amps were "Pentodes", assuming
you count the beam deflection 'cage' as one of the elements. Such tubes
would have 5 elements making them Pentodes. It wouldn't matter if this
cage was connected to the cathode or to it's own pin, it's still a
pentode.

Other special beam power tubes actually did NOT have a 5th element.
They had the turns of the first two grids aligned, and the spacing
between the grids and the screen grid and the plate were set to minimize
secondary emission. In addition, the plates had curved pockets in them
designed to enhance the capture of electrons to prevent any secondary
emission from ever reaching the screen grid. Tubes such as the 4-400
and 4cx250 were examples of such beam power tetrodes.