21 Jul
2010
21 Jul
'10
7:47 p.m.
That's true, but only in those cases where the
shell (or any other
element) is brought out to a base pin. The 1934 standard says that
an element has to be "useful" and have a connection to be counted.
Right,..
It's getting to the point where all you can do is look in the valve data
books....
I still much prefer the Philips coding scheme. it tells you a lot of
useful information, and exceptions are not common (put it this way, I've
never seen a valve which doesn't fit the scheme correctly).
So, a 6L6 has heater, cathode, control grid, screen grid, plate and
shell all connected to base pins. The beam-forming electrodes aren't
counted, so, for instance, the 6F5 triode has exactly one fewer
"useful" electrode, even though it has two fewer elements.
It appears that sometimes you count the tap on a heater (e.g. 35Z3
against 35Z4), sometimes you don't (12AX7 has 2 cathodes, 2 grids, 2
anodes and a tapped heater).
The other requirement is that the tube must have been introduced
initially in the metal shell form (I can't think of any that were
glass, then metal, but there may be some.)
My guess would be tuning indciators (magic eyes, or whatever you call
them). A glss window is essential for that type of device, and I would
assume many exist only in glass envelopes.
An "S" as the first part of a middle two-letter pair signifies a
single-ended tube, which was not universally followed. Rectifiers
There are many single-neded valves (no top cap) without the 'S'.
*usually* have a high-middle letter (e.g. 5U4, 5Z3),
but sadly, there
are many exceptions.
The most reliable part of the number is the suffix. e.g. G = glass,
GT = short glass, GA = improved glass version, GY = micanol base,
etc.
Which is not normally the most important piece of information about a
valve :-).
[I thought 'GT' was 'Glass, Tubular', meaning a straight-sided envelope]
In 1942, the RMA introduced a scheme for special-purpose and
transmitting tubes, that was called the "1A21" system. The first
number represents the power rating, the second, the tube type (e.g.
diode, triode, etc.), the second and third numbers are assigned in
the order of introduction, starting with 21. So the gas thyratron
2D21 tells us nothing more than it's a tetrode rated for 10 watts or
less.
Ah, so trhat's where that number comes from....
We call it an EN91 (6.3V heater, Thyratron, B7G base).
-tony