> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
> To: amps <amps@contesting.com>
> Subject: [AMPS] splatter
To: <amps@contesting.com>
> Date: Mon, 16 Jun 97 09:25:00 +0000
> Tom,
>
> I think that the confusion on 'maximum plate current at zero grid volts'
> occurs because the statement should say ' maximum plate current in the
> RF cycle flows at at zero grid volts.' The RCA Tube Manual TT5 (1962)
> uses the term as Rich put it, and that's what I read it as, but looking
> at it again, I can see that it easy to cause confusion. In AB1, where
> there isn't any grid current drawn, obviously the grid doesn't go
> positive, so the maximum voltage on it is zero. Minimum voltage (which
> is actually the maximum negative voltage excursion) is thus twice the
> bias.
No Peter, that still is not correct.
Minimum grid voltage is just that, minimum grid voltage. No where
does it say it has to be zero volts, and no where does it say the
plate current has to be the maximum rated current.
That confusion is probably why Rich is insisting an AB1 triode has
to use a low mu tube. The mu doesn't have anything to do with the
class of operation, or meeting that criteria.
Let me explain this a bit more.
The definition of classes from all my textbooks, including Reference
Data for Radio Engineers, Electronic Designers Handbook, and
Fundamentals of Vacuum Tube Design is:
A- Anode conduction angle of 360 degrees
AB- Anode conduction angle of less than 360 and more than 180 degrees
B- anode conduction angle of exactly 180 degrees
C- anode conduction angle of less than 180 degrees
An addition designation is sub class 1 for systems that never
draw grid current during a positive grid to cathode voltage
excursion , and sub class 2 for systems that produce grid current
during any portion of the grid waveform.
(Page 18-1 of Reference Data for Radio Engineers, Electron Tube
Circuits, Classification).
Nothing is mentioned about grid voltage OR current in those class
definitions, and nothing is stated about peak or average rated plate
current being reached in the system.
Class C PA's that have NO grid current are practical, and so
virtually ANY power grid tube can be used in any class. Just like
with the voltage regulator "rule", the actual real world is more
complex than a single "this tube works only for this application"
statement.
While it is true MAXIMUM plate current flows at minimum grid
voltage, there is NO definition that maximum plate current has to be
the rated current of the tube.
Nor is there any definition of what the minimum grid voltage is. It
might be zero, it might be positive five volts, or negative 1000
volts. The only requirement is the grid can NOT draw current during
the positive portion of the cycle.
A 3-500Z can be operated class AB1, as can an 8877. I used a 3CPX5000
class AB1 on SSB, and I've used 8877's class AB1. I built a series of
class AB1 triodes using 3CX15000's for industrial use. All three of
these tubes can be operated any class, from A to C. They can be
operated class A(normally A1) or A2, AB1 or 2, B1 or B2, or class
C(2) or C1. Normally the subscript is omitted in some classes,
because we "assume" class A never has grid current, and B and C do,
but that isn't always the case.
Claiming a tube works only one class is absolutely wrong. The class
of operation has as much to do with plate load impedance as it does
grid bias and other parameters.
It might sound "official" to make up our own rules, but it is
incorrect. The classes are defined as listed above.
> The guy who asked about two tone testing didn't get a reply.
Then someone should explain it. Too much time is wasted arguing
non-sense about esoteric and unimportant things, like trying to
incorrectly prove a certain mu tube can be used only for certain
things.
> Some naive questions:
>
> 1. I heard somewhere ( and I can't remember where) that running power
> tube heaters (4CX250's etc) on DC leads to failure because of an
> electrolytic type mechanism. This sounds unlikely to me: anyone got any
> information on this?
I'd call Eimac. They keep records of operating conditions and
failures. Otherwise, you are likely to get an answer like that
about the K2AW rectifiers.
In HF amateur use, the limiting factor is adjustability and operation
over several octaves of operation. The exciters aren't even all that
clean today. I think that rules out practical interest in complex
systems.
73, Tom W8JI
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