>> 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, ...
Not so in the Rauchian World, Peter.
>> 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.
I somewhat agree. Minimum grid voltage is not the peak negative value.
Minimum grid voltage is zero. However maximum anode current occurs at 0v
(in Class AB1). If max. anode current occurred at pos. 1v on the grid,
grid current would flow, and that's not Class AB1.
>Minimum grid voltage is just that, minimum grid voltage. No where
>does it say it has to be zero volts, ...
True, the grid V could be neg. 10v at the anode current peak---but if
another 10v of grid drive was available, increased peak anode current, as
well as increased RF output could be produced.
>...and no where does it say the
>plate current has to be the maximum rated current.
There is no rule that says people are prohibited from designing
amplifiers that do not utilize the tube's emissive capability.
>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.
According to the characteristic curves, Mu has much to do with selecting
the optimal class of operation. For those interested in producing RF
output, high Mu is best for cathode driven. Low Mu is best for grid
driven. However, there is a serious trade-off with the latter. Although
low Mu triodes produce the needed high anode currents with 0v on their
grids, low Mu triodes require high RF grid potentials.
>
>Let me explain this a bit more.
...snip...
>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.
.........nor should it be.
>Class C PA's that have NO grid current are practical,
....for those who are into QRP.
>and so
>virtually ANY power grid tube can be used in any class.
true, provided that output power is not a concern.
...snip...
>While it is true MAXIMUM plate current flows at minimum grid
>voltage,
Agreed.
>there is NO definition that maximum plate current has to be
>the rated current of the tube.
>
No, but when most folks design an amplifier, they try to utilize the peak
emissive capability of the tube.
For example, an 8877 (Mu=200) could be used to build a Class AB1
amplifier. However, according to the curves, with 0v peak on the grid,
the max. peak anode current would be approx. 0.03A instead of the usual
3A. Power out would be around 30w with an anode supply of 4kV. . A
seemingly more logical tube choice would be a tetrode like the 4CX1500A,
which can draw 3A peak with 0v on the grid. The only kind of triodes
that draw substantial anode current with 0v on the grid are those with a
Mu of 5 or less.
>Nor is there any definition of what the minimum grid voltage is. It
>might be zero, it might be positive five volts,
Positive 5 volts results in grid current.
>or negative 1000
>volts.
Algebraicly speaking, neg. 1000 is less than zero.
Electronically speaking, this ain't so.
>The only requirement is the grid can NOT draw current during
>the positive portion of the cycle.
Is this the same guy who said the grid potential could be positive 5V?
>A 3-500Z can be operated class AB1,
yes indeed. The average anode current would be about 0.04A under maximum
signal conditions. Power output would be roughly 100w with a 4kV anode
supply.
> as can an 8877
This is 'EXACTLY correct'. The max. output would be about 30w.
>. I used a 3CPX5000
>class AB1 on SSB,
I find it a bit curious that Mr. Rauch omitted the Mu designator suffix.
I'm guessing that he surmized that if the Mu designator had been
disclosed, someone just might know which Eimac constant current curves
sheet to start nosing around in. Hmmmm.
>and I've used 8877's class AB1. I built a series of
>class AB1 triodes using 3CX15000's for industrial use.
Building 30w 8877 amplifiers is not something I find particularily
impressive. However, building triodes from 3CX15,000 parts sounds pretty
awesome to me. .
...snip...
>Claiming a tube works only one class is absolutely wrong.
However, can low Mu triodes be interchanged with high Mu triodes in the
same application by merely adjusting bias, et cetera?
Mr. Rauch has not provided even one of the tube types used in the
"dozens" of Class AB1 triode amplifiers he alleges to have seen.
-someone named Rich---
R. L. Measures, 805-386-3734, AG6K
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