🚀 go-pugleaf

<36902872.9500D4B6@ieee.org>, Roy McCammon <rbmccammon@ieee.org> wrote:
>Doug wrote:
>
>> and valves can be thought of VOLTAGE CONTROLLING DEVICES.
>
>thats interesting.  I don't know anything about valves,
>but I always thought of them as voltage controlled
>current output devices characterized by gm.

That's true for pentodes, which have a high 'anode/plate resistance' ra,
like a BJT or FET. For pentodes, the traditional approach is indeed
VCCS, with stage gains expressed as voltage gain gm{Ra.ra/(Ra+ra)}, Ra
being the load resistor.

Triodes are a different matter. The ra is lowish, and the optimum load
resistance for best linearity is also in the order of ra. So they are
neither fish nor fowl. They were usually treated as VCVS, with
'amplification factor' [mu] = dVak/dVgk at constant Ia, and stage
voltage gain =[mu].Ra/(Ra+ra).
>
>Of course intuition is not always right.  It could be
>that the typical circuit to get the things working and
>not melting and capacitive feedback from plate to grid
>might very well mean that the devices have a fairly
>narrow voltage gain.  Any of that true?

For pentodes, yes, interpreting 'fairly narrow' fairly broadly! gm and
ra are more or less fixed by the geometry of the electrode structure,
provided the cathode temperature is stable. I don't think Cag has much
to do with it.

Langford-Smith (naturally, as basically a valve applications engineer)
has a great deal about this in the early chapters of 'Radio(tron)
Designer's Handbook', which has recently been re-printed (1953 edition).
Permissible tolerances on valve characteristics are said to be in the US
JAN-1A specification, which is presumably available from some library
archives.
-- 
Regards, John Woodgate, Phone +44 (0)1268 747839 Fax +44 (0)1268 777124. 
OOO - Own Opinions Only. You can fool all of the people some of the time, but 
you can't please some of the people any of the time.