"Chris Hornbeck" wrote in message
...
: On Thu, 12 Oct 2006 14:51:57 +0200, "Ruud Broens"
: wrote:
:
: : This is actually the approximation for a diode, but the
: : form for a triode is similar.
:
: actually, it's more general than that, it is applicable to all tubes
: under space charge conditions, subject to modification when there are
: more influences (such as grids). in a penthode, for instance, the diode
: curve comes back for both G1 and G2 voltage vs Ia.
:
: other interesting stuff snipped for bandwidth
:
: Another thing folks who take Terman's introductory college
: texts as Gospel is that Child actually is more interesting
: in the original (_Phys. Rev._, 32, p.498, 1911 [!} ).
:
: Child includes a term for distance between electrodes that
: we always just roll into k because it's outside our control.
: But he also specifies certain assumptions:
:
: 1) that the electrodes are parallel infinite planes
:
: 2) that the plate current is limited only by space charge
: (not by emission saturation)
:
: 3) that there is no gas within the tube (or at least not
: enough to cause retardation of the electrons due to collisions
: with gas molecules)
:
: 4) that equipotential electrodes are used
:
: 5) that the electrons are emitted with zero velocity
:
: 6) that the contact potential at the plate is negigible.
:
he, he forgot to include secundary emission fx ;-)
: Since most of these assumptions are never wholly realized
: in actual tubes, the exponent (*always* given without
: caveats) generally differs somewhat from 3/2, being
: somewhere between 1 and 5/2.
:
larger than ^3/2 you'll have to give me an example of that ;-)
but sure, actual electron emission is more like at some 0.5V 'speed',
the space charge is non uniform, trace gasses will be around,
DH will not have equipotential cathode when DC heated,
etc. etc. - the imperfections of the physical implementation world
: There's a very good description in Eastman _Fundamentals
: of Vacuum Tubes_ Third Edition, edited by Terman, 1949,
: probably still pretty available, from his famous MIT series.
:
:
: My thoughts are that interesting real-triode deviations are
: largely geometric, ei. from the first assumption above, and
: that a realistic triode model must begin in a geometric
: rather than an algebraic form.
:
: Much thanks, as always,
:
: Chris Hornbeck
with computing power now available, it should be possible
to model just about everything to ~perfection~ when the
physical sizes, shapes, chemical compositions, heat delivered
are known accurately. still, that 's a damn lot of work for
-somewhat better-
(although used -designing new tubes- would give it a whole
different meaning
you got me looking at some triode curves, first one
6BX7, 25V 10 mA, 100V 80 mA perfect ^3/2
then
the Pa 125W, 60W heater massive power triode
GM70, 100V 34 mA, 400V 204 mA
200V 54 mA, 800V 330 mA about ^1.3
btw, running it at 1400V, 80 mA 112W Pa,
will just do 44W SE in a 14K reflected OT,
40 % efficiency (for Stewart and Graham
ECC83 Mazda gave ^1.12 to ^0.97 from 10 to 160V
in general, low current will spoil the "space current
dominates all" approximation quite a bit.
cheers,
Rudy