[AMRadio] 807's triode connection question...

[Bob Bruhns]

Hi John,

All I know is that Tim said it happens.  I would not have
expected it either.

Just a wild guess:  Of course, when the grids go positive,
they begin to draw current.  Maybe the increasing grid
current at first mostly tends to suck up the cathode
current.   The more positive grid voltage attracts more
electrons from the cathode, but maybe with two grids in the
way, the electrons do not get to the plate at low voltages.
We know that in normal operation the inner elements of a
beam power tube such as the 807 are not strongly affected by
plate voltage; maybe the shielding effect of the screen is
the problem here.  So although the cathode current begins to
increase, the increase is more than offset at low levels by
the grid current, resulting overall in a plate current drop.

Whatever the case, I would have to think the dip-then-rise
effect would make the crossover characteristic less than
desirable.

  Bacon, WA3WDR

-----------------
(original message)
Subject: RE: [AMRadio] 807's triode connection question...
   Date: Thu, 4 Jul 2002 09:29:14 -0500
   From: "John E. Coleman" <colemanj at sbcglobal.net>
     To <amradio at mailman.qth.net>

That's very interesting Bacon, I had never done any
experimenting at low level on this circuit before, and to be
honest I can't see how the current could go down unless
there is a small average bias change at the grid caused by
the 20K resistor, but with out any capacitance in the
circuit where or how would it develop.  The control grid
would of course draw current through the 20 K resistor and
there would be a voltage drop across the resistor but the
drop would not be stored without capacitance.

I'm sorry I tend to get wound up in the minutia. I find the
reason for the problem to be more interesting that the fix
for it. HIHI