G3 to plate or cathode?

[Adam Stouffer]

When running a small signal pentode in triode mode should G3 be tied to
the plate or cathode? The Svetlana docs for the EF86 show it connected
to the plate while Amperex shows it connected to the cathode. What is
the difference?


Adam

[Tim Williams]

"Adam Stouffer" > wrote in message
...
> When running a small signal pentode in triode mode should G3 be tied to
> the plate or cathode? The Svetlana docs for the EF86 show it connected
> to the plate while Amperex shows it connected to the cathode. What is
> the difference?

Depends on the lunar position AFAIK. Makes a little difference but not much
IIRC (from putzing with a 6SJ7 on my ghetto curve tracer).

Tim

--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms

[Fabio Berutti]

The usual way of exploiting pentodes as triodes is by connecting G3 to the
cathode, AFAIK, particularly for power tubes. Nevertheless on many
references (the Electronic Handbook among them) I saw that many European
"signal" tubes like the PT49/TS49 are triode-strapped by linking G2 & G3 to
the anode. I suppose it's a matter of the way that specific tube was
engineered, but... well, Mullard was a part of Philips just like Amperex,
and I bet I saw a Mullard D/S relevant to the the EF86 showing the G3 tied
to the plate, just the opposite of what Amperex suggested.

Ciao

Fabio



"Tim Williams" > ha scritto nel messaggio
...
> "Adam Stouffer" > wrote in message
> ...
> > When running a small signal pentode in triode mode should G3 be tied to
> > the plate or cathode? The Svetlana docs for the EF86 show it connected
> > to the plate while Amperex shows it connected to the cathode. What is
> > the difference?
>
> Depends on the lunar position AFAIK. Makes a little difference but not
much
> IIRC (from putzing with a 6SJ7 on my ghetto curve tracer).
>
> Tim
>
> --
> "I've got more trophies than Wayne Gretsky and the Pope combined!"
> - Homer Simpson
> Website @ http://webpages.charter.net/dawill/tmoranwms
>
>

[Patrick Turner]

Fabio Berutti wrote:

> The usual way of exploiting pentodes as triodes is by connecting G3 to the
> cathode, AFAIK, particularly for power tubes. Nevertheless on many
> references (the Electronic Handbook among them) I saw that many European
> "signal" tubes like the PT49/TS49 are triode-strapped by linking G2 & G3 to
> the anode. I suppose it's a matter of the way that specific tube was
> engineered, but... well, Mullard was a part of Philips just like Amperex,
> and I bet I saw a Mullard D/S relevant to the the EF86 showing the G3 tied
> to the plate, just the opposite of what Amperex suggested.
>
> Ciao
>
> Fabio

I have not a great deal in mind to suggest that the G3 be connected always to
either the anode or plate.

But if connected to the anode, there is no secondary emmision suppression
by means of repelling the electrons which bounce off the anode after innitially
hitting it
but not being absorbed by it.
The suppressor acts to reduce screen currents somewhat, and presumably
there is a different amount of screen current in a triode connected pentode
when G3 is either
positive like the anode, and an "extension of the anode" or earthy, like the
cathode.
There would be no screening effect between anode and screen, so its closer to
real triode operation.

If the G3 is connected to ground or cathode, ie, at some potential which repels
electrons,
then the triode connection with G2 connected to anode is more like a pentode
with
screen NFB which happens to equal the anode signal voltage.
The outcome is similar, perhaps someone may like to tell what differences in
thd there may be either way.

In the case of beam tetrodes, the G3 suppressor action is done differently by
beam forming plates
which concentrate the electron streams into two beams each side of the tube.
These plates are connected internally to the cathode, so the beam action
continues with triode connection,
and it cannot be changed.
Nevertheless, KT66, 6L6, 807 all perform excellently when triode connected.
Some multigrid tubes have G3 fixed to the cathode, so there is no choice.

But a tube like the 6DT6 which is a mini 7 pin signal pentode used for
quadrature FM detection in FM receivers
and TV, the G3 has a quite high Gm, compared to the G1 and G2.
I would suggest that the G3 would have little effect though if at an earthy
potenial.
But when G3 is allowed to go positive, it can have a controlling effect in the
tube.
But with triode op this effect is complementary with G2 and the anode.

Patrick Turner.

[BFoelsch]

"Fabio Berutti" > wrote in message
...
> The usual way of exploiting pentodes as triodes is by connecting G3 to the
> cathode, AFAIK, particularly for power tubes.

I think this is primarily due to the fact that many connections have this
connection made internally, giving you no choice.

Really, the only time G3 does anything is when G2 is at a much higher
potential than the plate. In the triode connection, G2 and plate are at the
same potential, making the point pretty much moot.