Vacuum Tube "grids"

Discussion in 'Electronic Design' started by Jon Slaughter, Oct 22, 2008.

1. Jon SlaughterGuest

Anyone know if essentially the design of the grids(gird, screen, suppressor,
etc..) are virtually identical? i.e., could some application potentially use
the suppression grid for the normal grid and the circuit still work if it is
not too critical?

I have some pentodes I'd like to play around with for audio amplification
and want to try some ideas out such as tieing all the grids together. As far
as I can tell the construction for the different grids are electrically
equivalent? Maybe different voltage/current ratings and obviously different
geometrically but seems beyond that one could play around with it?

2. MooseFETGuest

The positions of the grids matters a lot. The suppressor grid is very
near the anode. It can't do much to control the current flow. You
need to make sure that it always stays more negative than the plate.
It usually is connected to the cathode to do this. If it comes out on
its own pin, you can run it at other voltages and capacitively couple
it to the plate.

The screen grid is sort of in the middle. Its voltage will control
the current somewhat. It normally is allowed to pass about 10% of the
plates current. You can run it at a lower voltage and a lower current
at the cost of some gain. The screen grid is normally at the same AC
voltage as the cathode so that it protects the control grid area from
the AC on the plate. You can drive it with a signal to modulate the
gain of the stage.

Contrary to the popular belief, it is not the grid which controls the
current from the cathode, it is the electric field in volts-per-metre
which the grid is partly responsible for creating.

This may seem like nit-picking, but you can deduce from the correct
definition that the closer the grid is to the cathode, the steeper is
the voltage gradient which it can create for a given signal voltage
swing (thus the greater the amplification of the valve).

The second and third grids of a pentode are a long way away from the
cathode and give very little amplification, they also have widely spaced
turns which give poor control of the current. They are there for a
different purpose and are custom-designed for that purpose only.

You could spend the rest of your life learning about valves and still
only know a fraction of what has been discovered about them - but a good
place to start for a proper understanding of the basics would be:

"Radio Engineering" F.E. Terman (McGraw-Hill 1932 or later editions)

4. Jon SlaughterGuest

Ok, thats all I needed to know. That tells me that the grids are
functionally identical. Doesn't matter the distance or anything because that
only modifies the output which I'm not worried about(well, in fact that's
why I want to know so I can actually modify the output).

If I can tie them all together that means I can play around with them and
treat them the same. i.e., treat each one as a control grid using the same
circuit topology and get different effects. If one grid, say, was
functionally different than another I couldn't do that. Having a different
geometry or physically being further away isn't a functional difference.
Two random resistors are physically and geometrically different yet are
functionally the same. (This is not to say that changing the resistance
value won't have a signficant effect on the circuit)

What you have told me is what I assumed but needed verification. (although
it's similar to Jan's "No" answer but since I have done my own reading and
concluded that you have only reinforced it. Jan's "No" contradicts what I
have read and he doesn't support his claim at all)

I know how the tube grids work in standard applications but I am using a
pentode for audio freq amplification which normally one uses a triode. I
could just disconnect the screen and suppressor but I was wondering what I
could do to take advantage of them.

The first idea would seem to be to tie them all together like you mentioned
as it would act like a "super grid". I do not know how it sounds but thats
the whole point. There are other possibilities too such as using the screen
for an attenuator and the supressor as a mirrored control. Again, I do not
know how well it would work or sound but the point is that I'd like to try
it but I needed to know if each grid was functionally equivalent.

5. Tim ShoppaGuest

Suppressor grids are sometimes used for AGC or for keying/gating in
pulse circuitry.

It is not too uncommon to use tetrodes or pentodes in "triode mode" by
tying the grid and screen together. That doesn't mean the screen grid
is the same as the control grid, in fact in beam power tubes the
screen grid is sometimes better identified as a "beam forming plate".
A good chunk of tube design for many years was heavily influenced on
whether you wanted to pay patents to company 1 or company 2, and part
of the distinction was whether the screen was a screen or a beam-
forming-plate, and sometimes the words have their definitions twisted
to mean something you wouldn't think they mean in an attempt to use
notation to sneak around patent issues.

I have seen some applications use beam power tubes with nothing - nada
- attached to the anode plate. They use the screen or beam forming
plate (which itself can have a respectable dissipation) as the anode.

There are some interesting heptode/mixer/computer tubes with multiple
grids for specialized purposes. Look up for example the 6BE6 and its
many variants for very common examples.

Tim N3QE

6. Richard HenryGuest

If they were "functionally identical", you could swap the signals on
the pins without any bad results. Trust me, that doesn't work.

7. kevin93Guest

....

A more common way is to tie screen to anode.

The suppressor grid is often not accessible and is tied to cathode
internally.

kevin

8. Rich GriseGuest

There are hundreds, maybe thousands, of "specific references", some of
which have been pointed out to you, but you don't like reading stuff from
people who are smarter than you.

And I think it's very rude to get all snitty with the people whom you're

Good Luck!
Rich

9. Rich GriseGuest

Everybody's been trying to tell you that they are NOT "functionally
identical". They have very specific purposes. The control grid is
generally the signal input. The screen grid reduces the interelectrode
capacitance between the control grid and plate, to prevent parasitic
oscillations. The suppressor grid is there to repel secondary emission
from the plate, so that it doesn't fall back into the screen, wasting

You can do anything you want, but it's a good idea to learn some
fundamentals so you have at least a shadow of an idea what you're trying
to accomplish.

Good Luck!
Rich

10. Rich GriseGuest

OOps! The screen grid is NEVER a "beam forming plate". The beam forming
plates replace the suppressor grid.

Hope This Helps!
Rich

11. EeyoreGuest

NO.

Their spacing is very important.

Graham

12. EeyoreGuest

Then you're reading the wrong books. Even the alignment of the various grid
wires can be inportant.

Graham

13. EeyoreGuest

Slaughter is what should happen to him. Aside from Jamie, he must be the
dumbest here.

Graham

14. EeyoreGuest

WTH have valves got to do with quality audio reproduction ?

Surely you mean 'audiophool' ?

Graham

15. EeyoreGuest

Utter BOLLOCKS.

16. EeyoreGuest

Your problen is that you're STUPID.

17. Jon SlaughterGuest

You guys don't get it. I don't care about the output. It is for an audio
project and and I am looking for different effects I can get which want
those different possibilities. I don't know how many times I have to say it.
I'm only concerned with the circuit topology used to drive the grids as
somehow having to be different or I'll end up fucking up the tube. But now
that I think about it it's quite obvious that the circuit topology can be
the same.

E = -phi_n+1 - sum(grad(phi_k)) - phi_0

where phi_k is the potential of the kth grid, phi_0 is the potential of the
cathode, and phi_n+1 is the potential of the anode.

phi_k might depend on material, temperature, geometry, etc... but since the
ultimate result is the potential(which is basically what I was trying to
ask) it doesn't matter. I just have to put a voltage across it and only make
sure I don't go over the current rating. That means that I'll get some
effect. THE TUBE MIGHT NOT OUTPUT ANYTHING but that is of no concern to me
as I mentioned I will be playing with "ideas". You guys seem to think I'm
going to randomly hook up the grids in some configuration for some
application then run it into mass production. You don't get that I am
simply going to play around with some idea(And it shouldn't matter) and see
what kinda outputs I get.

What is important too me is that I don't blow up the tube because one of hte
grids, say, happen to be created in a way that resonates at some frequency
and when I hook up my circuitry too it I end up hiting that resonance and it
creates an atomic explosion. I've mentioned many times before I'm not
interested in the signal output but the not ruining the components.

so, for you what works it's totally different from me. If I hook up
something to the grid and I don't ruin the tubes then it works... regardless
of the output. I guess this is a very difficult concept for people in this
group to comprehend but I guess then again they never "play around" with
circuit ideas and just copy shit out the book.

18. EeyoreGuest

Slaughter has an aversion to that.

Graham

20. EeyoreGuest

No we get it EXACTLY.

You're STUPID and ineducable.

Graham