I'm looking at a single-ended output circuit that uses a 7591.

The anode of the 7591 gets its juice through a SE tranny connected to 280
volts. The resistance of the tranny is 150 ohms and the voltage at the
plate is 265.

The screen grid is connected directly to the 280V source. So in operation
the screen is at a higher DC potential.

Does this force the 7591 into triode mode, essentially? The rated output is
5W RMS. I'm always puzzled with the plate is at a lower potential . . .

The screen grid is connected directly to the 280V source. So in operation
the screen is at a higher DC potential.

Does this force the 7591 into triode mode, essentially? The rated output is
5W RMS. I'm always puzzled with the plate is at a lower potential . . .


I wouldn't call that "triode mode". For triode mode the screen
needs to be tied to the plate, such that the tube acquires that
triode "feedback" effect. Where the plate voltage directly effects
the gain on signals on the control grid.

in article t, robert casey
at wrote on 4/29/05 4:32 PM:



The screen grid is connected directly to the 280V source. So in operation
the screen is at a higher DC potential.

Does this force the 7591 into triode mode, essentially? The rated output is
5W RMS. I'm always puzzled with the plate is at a lower potential . . .


I wouldn't call that "triode mode". For triode mode the screen
needs to be tied to the plate, such that the tube acquires that
triode "feedback" effect. Where the plate voltage directly effects
the gain on signals on the control grid.


I'm still unclear on it.

In typical "triode" mode, a pentode screen grid is tied to the plate (at B+)
through a 100 ohm resistor.

In this case, the plate connects to the screen grid through a 150 ohm
winding, but the screen grid is at B+.

Jon Yaeger wrote:

I'm looking at a single-ended output circuit that uses a 7591.

The anode of the 7591 gets its juice through a SE tranny connected to 280
volts. The resistance of the tranny is 150 ohms and the voltage at the
plate is 265.

The screen grid is connected directly to the 280V source. So in operation
the screen is at a higher DC potential.

Does this force the 7591 into triode mode, essentially? The rated output is
5W RMS. I'm always puzzled with the plate is at a lower potential . . .

The 7591 is in pentode mode.
The slightly higher fixed screen voltage is only of concern
if that is ok for the tube and within screen dissipation ratings.

Triode operation is only possible with the screen tied to the anode
so signal at the screen is the same as at the anode.

Patrick Turner.

Jon Yaeger wrote:

in article t, robert casey
at wrote on 4/29/05 4:32 PM:



The screen grid is connected directly to the 280V source. So in operation
the screen is at a higher DC potential.

Does this force the 7591 into triode mode, essentially? The rated output is
5W RMS. I'm always puzzled with the plate is at a lower potential . . .


I wouldn't call that "triode mode". For triode mode the screen
needs to be tied to the plate, such that the tube acquires that
triode "feedback" effect. Where the plate voltage directly effects
the gain on signals on the control grid.

I'm still unclear on it.

In typical "triode" mode, a pentode screen grid is tied to the plate (at B+)
through a 100 ohm resistor.

The 100 ohms is to prevent oscillation and to limit screen current a little.
The screen signal will still be identical to the anode signal.



In this case, the plate connects to the screen grid through a 150 ohm
winding, but the screen grid is at B+.

That's pentode mode.
Regardless of the supply voltage level, when the screen is kept at the same
**signal** potential as the cathode, its pentode mode.

A screen is a sort of secondary control grid, and may be used as such,
and intriode mode it **is** used as such to
transfer the signal of the anode directly back into to the tube to allow
the electric field to work on the electron stream.
When G2 has a fixed voltage, it has no dynamic effect on the electron stream
which flows in the presence of the fixed screen voltage.
Electron flow is controlled only by the G1 control grid.
The effect on the electron stream by the anode anode voltage is **screened** off
by the screen, which acts almost as well as if it were a tube of sheet metal.
But its not a piece of sheet metal, its a helical coil of wire like G1, and about
90%
of the electrons heading towards it due to the G2 fixed positive charge miss the
wires,
and head out into the gap between the screen and anode and are then attracted to
the
nest best positive thing, the anode, which is either more or less positive than the
screen,
but at least positive.
Quite a few electrons bounce off the anode, or displace other electrons
which try to then head back to the screen, so a **third** grid with a coarser
wire winding and held at cathode or a low voltage potential is
is installed betweenn the screen and anode which deflects the bouncing *secondary*
emission electrons back to the plate.
This 3rd grid is called the supressor grid, G3.

We can allow the anode voltage to not be screened off from the rest of the tube
by connecting the screen to the anode, so it becomes virtually part of the anode.

This results in a dramatic lowering of dynamic anode resistance and a
very dramatic fall in the µ of the pentode, and in the case of EL84,
Ra falls from 38k to 2k, and µ falls from 400 to 20.

You see, when the *triode* anode voltage falls by 10v because of increasing Ia into
some load,
the G1 voltage may have risen +2V.

The anode drop in voltage tends to reduce the electron flow to itself.

With a fixed screen voltage, we could get the same -10v anode voltage change,
but perhaps only need +0.1 G1 voltage change to get it, since the
µ ( or amplification factor ) is so much higher because the anode voltage change
has not been allowed to cause its own Ia reduction.

So the electron flow in a *triode* is the result of **TWO** applied
electric fields, or voltages, applied virtually simultaneously,
and the G1 and anode voltages both effect the signal outcome.
It is an electrostatic shunt FB loop.
G1 voltage is controlled by the input voltage and any thd appearing at the anode
have an effect to reduce the distortion and that's why triodes have less distortion

than pentodes especially when RL is high.

From your initial question, your understanding of the basic operation
of a tube must be limited.
You should consider to get the basics clearly understood
to make schematic analysis easier, and allow you to appreciate the
pros and cons of a schematic in a glance.

A tube is just a device to get a small voltage change to make a
bigger voltage change and perhaps considerable current change elsewhere,
and the mechanics are quite simple, really, once understood.

Patrick Turner.