[Patrick Turner]

John Byrns wrote:

In article ,
"Alex" wrote:

"Patrick Turner" wrote in message
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Now the screen should be considered for what it really is, ie, an
additional control grid, although one operating at the same potential as
Ea. What does it really do? Well, the data pages in old books are almost
dead useless in describing what happens with the screen.

Consider the EL34 set up and idling away with cathode bias and about
50mA for Ia, and Ea = 375V, and a fixed Eg2 screen voltage of +375V.
Let us load the anode with the high primary inductance only which at
1kHz is a load so high its high enough to be considered a CCS. Let us
connect the g1 control grid to 0V. Now consider we apply an ac 1kHz
signal voltage to the g2 via a transformer fed by low impedance,
and thus modulate the anode current similarly to how g1 does it, by
means of changing the control voltage.

That is interesting, Patrick -- to use a pentode as a triode with the g1
connected to the cathode and with g2 as a control grid. Such mode will
always involve grid current and will require quite a large drive to get a
decent peak anode current at low anode voltage.

However, line deflection pentodes can be chosen. Those can produce enormous
plate currents from a relatively small g2 voltages, about 50..100V (Vg1=0).
Therefore, to drive such a "triode" by g2 one will need a cathode follower
on the basis of a smaller tube, capable of delivering peak screen current
(when anode is going low), I guess up to 30mA. For that driver a 6AQ5 can be
used (in a triode connection) or even 6N6P triode.

I am wondering, has anyone tried to use pentodes as screen grid controlled
triodes with g1 permanently connected to cathode?

Why would one want to do this vs. simply driving the control grid as
intended? The only reason I can see is higher potential power output
due to increased peak plate current, assuming the tube can tolerate
abnormally high peak screen voltages while the plate voltage is low.

Indeed John, why?.

But I raised the issue in my OP to give some idea about how the screen
g2 works.

Patrick Turner.

--
Regards,

John Byrns

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