[ARC5] Smart People: Triode Connecting Pentodes

[Dennis Monticelli]

Dave,

I have no experience in wiring tetrodes or pentodes to make triodes but
connecting plate to screen seems like a recipe for excessive screen
dissipation.  With each electrode having the same accelerating field and
the screen positioned much closer to the grid than the plate, guess who is
going to get first crack at the electrons?  Of course many will get past
the screen wire but I'll bet not all of those that do will eventually make
it to the plate because there is no net accelerating field between the
screen and plate.  My guess is that most of those passing electrons will
fall back of the screen. The screen failure will not be due to voltage over
max but excess dissipation.

Dennis AE6C


On Tue, Nov 12, 2013 at 12:08 PM, David Stinson <arc5 at ix.netcom.com> wrote:

> Home for lunch with my beautiful bride and a hot cup of Joe...ahhhh...
>
> Working on a project where I must triode-connect a 7984 pentode.  I have
> some well-used ones here which tested "good" that I'm trying in these
> "destruction" tests.
> I've been told that the screen should be directly connected to the
> plate through a parasitic suppressor (100 ohms) and that the
> screen area is effectively added to the plate.
> These are running at 500 volts on the plate at about 100 mils,
> which is well within the tube *plate* ratings.
> However, the screen is max rated at 250 volts 12 mA, or 3W max input.  My
> source says this isn't an issue when the screen is connected to the plate,
> but I'm not so sure.
> The 7984s work OK for the first few minutes, then start folding on me.
> I'm betting the screens are going.   Who are the experts in this, and what
> are your points of view?
>
> Back to work!
> 73 Dave AB5S
>
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