[ARC5] Smart People: Triode Connecting Pentodes

[Fuqua, Bill L]

  If the screen is the same potential as the plate the electrons momentum or kinetic energy will make them continue to the plate.
The plate may emit some secondary electrons that may get back to the screen but they will have no more than a few tens of electron
volts of kinetic energy and will not produce much heat. However, the screen should not be operated above the specified voltages and
currents. So that limits the plate voltage you may apply. You will still have about the same number of electrons striking the screen and
if you increase the voltage of the screen-plate you may exceed the screen's dissipation. So, the triode connected tube should be derated  
to perhaps to 1/4 or 1/3  of the of the plate voltage that you may otherwise apply if it was a tetrode. 
  I never much cared for this particular way of triode wiring a tetrode but it has been done for quite some time with success, mostly
for audio applications. It is the simplest way to get a tetrode or pentode to behave as a non-zerobias triode.
  You can make a tetrode or pentode behave as a triode if you have some means to assure that the screen voltage changes proportional
to the plate voltage. But this is not always simple. And at RF frequencies it could be very complicated if not impossible. 

73
Bill wa4lav
________________________________________
From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on behalf of Dennis Monticelli [dennis.monticelli at gmail.com]
Sent: Wednesday, November 13, 2013 11:56 PM
To: David Stinson
Cc: ARC-5 List
Subject: Re: [ARC5] Smart People: Triode Connecting Pentodes

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
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html