[ARC5] Smart People: Triode Connecting Pentodes

Thu Nov 14 11:26:11 EST 2013

It is rather common to connect control and screen grid together and 
sometimes with a resistor in the screen lead.
The end result is a low mu triode that can use whatever plate voltage it can 
handle. I dont recall a pentode being used however but a 813 would have been 
a natural.

There have been lots of writeups on that in ham literature since the 30's.

Carl

----- Original Message ----- 
From: "Fuqua, Bill L" <wlfuqu00 at uky.edu>
To: "Dennis Monticelli" <dennis.monticelli at gmail.com>; "David Stinson" 
<arc5 at ix.netcom.com>
Cc: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Thursday, November 14, 2013 12:23 AM
Subject: Re: [ARC5] Smart People: Triode Connecting Pentodes

>
>
>  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
>>
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