Pure tetrode and negative g2 - diyAudio
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Old 12th July 2012, 06:05 PM   #1
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Default Pure tetrode and negative g2

I have a question for the forum. I read all I could find on the matter but there is an idea (possibly a very stupid one) which has not been brought up.

A pure tetrode is almost always unusable for audio purposes as it displays a kink in curves due to secodnary emission from the plate. The development of kinkless tetrodes, beam forming plates and other very interesting designs "ironed" out the kink.

There are also peculiar tetrodes where physical distance between anode and g2 seem to counteract secondary emission (charge buildup around the anode?)

What I thought of is using g2 at a negative potential. Even though it stands in the way of travelling electrons the theory of scondary emission seems to suggest (I am not fully qualified to draw any certain conclusion) that emitted or dislodged electron(s) display an energy level which is lower than colliding (primary) electrons.
If this is the case could there be a negative voltage threshold on g2 which, in conjunction with a high enough anode voltage, could effectively straighten the kink in a pure tetrode's plate curves?

Secondary emission would be counteracted but the negative charge would not be enough to hinder primary electron flow.

Just a question.

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Old 12th July 2012, 06:32 PM   #2
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Tetrodes are used all the time for Audio power amps, especially.
Many power amps that we think of as pentode are tetrode. Although pentode tubes are used, they are used in what is considered a tetrode mode...
Straightening the curves did not matter a bunch to guitar amp builders.
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Old 12th July 2012, 06:36 PM   #3
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Yes tetrodes are used but KT tetrodes (kinkless tetrodes), radial beam tetrodes (Eimac or amperex) or beam power tetrodes. I am not aware of a widespread use of pure (with "kink") tetrodes for linear audio use.

For "tetrode mode" you mean grounding g3? AFAIK that is the way pentodes are supposed to be used.

finally you suggest that pure tetrodes can be used for guitar amplification?
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Old 12th July 2012, 06:44 PM   #4
DF96 is offline DF96  England
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With g2 negative you won't get much anode current however much voltage you put on it. g2 and g3 do different jobs so have different wire spacings and are in different positions. You can't turn g2 into a suppressor grid just by changing its potential.
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Old 12th July 2012, 06:48 PM   #5
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A negative potential on G2 will stop the current flow.
Flatline. Dead.
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Old 12th July 2012, 06:57 PM   #6
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I suspected it was a stupid idea

I guess it depends on the spacing between grids but I get the point.

Thanks for solving that
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Old 12th July 2012, 07:45 PM   #7
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You could, however, tie the two grids together and make a high impedance triode if you want to use one of these tubes and are afraid of the kink. You would probably have to drive the grids positive to get much output, which is easy enough to do with a mosfet these days.

Edit: This would be a good option if you don't want to tie screen to plate like usual, like if you are dealing with a tube with a fragile screen grid (one with a low max voltage rating).

Last edited by SpreadSpectrum; 12th July 2012 at 07:50 PM.
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Old 12th July 2012, 07:49 PM   #8
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or simply skip the kink by increasing anode voltage, correct?

The point is I found some really cool tetrodes most NOS/NIB which are absolutely rejected by the audiophile world.
Since i can have any power/output transformer wound, I hought of tapping on his "reserve". Honesstly I still have to test my 1700w OPT extensively, but if it checks out I can think of grabbing some of these large tetrodes.

AB2 operation is already what I am seeking with my gu81m so the experience I gatherr with that project will prove useful.

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Old 12th July 2012, 10:00 PM   #9
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Originally Posted by Alexontherocks View Post
or simply skip the kink by increasing anode voltage, correct?
I would think that as long as your load line crosses well above the knee of the curves you might be able to get away with it. The tricky part would be with the varying load that the speaker presents (depending on frequency). You would have to be careful that you don't get into the negative slope zone at any frequency, which totally depends on the speaker. I have no experience using tetrodes as AF power amps, so I'm not talking from experience.

The purpose of the screen grid is to make it easier(at the control grid) to drive the tube to saturation. If I were to do this, I would go for a triode connection (whichever made more sense depending on the tube data) and use a mosfet to drive it, that way there is no possiblity of accidently hitting the negative slope region. Grids tied together will give you high mu and high rp, screen tied to plate will give low mu and low rp. With the high mu connection, you could use external local feedback to lower rp.
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Old 12th July 2012, 10:33 PM   #10
7N7 is offline 7N7  United Kingdom
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Pure tetrodes are quite interesting to me. It seems that the pure tetrode was more or less a dead duck after the invention of the pentode and the beam tetrode, but the pure tetrode did make a comeback (as far as I know it was a comeback) late in the history of valves; the WE448 (the subject of a recent thread here is an example) I had some Ericsson 7150s which appear to have been identical to the WE valves. This was a pure tetrode designed for high-frequency telephony applications. The 7150 had high gm - about 33, and incidentally made a nice triode when strapped, with mu of about 30.

I should love to know what it is about tetrodes that made them suitable for these applications

Plug them in and light them up
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