Triode-wired small signal pentode question - diyAudio
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Old 27th August 2010, 05:55 PM   #1
rongon is offline rongon  United States
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Default Triode-wired small signal pentode question

In phono and line level preamps, I've seen triode-wired pentodes wired up with the suppressor grid tied to the cathode or the suppressor grid tied to the plate (along with the screen grid).

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I can't seem to find a discussion of why you'd choose one over the other. I've seen that power output pentodes or beam tubes like EL34, 6L6GC, etc. are always wired with the suppressor grid to the cathode. But 5879, EF86, etc. are often wired with the suppressor to the plate, along with the screen grid.

Can anyone explain the differences, advantages, disadvantages of the two? Are there situations where one might be preferred over the other?

Thanks...

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Old 27th August 2010, 07:08 PM   #2
DF96 is offline DF96  England
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I'm sure I read something about this, but I can't remember where. Sorry!
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Old 27th August 2010, 07:16 PM   #3
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Triode connecting a pentode?

EL34 Triode Mode - G3 where?
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Old 27th August 2010, 07:31 PM   #4
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The effect would be similar to remote cut-off triode. Triode strapped pentode is a little bit remote cut-off. The more uniform distances between grid and cathode and grid to anode are, the more uniforn is field density where grid controls electric field, the more smooth is resulting tube. I also prefer tubes with longer structures. Like, 6N6P VS 6N3P: measure and compare results.
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Old 27th August 2010, 09:03 PM   #5
rongon is offline rongon  United States
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Wow, I read thru all that stuff in the other threads and could not come to a definitive conclusion either way. Testing doesn't show much of a difference between the two. The "spray" of low-level upper harmonic distortions generated in an EL34 triode with G3 connected to the plate does give me pause. I might leave things with G3 connected to cathode. That saves the bother of installing one more resistor (G3 grid stopper to plate).

I guess the jury's still out on this one.

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Old 27th August 2010, 09:09 PM   #6
SY is offline SY  United States
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For grins, go look at the D3a datasheet. It shows triode data connected both ways. The differences are subtle to say the least.
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Old 27th August 2010, 09:32 PM   #7
piano3 is offline piano3  United Kingdom
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I have tried both ways many times with small signal pentodes,but not with power pentodes(yet!). I could not hear any difference or observe any in operation. There is,or was, somewhere on the internet, a substantial article, with lots of measurements and graphs, of the various possibilities for connecting pentodes.I remember it being quite hard to find-very sensitive to the wording of the search. One might feel instinctively that if you want a triode,more or less,then it will be more of a triode if all the superfluous grids are co-opted to the anode. It is entirely possible that what I have just said is unadulterated nonsense and I am prepared to be shot down in flames by any of the learned gentlemen above!
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Old 27th August 2010, 09:32 PM   #8
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by rongon View Post
In phono and line level preamps, I've seen triode-wired pentodes wired up with the suppressor grid tied to the cathode or the suppressor grid tied to the plate (along with the screen grid).
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In one case you have a triode wired pentode and in another you have a tetrode wirted pentode.
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Old 27th August 2010, 10:42 PM   #9
DF96 is offline DF96  England
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This topic was discussed here. They remind us that g3 is there to deal with secondary emission from the anode, especially when the anode is at a lower voltage than g2, and stop secondaries from going to g2. If g2 and the anode are connected together, then it doesn't really matter which electrode catches the secondary electrons. So the original purpose of g3 has gone.

It is usually a fairly open grid, with little effect on the electron stream, so it doesn't matter too much for small signal valves. For power valves there is the complication that g3 is not designed to dissipate any power so it ought not to be used for catching electrons, so connect it to the cathode.

piano3 may have seen the same article that I saw, but I can't find it either. Possibly Radiotron handbook?
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Old 28th August 2010, 12:20 AM   #10
piano3 is offline piano3  United Kingdom
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Would it perhaps be wishful thinking to suppose that if electrodes are connected to the same potential then they will capture electrons in proportion to the area which they present? Therefore in practice they would be unlikely to exceed their power rating?
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