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Old 17th April 2012, 05:23 PM   #1
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Default To vary a signal pentode to triode in stages

Has anyone tried to make a circuit that varied a small signal pentode like EF86 or 6SJ7 to points between full triode and full pentode operation ? I would happily accept that local feedback is the better route and views on that . The application is for the input of any amplifier where loop feedback is used ( no specific amp ) . I have in the past removed the feedback loop from the output transformer and strapped G2 to the anode of the input pentode valve with good results ( triode ) . Having a distortion analyzer these days it would be nice to try options between full pentode and full triode along with less loop feedback . I suspect that this is a little more complicated than just a potential divider to G2 and there are risks to the tube . I note most circuits show G2 connected to HT and a capacitor to ground . Simply changing this resistor seems too simple ?

Some one must have asked this question before ? Any help greatly appreciated .
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Old 17th April 2012, 08:52 PM   #2
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Is this what you are looking for ?
Ultra-linear Aikido Amplifers
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Old 17th April 2012, 09:25 PM   #3
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Difficult to do without a tapped transformer. A resistive divider adds resistance so modifies things.
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Old 17th April 2012, 10:52 PM   #4
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You may also want to read the Frank Blohbaum (mit der umlaut) article in Linear Audio Vol. 0, which explores some options. (And is very interesting anyway.)

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Old 18th April 2012, 12:25 AM   #5
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Morph control from Merlin Blencowe's first book.

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Old 18th April 2012, 02:13 AM   #6
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Problem is that the screen grid draws current and is not a linear resistance either (under most circumstances) versus signal or control adjustment. For a small signal pentode, operating over a limited range, the variation of resistance may be tolerable for a pot control as these schemes use. For driver or output stages you likely will need to buffer the screen with a Mosfet source follower. A series grid damper resistor and an RF (MHz, so tiny C) cap to ground or cathode may be needed to prevent parasitic oscillation.
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Old 18th April 2012, 10:25 AM   #7
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Thanks to everyone who replied . The Akido is fascinating , I will have to find some time to study that in detail . I had also thought of a transformer type of UL feedback which could be used with interstage transformers . A little unlikely as it seems against the religion to do that . However my joke to a freind that EL 84 would drive a 300B correctly seems to have been done and supported . Glad to see some people think outside the box . Forgive my drawing . It is simply how Quad configured it's preamp using EF 86 . If something this simple has any merit the next question is can the anode coupling capacitor be common to both input and EL 34 ? The Blencowe's example is very useful . Also , I saw someone running a 6SJ7 at 127 V / 4mA anode and 1.1 ma g2 . That seems twice it's rating ?

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Old 20th April 2012, 09:49 AM   #8
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If anyone returns to this thread I found a very early description of UL feedback which although probably a bit optimistic is very simple . It shows pentode and triode curves and states that they bend in opposite direction . It then shows perfectly straight curves and calls them Ultra Linear . Statements of the bleeding obvious one might say . The author was convinced that this exact point was inside every pentode one felt . I did find one example of an amplifier using EF86 with an UL intestage transformer . I found a few guitar amps with EF86 and Morph control . That is interesting as one would think a simple pentode / triode switch good enough for them , obviously not ! For the duties of a perfect UL device I would say EF184 . It is said to mimic ECC81 with greater linearity in triode . I said to my late freind Sidney Smith " inside every good triode there is a good pentode trying to get out " . He nearly crashed the car . He then spent some time with me showing triode curves . I haven't looked much at valves since his death which is obvious from my incomplete drawing . Sidney did agree and we talked about UL feedback although we both thought it a concept rather than workable ( Sid's amps had it and triode switchable ) . I have recently changed my view of UL . In SE amps it sounds rather good . Perhaps the air gap helps ? Is it only global feedback that spoils it ? Sorry about my drawing . I had rather hoped someone would show me how to do it properly . If drawn as a Leak pre-amp where feedback is taken from the output coupling cap via a resistor to g1 and a resistor to signal in ( shunt feedback ) it looks right . I feel any triode you want is available from any pentode using simple shunt feedback . I guess that perfect curve is in there somewhere ? As the capacitor is there anyway I see no extra mechanism for sound quality degradation ? I could even imagine in some conditions feed forward would happen and might be useful ? If it works who cares ? Returning to EF 184 I wondered if the Quad 11 power design using it and KT 88 might work out rather well with a bit of a redesign ? To anyone who looks at this do read all of the Akido .
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Old 20th April 2012, 10:34 AM   #9
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To a first approximation a pentode will give you whatever distortion is set by signal level and bias point, due to the grid-cathode behaviour. It doesn't really notice the anode load, as g2 screens this.

To a first approximation a triode can give you very low distortion, provided that you give it a sufficiently high anode load. This is because of feedback from the anode, which can exactly counteract the non-linearity from the grid.

What UL provides is lowish distortion for a much lower load than would normally be needed for triode linearity. Hence UL is useful for output stages. It is less useful for small signal stages, as here it is usually easy enough to provide a high anode load and get triode linearity. In UL arrangements where g2 current contributes to the output then it is possible that partition noise will be reduced. This is less useful than it sounds because most valve audio noise is 1/f, not partition.

A useful generalisation is that if an apparently good circuit idea has not been used down the decades then it probably means that it offers no real advantage over simpler circuits. Our forerunners did know what they were doing!
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Old 20th April 2012, 11:25 AM   #10
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I absolutely agree with that . I did feel the the writer of the UL explanation had hit upon a point we all of us miss . He said neither the pentode nor triode curves are ideal . Inside the valve there will always be a better curve if pentode to start with ( perhaps ) . I said to Sid that triodes are linear due to internal feedback . It is a defect which we rather like . By experimenting we can have linear or not so linear triodes if a designer of valves . Sid rather preferred EL34's strapped as triodes compared with 300B's . My friend Paul likes his TR34's as I call them and not 300 B . He likes PX25 which I think is a bit more pentode like . All very fascinating . As you sayfor an input stage probably use triodes . This curiosity of mine came about because after Sid's death I was asked to help convert a Leak TL12 ( not plus ) for a mutual friend . I had never done this in my life and had to guess some of it . I also felt it a bit wrong to alter a classic design . However it was said to be transformed . In both cases all was converted to triode . Many years ago Jean Hiraga who I long to meet one day pointed out that linear reducing harmonics is what a zero distrotion amp must have . Not absolute zero , just the reduction in harmonics on an exponential scale is more correctly a perceived zero . OK , we all know that now . Having ways to get there is useful . Ricardo Kron mistook me for someone who owed him money . I wasn't bothered and we got on rather well . Sitting together and having never met him before he says dryly " surely transistors are better " ? to which I replied " yes " . He smiled and relaxed " that's what I thought .......you know , I once made a dull emission 300B , I couldn't give them away . It was the best valve ever made " he said . Alas Ricardo is not with us now . The PX 25 I refer to is one of his . I would say EL 84 or 34 with a UL transformer might drive 300B well . I am not a fan of 300B . However I always suspected it needed more drive .
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