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Old 20th April 2012, 12:40 PM   #11
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BTW DF96 . I/f noise in pentodes and not partition noise . That is very significant and logical . 1/f noise was first talked about by Tim de Paravacini when I heard of it . Specifically in transistors . I saw good EF 86's quoted as about 4 nV per root Hz in triode ( like a BC 108 I guess ) . If I understand correctly valve microphones use pentodes as pentodes .

To Smoking amp . I appreciate your description . In all of this the complexity gets out of hand .

I love valves because with just two we can have a proper power amplifier . Sometimes with only one capacitor in the forward signal path ( if very brave we even loose that ) . My friend is running a Fostex full range unit with just such an amp . He uses a 300 watt class D amp to drive the old Leak 12 inch Sandwich unit to give the lower octaves . It has an adapted car stereo active crossover . The Fostex only shows it's point source abilities when driven by a nice SE amplifier . The car stereo crossover has been the thing we have found most transparent , it cost pennies . It drives long cables very well . Many DAC's have failed to deliver when listened to through that crossover . Eventually the crossover could be replaced by carefully designed output transformers and passive LF bass filter . There would still be the need for a buffer amp .

My thoughts for that perfect power amp would be EF 184 and EL 36 . Less than 1% second harmonic at full power . Likelihood is KT 88 will be used . Most likely is that none will be run as absolute triodes . I suspect it will have many small feedback loops . I doubt it will have global feedback .

Thank you everyone who has replied . Hope someone keeps this going . I think it is a deeply interesting subject . In 1927 Philips gave use this chance , they lost out commersial due to pushing of the beam tetrode . The pentode is almost a box of crayons if we understand it correctly .
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Old 20th April 2012, 12:51 PM   #12
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From reading so far I get the impression that you are not interested in varying over the full range from Pentode to Triode, but rather the ability to vary over a wide enough range to be able to select the optimal operating point for linear operation.

To that end, some variation of the mentioned circuits should work satisfactorily.
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Old 20th April 2012, 01:31 PM   #13
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I think unintentionally we have arrived at 3 states of operation . With a hypothetical minimum distortion in a fictional UL feedback arrangement for small and medium sized valves ( EF 86 EF 184 EL84 perhaps ) . If you are saying all points of operation between would be better ? Then I absolutely agree as it might find the exact sweet spot a design needs . I did get the impression reading early UL for power pentodes that we all might have lost the point . It seemed if I understood correctly that a power valve will produce it's lowest distortion if UL . This has somehow become , almost as low as triode and almost as powerfull as pentode as we now see it . If correct and not a misunderstanding by that writer ( forgive him it was early days ) there is mileage in this we never found . I never remember a UL power amp having less distrotion than when in triode . However I never had the transformer reconfigured for the same maximum output power as triode . Returning to your question . As far I can see we can have any pentode to triode curve we want if using simple shunt feedback . It does no great harm and is only doing what a triode internally does . Admittedly a triode is the fastest feedback possible . The idea we could play with the pentode curves by altering something to do with g2 seems unlikely to work very well . UL feedback is a bit like turbo chargers .It all seems very wrong yet it sort of works .
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Old 20th April 2012, 03:43 PM   #14
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There may be more to the UL optimisation than just the % feedback. The % UL is really just varying the effective Mu of the resultant triode. (the pentode end is just a high Mu triode). This plays off power versus distortion for a fixed load as the effective Rp varies from the UL modded Mu.
Mu_eff = gm1/(UL% x gm2 + 1/rp)
Rp_eff = Mu_eff/gm1 (I think?)

Consider the pure triode with its heavy plate feedback. It would linearize the 3/2 power emission to Mu voltage gain if the load Z were sufficiently high. But it's varying Rp sabotages this.

Now consider the pentode, if the screen were operated at a constant % V (both AC and DC) of the plate, it would intercept a constant % of the plate current at all signal levels by symmetry (geometrical construction determines the % interception, the scaled tracking voltages determine the level of current that is being partitioned).

That would mean that the screen would be a constant % impedance of the plate Rp (actually by a higher ratio). But since the plate Rp varies with signal, we don't have a contant screen Rscrn yet. Now consider using a feedback network from the plate back to the screen that has a similar ratio of effective Rfdbk to Rscrn as the plate Rload is to the tube's effective Rp. (not just the pentode Rp any more, but the UL effective Rp which is much lower) As the Rp drops versus the Rload with signal, so does the Rscrn versus the Rfdbk with signal. So Vscrn drops disproportionately with signal as Rload drops. Could this compensate for Rload variation? Eliminate loading distortion? As the plate Rp drops, the screen V ratio drops to compensate. It's doing the right thing at least. Remember, both AC and DC must be scaled down by the same % here. A pot or R network does this conveniently.

Some math/formula solving for 3/2 power will be required to see if this is working correctly. Unfortunately this won't compensate for g1 island effect distortion, but maybe the ratios can be tweeked off some to improve that too over some range.

Note:
This scheme could be working for the linearization of small signal stage pentodes, but would lower the power output unacceptably for power stages due to the % DC drop required on the screen for tracking, unless rather high plate voltages were used, or low screen V Sweeps were used.
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Last edited by smoking-amp; 20th April 2012 at 04:10 PM.
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Old 20th April 2012, 05:00 PM   #15
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"The pentode end is just a high mu triode " is exactly how I see it . David Hafler put it very simply and although slightly to one side of your point reinforces it . Hafler says for UL to work current is involved and that resistor chains do not provide similar conditions . I was saying to myself what a shame we don't have the ability to do this with transistors . Then maybe we do with a cascode ? When I have time I will look at this with real life devices . No transformer's , that's for others . I think the most useful thing is to have found an interesting device ( EF184 ) which looks to me to be many different things especailly where high current is required . I suppose UL is a tracking triode connection that moves almost as bootstraps do . It has time lag and that must be where it looses out . As said a SE UL seems to work well . I suspect that the transformer air gap helps ( anyone want to say , hysteresis and constant DC bias to magnetic circuit perhaps ) ? Thank you for a journey of discovery . I have a lot of reading to do . It's been 37 years since I was taught any of this . The teacher failed to say about depletion and enhancement , however he did teach Kirchhoff . He tried to teach Fermi probability gradients without an idea why , at least he was honest . The next year never got valves at all . It's far more challenging than transistors ( forgetting JFET's that is , they are challenging ) . It's almost a religion and yet a science . I should clarify a point . I have had extreme prejudice against UL in the past .
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Old 20th April 2012, 05:20 PM   #16
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The usual UL output mode allows the plate voltage to drop below the screen voltage, causing noticeable screen current distortion. The OT xfmr tap helps ameliorate that problem by making some use of the screen current at least. I suspect the "optimum" UL tapping is a result of playing off the screen current distortion against the 3/2 power distortion of the pentode mode. They are in opposition. Also the effective Rp is being lowered in UL mode, which lowers loading distortion. Power versus distortion tradeoffs as well. Unfortunately, the pentode 3/2 power distortion (variable Rp and emission) is current related and the screen current distortion is voltage related. A resistive load puts them in phase to cancel. A reactive load causes them to go out of phase and not cancel.

By scaling BOTH the DC and AC %UL, as I mentioned in the above scheme, the screen current distortion factor is eliminated. (constant fraction of plate current diverted to the screen, plate V always above the screen V by a fixed ratio) By using the same R loading distortion factor in the screen feedback, as the output already suffers from, it should generate a compensating correction of the plate loading distortion, if adjusted correctly. (hopefully anyway, that's the idea at least)

Of course, this only works for a resistive load also. But if the load is a fairly constant reactance, then a similar reactive screen feedback network could be used. (which brings up the interesting idea of phase compensating standard UL too. Either scheme would want a speaker reactance model in the screen feedback path.)
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Last edited by smoking-amp; 20th April 2012 at 05:50 PM.
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Old 20th April 2012, 06:29 PM   #17
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That is very interesting . I must put that on a scope and look at it . I will try a real speaker ( something inexpensive ) and the 8R 2uF . I will be looking at the UL tap ( hopefully it stays stable ) . Hafler stated clipping distortion is superior in UL . I never found that . One thing the teacher said which is interesting " you audio people talk about things in the signal path , you obviously don't know Kirchhoff current laws , everything is in the signal path " . How true . As the UL tap has a series resistor I will look in the current mode also . As anyone will guess it is not always possible to measure without influencing results . I am cautions of current mode . in a trans-conductance transistor amplifier the voltage waveform will seem to show gross distortion . However at the collector of the transistor we see nothing . Inserting a small resistor in the transconductance path allows us to see it was a ghost in the machine . However it never seems completely satisfactory as evidence . For what it's worth I don't like transconducatce amplifiers and still operate a 1: 5 ratio of impedance ( or the Williamson 1 to 3 if forced ) . If you don't mind a deviation that is an interesting point . A British amplifier of the 1970's had an output resistance of 2K4 feeding the next stage . That stage the voltage amplifier stage ( VAS ) was at 8 mA and Beta of 100 . The input impedance of the VAS 312 ohms .The emitter sat on a diode at 0.6V . The designer must have thought it is like a 75 ohms resistor which would offer 7K8 input impedance . Replacing the diode with that resistor transformed that amplifier ( measured IM distortion reduced ) . Yet DC conditions remained the same . I suspect the ghosts we find are not ghost and why not eliminate them . That amplifier had the usual bootstrapped constant current source to the collector which works a treat . I notice the ratio of triode output inpedance to transformer is about the same ratio as an optimum . Probably a coincidence ?
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Old 20th April 2012, 07:52 PM   #18
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Quote:
Originally Posted by smoking-amp
the pentode end is just a high Mu triode)
No, not really. The pentode is more like a triode in which the functions of majority current collection (anode) and voltage feedback (g2) have been separated. You don't get low distortion by giving a pentode an infinite anode load, as the distortion is set by the grid-cathode behaviour. Remember, pentode curves go the opposite way - increasing mu will not make a triode curve like a pentode.
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Old 20th April 2012, 08:17 PM   #19
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That's a very good way to put it and logical . I was debating something . Supposing we could easily make 100 % anode to g1 feedback on a pentode some will say we don't get exactly what we get from joining g2 to the anode . Could I be incredibly simplistic and say that's because g2 is closer to the anode ? I can see an attractive argument for using a high mu triode in parallel with a pentode . As it's a reasonable idea I am sure it's been done . I am glad you said about high mu triodes . Without looking at any curves I assumed them to look more like pentodes . My friend likes his triode connected EL 34 and PX 25 triodes . I always assumed because ra was similar . Apparently sending signal to g2 of EL 34 makes a low mu triode more like 300 B . Taking my last point an EL34 triode strapped and a EL 34 pentode might be getting somewhere in paired parallel . Might we have just described UL feedback ? The difference is no time lag if done this way ?
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Old 20th April 2012, 09:26 PM   #20
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There is no significant time lag in any normal audio feedback. If there was a time delay then feedback would be difficult to apply, as a servo loop with a genuine delay can be difficult to stabilise.
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