Why do some people dislike ultralinear?

[Tubes4e4]

Hi smoking-amp,

"Optimised?"
My working hypothesis (working on data measurements to verify this yet) is that the "magic" 43% tap is just the point where screen grid current distortion (using B+ DC on the screens) = the 3/2 power law plate current distortion component, due to a low load impedance.

Since they are out of phase they cancel largely. The 43% figure is specific to tube construction (mainly screen grid spacing or equiv. triode configured Mu), so will vary with tube. Nothing magic about 43%.

My hypothesis is that all those usual values like 20%, 25%, 40% and even 43% just fit very nicely to equal turn number interleaving schemes (f.e, 43% is at 3rd connection of a 7 parts (equal turns!) primary winding scheme; of course 14 parts for PP).

Fits very nicely to effective production methods, eh. Honi soit qui mal y pense. Really, nothing magic about 43%.

Regards,

Tom

 

[ray_moth]

Using a 43% lower DC screen voltage will just get you less power output like the triode case. (or a lot of distortion if power is not reduced)

Johan, I think Smoking-amp's response was to the link provided by Cobra-2, namely, the 'Web Ace' site of Dennis Grimwood on the subject of what he calls 'Optimised Ultralinear'. Have you seen it?

Dennis has strong beliefs about how the voltages and taps applied to screens should be optimised. He says that the ratio of screen voltage and screen primary turns to the voltage and turns at the plates should be in proportion to the inter-electrode distances in the construction of the tube itself, so as to conform to the voltage gradient across the tube. Of course, that necessitates a separate winding for the screens, in order that a lesser voltage can be applied to them.

For example, according to Dennis, if the distance from the cathode to the screen is 50% of the distance from the cathode to the plate, then both the screen turns and the screen voltage should be 50% of those at the plate.

The way Dennis argues it makes it seem reasonable but, of course, that doesn't necessarily mean it's right! If you have read what he says, do you agree with him?

 

[Miles Prower]

The way Dennis argues it makes it seem reasonable but, of course, that doesn't necessarily mean it's right! If you have read what he says, do you agree with him?

Yes, it does seem reasonable, however, I strongly suspect that this is a bit of self promotion. Here's what you'll find at the very bottom of the page:

IMPORTANT NOTICE

THE AUTHOR MAKES NO CLAIM WHATSOEVER AS TO THE VALIDITY OR ACCURACY OF ANY STATEMENT, INFORMATION OR OPINION CONTAINED IN THESE PAGES AND NO LIABILITY WILL BE ACCEPTED FOR ANY ERROR OR OMISSION OF ANY KIND WHATSOEVER.

That says it all, dontachathink?

Now, we've had some 100 years of experience with VTs. I highly doubt that there is something radically new and undiscovered just waiting out there. As for maintaining a plate voltage to screen voltage ratio that's porportional to the ratios of cathode to element spacing, the suggested Q-Point for the least THD operation of the 807/6L6 defies this:

Vpk= 365Vdc
Vsgk= 270Vdc
Rl= 6K6 (P-2-P)
THD= 1.8% (which is spot-on, as that's very close to what I actually measured with a pair of PP 807s)

270/365= ~74%, and he claims that the spacing ratio is closer to 50%. You also have a whole boat load of audio power finals where the screen and plate are suggested to operate at the same voltage: 6V6, 6AQ5, 50C5, 50L6, 6CU5, etc. The 6V6 & Co. are some of the best sounding VTs going. Of course, sometimes you can do better if you get that screen voltage down. Busting the specs a bit, and running at Vpk= 150Vdc -- 180Vdc, and getting Vsgk down to 90Vdc will improve the linearity of a 50C5 considerably. Given that the 50C5 is most frequently operated off a transformerless power supply connected directly to the AC mains, it's not surprising that the Q-Point for max audio power under those conditions isn't the same as that for least THD.

Here he claims:

Another and previously unpublished option to creating an operating environment where the Screen Grids will be at a DC potential sufficiently high enough to attract and accelerate electrons towards the Plates but, to maximise power output, not to collect and divert them to earth through the B+ supply, is the humble silicon diode semi-conductor rectifier.

By inserting a standard half-wave silicon rectifier diode in series with the Grid Stopper resistor, an electronic control circuit is created whereby the Screen Grid will be able to be energised at DC potential attracting and accelerating electrons towards the Plate - still electrostatically controlling current flow in the normal way - but blocking the flow of AC current from the Screen Grid back to the DC source - ie "one way traffic"

It's been tried and what it mainly does is poof VTs.

It's all part of his schtick to make you think he has something grand and glorious so's you'll buy his stuff. Self promotion, pure and simple.

 

[Johan Potgieter]

The way Dennis argues it makes it seem reasonable but, of course, that doesn't necessarily mean it's right! If you have read what he says, do you agree with him?

Thanks Ray - and Miles!

To start with a guilty feeling ... I have read the Dennis document some time ago (before this correspondence), and should really have done so again! In that sense Smoking Amp may have been influenced and also been a messenger like I claimed before, and perhaps I owe him an apology for being harsh - not intended.

To keep this brief and yet say something meaningful..... The best I can do is to refer to Dennis' remarks by page - I have printed it all out over 26 pages; hope that helps.

No, I carefully do not agree with some of his major assumptions; I simply don't see the grounds for some, after I sat and pondered the document this evening. (Us losing the cricket against the no. 9 seed did not encourage further tv watching!) Perhaps the "bottom line" is that from tube basics he makes assumptions, and then go on from there.

His departing point, that geometric position of the G2 must correspond with the amplitude of signal entered there, does not necessarily follow; I can't see that. His argument is sound (and I must compliment him on an excellent description of what goes on inside a tube, for novices) as far as it goes, but the effect of an electrostatic field as generated by G2 is more complex than its geometric position. We are familiar with rp, the so-called plate resistance. Similarly there will also be an r.g2, measureable in the same way- that will play a role. G2 is not just another control grid at very high input impedance because it is negative; it has different characteristics. There is the grid wire pitch, wire thickness, etc. (It is not new to feed an analogue signal into G2, e.g. for control purposes, and one has to feed it right.) Since inside a tube everything is rather interrelated, I will not be surprised if part of his thoughts are true, but not in that simple way. [One can generate an electrostatic field without drawing a single mA (very high impedance electrode), or drawing quite a current. Consider e.g. the difference between the effect of G2 in pentodes and beam tubes, not to mention electrometer tubes. It's a subject on its own.]

Then there is the blank disagreement with published tube data. He veers towards transmitter tubes where Va of 600 - 1KV is normal: subsequently Vg2 must be lower. But in his eventual list there is not one audio tube! Have everybody been dense all these years?? Sorry. He stated that lower max. Vg2 than Va, so happening to be 43%, is common in tube specs. That is plainly not so for any audio tube, they are mostly equal! Not that max. specs have anything to do with typical operation - there he again goes awry. He alluded that as a result ratings are exceeded - again NO! That clearly depends on the designer; there is no danger to having Vg2 = Va provided the max. dissipation is not exceeded, which is basic design.

Regarding the "necessity" to never let Va go lower than Vg2 (signal-wise), also nonsense. It does so all the time in straight pentode designs, and proper designs never go unstable or berserk as a result of that alone. Again this practice veers toward transmitter-type voltages. You keep to that philosophy with any audio power tube as we know and use them, and you will be hard-put to get over 20W even from a pair of 6550s!

Then (where do I stop) there is the optimum of 43%. This has been debunked before by Smoking Amp - just to confirm, the GEC graph on p. 3 for KT88 shows different: The respective max. outputs as well as D remains constant from 20% - 41% taps. One tends towards the 40% side simply because the rp is lower there, other things being equal; nothing to do with distortion. The graph on p.17, also by GEC, is again contradictory to the above - strange; probably different operating conditions, not given.

By the way, that is another major problem I have with this epistle: Not a single result is given that I could find; it simply sounded excellent to the writer. Sorry, that does not constitute proof. At least the operations that he "disapproved" of, is well-documented in tube data from several manufacturers. One is inclined to ask: Debunk those and give your own results, or else.

Finally perhaps, I don't want to sound wise, but I cannot see the great mystery of how UL works - it is quite intuitive! Taking G2 from pentode to triode operation, must surely follow about the way it happens? The exact physics/maths might interest some - fair enough. But there are times that an engineering approach by measuring and drawing graphs like the KT88-example (they also exist for EL34), give a better picture than symbols. ("A picture is worth ... ") It not only shows tendencies, but also sharp changes/parameter (to stay clear of), etc.- I think you know what I mean.

There are other irritating points; one must stop some time. To sum up, regrettably Dennis' arguments start to unravel from about p. 16. As I said before, Smoking Amp seems to investigate all this, his methodology looks sound, and I for one would appreciate seeing his findings. I would just urge him to keep an open mind, not just tied to "Vg2-must-be-lower-than Va".

There - see what you have done, Ray_Moth. My longest contribution. Time for others to take it further.

Regards

 

[ray_moth]

Thanks, Johan! This is well on-topic, since it examines someone's ideas about how to do UL right (in his view). I've been wondering, ever since I first saw Dennis's articles a few years ago (he's written several), what other practitioners would think of his ideas. As you say, he doesn't provide any proof that he's correct.

Another Dennis Grimwood article is on 'Optimised Electron Stream', where he ropes the suppressor into the UL argument on the grounds that it, too, sits part way along the path from cathode to plate. Any thoughts on that?

 

[Miles Prower]

Another Dennis Grimwood article is on 'Optimised Electron Stream', where he ropes the suppressor into the UL argument on the grounds that it, too, sits part way along the path from cathode to plate. Any thoughts on that?

"It's ridiculous", comes to mind here. 😀

 

[Tubelab_com]

I read through this web site a while back. I tried the diode in the screen grid trick, and it does provoke s particularly nasty sounding distortion near full power. I have not tested the optimized electron stream technology ( I am not sure he has either), but I would expect distortion due to secondary emision.

But I found these humorous quotes" It will be seen also from tube plate characteristic curves that the values of grid bias within the above range are such that most tube types cannot operate linearly in Class A - ie the positive alternation of signal will not produce the same plate current change as an equal negative alternation of signal.

Furthermore, any transient peak signal having an amplitude several times the RMS value will not be reproduced in the output stage load unless it is in the positive alternation of the AC signal - because any signal greater than plate current cutoff cannot be reproduced - ie the negative signal alternation will be truncated.

This is why a single-ended amplifier is fundamentally a waste of time and money for hi-fi.

This is also why a Class A push-pull amplifier is undesirable - unless it has sufficient headroom power to cater for all power output conditions required for high fidelity reproduction at the required listening level.

See paper by Williamson and Walker on the above topics.

I guess that I have been wasting my time and money for the last few years. So have many of us!

 

[Tweeker]

Well, it is a good reason to strive for fast overload recovery in any amp. But, theres a price to pay for more watts, and not just economic. Ei- Cmiller growing as you parallel devices and then having to get them biased right etc. In AB you have gm doubling or crossover distortion to contend with.

 

[Dave Cigna]

By the way, that is another major problem I have with this epistle: Not a single result is given that I could find; it simply sounded excellent to the writer.

Exactly! It seems that he has developed in his own mind an image of what the electric field looks like inside a pentode. Unfortunately, as evidenced by his conclusions, (UL tap ratio, Vg2 must be less than Va, etc...) his mental image is wrong and too simplistic. There are many textbooks that give good descriptions with diagrams of the voltage gradient, in particular how it is changed by grid wires. Apparently he has not seen them or did not understand them, or simply chooses to believe that he knows better!

The RCA Engineering Bulletin on Beam Power Tubes by O.H. Shade has the diagrams. A scan can be downloaded from tubezone.net. (Scroll down to the bottom of the page.) Just don't fall for the advertising hype. The publication was meant to sell RCA's new beam tubes, so it paints them in especially pretty colors.

-- Dave

 

[Salas]

After all this excellent discussion I tend to conclude that what we like or don't like about UL sound is the sound of local feedback and OPT quality which becomes even more central?

 

[Johan Potgieter]

MA!!! Tubelab has stolen my thunder!

Seriously, thanks for saving me much trouble.

Ray,
I did glance through that G3-Grimbeek paper, until I came to the diode-in-G2 arguments and stopped. (Frankly, did not remember that those were also from this gentleman.)

Since you asked me, the G3 argument is much the same as the G2 one; again unraveling somewhere down the line. Since I got a salary for being inquisitive and going down roads less travelled (as a research engineer I was not paid to re-invent the wheel), I was rather like Dennis. Only one cannot simply declare the work of others also having been of that inclination void - I am referring to manufacturer's data. I cannot simply call such data "a barrier to further development". They want to sell their product after all! They must give reliable data regarding normal (safe) use. The only barrier to further developmind is in your mind, not their data.

After that philosophy, my remarks would be the same as before. Again I find funny statements, like "electrons attracted to G3 because it is at lower potential than the plate" (or similar). In normal operation, electrons are not attracted to G3 - how does a negative electrode attract negatively charged electrons?? (We then seem to need a whole lot more re-research than about where one simple grid should go. Like Tubelab said.) Electrons which bounce off the anode because of arriving there at speed, are prevented from flying back to the positively charged screen lurking just round the corner, by G3 exerting a repelling field to send them back home (the anode). In the process a few do get stuck on G3, theoretically causing a current, but that is totally negligible.

And that is about it! Again, not desiring to sound like an oracle, I also used G3 for signal purposes long ago. (The Mu is about 1!).
And extra grids could be used for a purpose - we all know about frequency converters. In exactly the same way the normal G3 was also designed for purpose: to prevent secondary or "back" emission from the anode. Fiddle around by all means, but do not expect to suddenly find a greater use for G3 than just what it was designed for. All engineers in the hey-day of tubes (never mind me) were not fools.

Again enough other statements popped up to keep a more or less fixed frown on this forehead. Sorry, no go.

Salas,
No; well not so critical as to be frightening. UL has been used for so long with mostly laudible results. Sure, there are the poor designs/output transformers; they are not the norm. Also, yet again, the "liking/not liking" of the sound of UL, ditto the above, as well as: You are also listening to a complete circuit, etc. etc. One cannot single out one element without examining the whole.

Mainly, there is no reason to be scared of it - least of all the urban legends that aboud. (I used to call them old wives' tales, but I suppose these days one must be politically correct ....)

Regards.

Edited: Typos galore - sorry

 

[ray_moth]

The only barrier to further developmind is in your mind, not their data.

Why pick on me? 😀

Again, thanks, Johan. I find your response to those articles by Dennis Grimwood (not Grimbeek!), together with Tubelab's response, to make good sense. I suppose it all boils down to Dennis's assertion that engineers over the decades have got it wrong, while at the same time failing to provide any acceptable justification of why one should believe that he's got it right. (There's someone like that at AA, with whom I have had several altercations, but let's not go there!)

Dennis's 'Optimised UL' is, perhaps, somewhat plausible; however, his 'Optimised Electron Stream' is definitely not. Connecting the suppressor to a positive potential between plate and screen turns a pentode into a tetrode, and back comes the dreaded kink! Of course, he can apply his OES approach only to pure pentodes that have a separate pin for g3, so his beloved 6146s and 807s and other beam power tubes can't be considered.

Another proponent of UL with somewhat unusual ideas is Stan White, who designed the POWRTRON amplifier over 55 years ago. In his criticism of the Williamson, he claims that triode OP stages try to give constant voltage, pentodes give constant current, while UL gives constant power, which is what he says speakers need.

 

[Tweeker]

What speakers need is a bit too nebulous for me. Some amps match well with some speakers, as different speakers want different amounts of damping. Some triode OP stages arent really famous for their damping either, no NFB SE 300B amps anyone?

Of course 55 years ago feedback was the bees knees and SE amps were for cheapskates.

 

[ray_moth]

Of course 55 years ago feedback was the bees knees and SE amps were for cheapskates.

So what's changed? I never liked the sound of table-top radios!
(Now ducking for cover 😀 )

 

[phn]

So what's changed? I never liked the sound of table-top radios!
(Now ducking for cover 😀 )

You better. I'm on the lookout for a new tabletop radio. Not easy finding a good one with FM. FM came around the time of the transistor. So many great, and not least great-looking, 1920-40s tabletop radios and no use for them.

Real radios

 

[Johan Potgieter]

Why pick on me? 😀

Och Man, sorry! It is this peculiar English language to us Boertjies - never could understand why "daughter" was not pronounced dafter or "laughter" lawter. Guess I'm just daft.

The "you" in Afrikaans also means an indeterminate person. I should have said "it is in one's mind".

Regards

 

[Johan Potgieter]

Another proponent of UL with somewhat unusual ideas is Stan White, who designed the POWRTRON amplifier over 55 years ago. In his criticism of the Williamson, he claims that triode OP stages try to give constant voltage, pentodes give constant current, while UL gives constant power, which is what he says speakers need.

Unfortunately I clocked this article up. 😡 😡 😡

Got right back at me, Ray! 🙂

I was just about to wonder whether to say something about it (I had a brandy first), when I found that several fundi's on this site did that some time ago in another thread. So I will defer. (The brandy was good, anyway.) I tried to Google on Powertron but was unsuccessful. Perhaps someone can direct me to that lofty example of a 50s design.

So perhaps just on the voltage/current/power issue. Practical loudspeakers require constant nothing. Ideally a moving coil motor is driven by ampere-turns. Thus a constant current is indicated! In fact, a Danish experiment showed that a constant I feed was slightly preferred to a constant V feed by a group of listeners (bass damping excluded). But we are normally using constant V output amplifiers and they also sound excellent.

Point is that owing to the influence of mechanical entities on any driver (including box contribution), one cannot make a simple pronouncement that constant power is required. That is, anyway, not nearly given by UL. As I see it, the closest one can come to that is with some form of motional feedback.

I think it has been well established by now on several threads, that UL come quite close to triode operation. (As I recall, KT88 UL is very similar to a KT66 triode operation.)

I find the White criticism of the Williamson preposterous. There - must be that good brandy.

Regards.

 

[Tubelab_com]

MA!!! Tubelab has stolen my thunder!

Sorry about that. I don't like to comment on things that I have not actually tried, and I have not tried the Optimized Stream. The untried ideas on my web site are clearly labeled as such. The tested circuits have test data, and often simulations, and pictures. Pictures at least prove that I actually built it, even if it didn't work! It would be nice to see some evidence that these concepts were actually tried.

I did stick the "magic diode" in a SE amp and tried it in both triode and UL mode with an EL34. Distortion was clearly audible (a crackling sound) as the amp approached clipping. The distortion dissapeared when the diode was jumped. It was worse in UL.

I have not actually tried the optimized stream stuff, but I did try puting a positive voltage on G3. As mentioned there are only a few tubes that have G3 seperately pinned out. The EL34 is one of them. There was a thread a while back where the question was asked whether G3 should be connected to the plate or the cathode in triode mode when using an EL34. I connected an EL34 up both ways and manually plotted the curves. I found that when G3 was connected to the plate the secondary emission "kink" became clearly visible. Even though I presented real measured data, the results were disputed by some. The graphs were slammed because they were manually plotted. The thread degenerated to AA quality.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=67422&highlight=

Some sweep tubes have G3 pinned out seperately and it is common to apply a small positive voltage to G3 in sweep tube service to prevent Barkhausen Oscillation. Where this is specified in the tube data a G3 maximum voltage is specced. This is 30 volts max for most sweep tubes. I usually have no problem experimenting with operation in excess of the specs but Grimwood stresses adherance to the specs.

There is a long dissertation on how to build a guitar amplifier (on the screen grid page). Some of the advice is in direct conflict with my observations, and the accepted norm. For grunge or heavy metal, a class A amp is recommended. An amplifer for lead guitar use ("minimum distortion" ??) sould use class AB2 or class B. This is news to me. The only class B guitar amp I have ever seen is the one that I made. AB2 amps are very rare. Most are AB1.

I have built quite a few UL amplifiers, both SE and P-P. Most of them include a switch to flip from UL to triode. Some include switchable cathode feedback. I have found that my preference for mode and feedback depends on the choice of music and the speakers being used. I took one of my SimpleSE amplifiers to a friends house who has some 106 db horn speakers that use Lowther drivers. UL sounded absolutely awful on his speakers with any music, and any tube that we tried. It souded sweet in triode. If I had only that experience, I would never try UL again.

http://www.tubelab.com/Sound Checks.htm

I read about the POWRTRON amplifer and came to the same conclusion that Tweeker did. Some speakers may like a "power source". Most speakers today are optimized for a voltage source since that is what you get from a solid state amp. However Nelson Pass and his First Watt series is bucking the trend with a current source amp. He is the first to tell you that it requires a special speaker.

I have a collection of old radios. Most of them are from the 20's to 40's era. In this area there is only one AM station that is in English, and it talks extreme politics. They don't get much use. I have found a few tube radios that have FM. It seems that FM came a while before the transistor, but FM came about the same time that radios were made out of plastic instead of wood. I have a few wooden radios that have FM, none of them sound great. I have two bakelite Grundigs that do FM and both of them sound nice.

 

[Tweeker]

I pretty much gave up on the Williamson critique after these pearls of wisdom:

The capacitor-input system rendered marginal the use of the first choke as a filter element. Capacitor input systems defeat the advantage of using chokes in a power supply.

When the Williamson circuit was published, inexpensive electrolytic capacitors were already available in the post war market. Oil capacitors are very expensive per mf. and as obsolete as copper oxide rectifiers. It was a poor choice for home use. Oil capacitors make very poor power supply filters, for they lack enough capacity to do the job. Electrolytic capacitors are much preferred in good designs, being available in sizes to 500 mf. or more.

I like my even more obsolete now oil filled capacitors.

The Williamson PSU may well be flawed, but youl not convince me while arguing chokes are a waste. Is he suggesting to just use a 500uF capacitor on the input and leave it at that?

When a loudspeaker is attached to an output transformer instead of a resistive load, the load line of the output tubes goes crazy, whether the tubes are triode or pentode connected. Neither triode or pentode mode operate well with loudspeakers. This is why all performance tests are carried out with resistive loads.

If both pentodes and triodes are discombobulated by it, why should a UL connected tube be fine? Besides, I thought triodes were the most tolerant of reactive loads.

 

[Miles Prower]

Web sites like White's and Grimwood's are all over the 'net, and are filled with all sorts of nonsense. White even includes ideas on physics that are pretty whoo-hoo. Of course, White is trying to get you to buy his stuff, and so needs some sort of self-promoting schtick.

You can go to This Website and get as much of that as you want.