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Old 3rd June 2013, 11:32 PM   #51
SY is offline SY  United States
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Originally Posted by ilimzn View Post
A Add to that that ECC81/12AT7 was never intended to be tightly specified, and is nonlinear to begin with.
I have some measurements that disagree.
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Old 4th June 2013, 03:30 AM   #52
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It is not harmonic cancellation, it is part of distortion waveform cancellation. As the result it cancels 2'nd order errors to some degree.
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Old 4th June 2013, 10:49 AM   #53
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ECC81 spec is as tight/loose as most other valves. It was originally intended for VHF receivers, but useful for other things too. Not as linear as an ECC83, of course, but in the wrong hands the ECC81 could do as well since it is less fussy about details of bias and anode load.
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Old 11th June 2013, 03:38 PM   #54
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Originally Posted by TheGimp View Post
A caveat of harmonic cancellation is that you may end up with lower total harmonic content at the expense of listen-ability.

In my experience the rate at which each harmonic is cancelled is very bias point dependent.

For example, I have seen lower second and fourth harmonics at the expense of greater fifth harmonic and marginal lower third harmonic. Change bias by 5% and everything changes.

That is why an analyzer is important , I 100 % agree . 90% of things I tried were bad at best . They had to be tried as I wanted to honestly say I had . I don't use a simulator , perhaps I waste more time on this than most ? 70% of the time things work out as one might think . 20 % has an easy answer . 10% has a slightly unexpected outcome .

Joy of joy in my design I can repeat results with many different makes of valve and some variation of type . EL 34's of different makes do sound different yet measure about the same . A slight mystery .

Some have been a bit pedantic I feel about what is a pentode . Sure there will be areas where the Beam Tetrode and Power Pentode part company . To me they all offer pentode possibilities . FET's , Triodes , Transistors don't offer these possibilities .Sometimes gaining that advantage is not seen by others as an advantage . I put up a pentode and triode curve . No one commented on the fact that neither is a great curve . Happens to be that triode is the least bad easy choice . The UL curve is a nice curve . It is in the " pentode "devices to have that . If an inter-stage transformer can be used the UL pentode is an exciting choice . It even allows phase change if that would be useful ( I could list a few uses ) .

I am told that 6L6 and 807 were a little different in reality . There were many 6L6 types ( I think 6L6GC was nearest to KT 66 ) . Some say 5881 = 6L6GC , others say nonsense . The renumbering of devices in a shrinking market probably is to blame ?

Anyone who is contemplating a " zero feedback " amplifier ( loop ) do not dismiss UL . If you learnt triode sounds better than UL , better still with no loop you might say you know something you don't . Just give UL a try with no loop .

Not recognizing pentodes would be like ignoring athletes from Kenya . There is something remarkable to know about pentodes even if everyday life is not asking for that these days .

Last edited by nigel pearson; 11th June 2013 at 03:40 PM.
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Old 12th June 2013, 12:57 AM   #55
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Originally Posted by nigel pearson View Post
I am told that 6L6 and 807 were a little different in reality . There were many 6L6 types ( I think 6L6GC was nearest to KT 66 ) . Some say 5881 = 6L6GC , others say nonsense . The renumbering of devices in a shrinking market probably is to blame ?
Actually, the numbers are different for NOS US and Western Europe tubes, and for Russian equivalents. Mr. Wavebourn may weigh in with better info on the Russian types.

The US-made types:

6L6 was the original metal type. 19W max plate dissipation, 360 max plate volt rating, iirc.

6L6G was the version with a large ST shaped glass bulb ("Coke bottle" shape). I think the 6L6GA had a smaller ST shaped glass bulb.

6L6GB was the GT shaped glass bulb version (straight sided bottle). The 5881 was the industrial version of these, also labeled 6L6WGB. (The "W" = "ruggedized")

6L6GC was the last version of glass bottled 6L6 type. The GC version has higher max plate voltage and max plate dissipation of 30 watts. This version looks very similar to type 7027A.

The Russian types:

6P3S is a GT bottle type (straight sided glass), equivalent to 6L6GB. About 20 watts max plate dissipation, about 400V max plate volts. That's the one usually sold as a 6L6GC, but it's not quite up to those specs. I think that's why you see so many of these 'red plate' in Fender Twin Reverb guitar amps with 450V or higher on their plates.

63PS-E is the heavy glass bottled, wafer bottom type that Sovtek was selling as 5881 (also called 6L6WGC, which is a type that was never made by the US companies). But the 6P3S-E is a quite different tube from the old Tung Sol 5881. 63PS-E seems to perform fine at over 450V on the plates, and 30 watts plate dissipation. I've used 6P3S-E to replace EL34 in a Dynaco Stereo 70. 63PS-E is not equivalent at all to EL34, but it does bias up nearly the same as an EL34 in that circuit. Put a real 6L6GC (or a 63PS with no "E") in that circuit, and that tube will bias up with a much higher quiescent current draw. Obviously, if you use 63PS-E (Sovtek 5881) as a drop in replacement for a NOS 6L6GC, it will make your 6L6 amp perform very differently.

Originally Posted by nigel pearson View Post
Anyone who is contemplating a " zero feedback " amplifier ( loop ) do not dismiss UL . If you learnt triode sounds better than UL , better still with no loop you might say you know something you don't . Just give UL a try with no loop .
I have. Both in a Dyna ST35 and in a Dyna ST70, and in a new circuit I built in the ST70. The "sound" of the UL output stage is sort of intermediate between triode and pentode. The curves I've seen for KT88 in triode and in UL seem to bear this out. I couldn't live with the sound of a UL stage with no gNFB, unless there was some other kind of NFB employed to decrease distortion. It could be that the A470 OPT's in the ST70 are designed to be used with gNFB, so it's actually necessary in any amp using those OPT's. But I can tell you that the amp with triode-wired EL34's sounded cleaner than the same amp wired UL.

Maybe some other folks have tried this too.

A friend made an amp with EL34's into a pair of Hashimoto OPT's (very nice), with a triode/UL switch. Once -6dB of gNFB was applied, we could not hear the difference between triode and UL, although the UL position delivered about 10 more watts per channel.

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Old 12th June 2013, 02:10 AM   #56
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I am told that 6L6 and 807 were a little different in reality .
I was given over 100,000 tubes about ten years ago. They were all loose tubes that had been dumped into about 30 eight cubic foot boxes and then tossed around with fork lift trucks until they were headed to scrap when my friend bought the entire warehouse. There were thousands of broken tubes, most of which I took apart to see how they were made.

The 6L6 / 807/ 6BG6 types have been around for a long time. Some variation has been in continuous production since 1939. Over the years a lot of different things were stuffed into the glass with these numbers on them.

The original 807 and 1625 contained the exact same plate structure as the 6L6GB with some additional shielding around the bottom mica insulator. The 6BG6G had exactly the same structure as the 6L6GB without the extra shielding. After looking at enough tubes, including several hundred rush produced for WWII I have seen just about every permutation of 6L6 type guts inside an "807". So there can be differences, but it is possible to find 6L6GC guts in an 807 and 7027 guts in a 6GB6GA. I have working examples of both.

Just give UL a try with no loop .
My experiences are all with a Tubelab SSE. I like the sound of a triode EL34 for some music, in fact most music that does not have extreme dynamics or where 5 watts is enough. I have tried a UL EL34 with and without cathode feedback, and it lacks something. I can't say just what is missing, but it is louder, but the SE magic is missing. A UL KT88 with zero feedback is better, but a UL KT88 with CFB just rocks the house. Feed these 15 watts to some 15 inch OB coaxial speakers with 96 db sensitivity and the bass shakes the house across the street. Want to play Pink Floyd or Depeche Mode through an SE amp? Use a low impedance tube like a KT88 in UL with CFB through a big OPT, GNFB is not needed.
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Old 12th June 2013, 10:08 AM   #57
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Originally Posted by tubelab.com View Post
Use a low impedance tube like a KT88 in UL with CFB through a big OPT, GNFB is not needed.
Ok, I'm sold, which OPT do you recommend?
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Old 12th June 2013, 10:45 AM   #58
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Thanks for valve info .

UL without a loop wouldn't be modifying existing UL amplifiers . Also UL in every stage is a suggestion . I see a transformer as the purists route if so . Morph controls seem too complex . I have to stress that many loudspeakers demand a triode solution . On paper even that won't work . Works quite well in practice . Loudness control ? Perahps . A 1950's Wireless World ( 55 ? ) I read suggested a damping factor of 3 as a minimum . This was by experiment using a lightweight foil contact on the cone . The amplifier had variable damping factor . The cone of the speaker would have a stiffer suspension than now is my guess ?

Dynaco is an excellent example of the seemingly impossible made easy . The output transformers look too small . The rectifier valve is run hard . The input pentode does all the work in the amp . Only UL and loop feedback make it possible . The fact it works so well is testament to the pentode and Hafler . The inspiration for it comes from the maker of the 7199 if I am right ? The same generic idea in other amps I have seen . What the Dynaco proves is that mathematically correct solutions work exactly as predicted if the machine is a good example . Having built transistor amps all my life I am astonished any loop feedback valve amp will work , to work well is a miracle . My total respect for those who got it right .

I found by accident that UL of about 80% is nice with an EL 34 ( almost triode ) . My transformer had a spare tap ( SE ) . It was a bit like the high jump . It gave me enough to rattle the pole I had set as my target . I used KT 88 also . I have a mild preference for EL 34 . I use conventional cathode bias with a resistor and big electrolytic . I read somewhere that this bias method is to an extent auto correcting . My observation is it must be . Some of my valves are so old as to have no markings . My brother put them in ice cream tubs , otherwise I wouldn't have been able to tell . I run one valve at about 25% full power . There are no obvious differences except gain . Output valves if KT 88 varied between 39 and 45 mA . The spectra remained about the same . The EL 34 samples were closer , all about 65 mA . I would suggest the EL34 is a better understood device where cloning is concerned . The amp has no plate to plate nor loop feedback . All it has is the interplay between devices .

A question someone might answer . The friend who I made the amplifier for said that when valves die they go Super Nova . He said some designers even solder the cathode resistors in such a way so as to unsolder themselves . My cathode resistor is either 560 or 470 R ( 7 W ) . I propose to put a simple heat fuse next to it . Without going the transistor route any better solution ? I have proposed a meter with a red zone as I feel this doesn't happen overnight . I have seen this in the past and assumed it was grid failure .

Fixed bias is strange . I won't give up with it . I used a mixture of fixed and cathode bias to have an overdrive facility . It allows the valves to idle at about 70% the usual . It takes them to about 150 % if clipping . No positive or negative outcome . It did do one thing . It allowed an EL 37 to keep up with a KT 88 . As a transistor class B addict I know music is about 15% of the full sine-wave power . Valve amps are class A ( add numbers if you like ) . The rules can be bent a little . I would expect the EL 37 to have a long life in this arrangement if real music . The circuit I used is very much simpler than the TubeCad version and uses no semiconductors . Time constants are it's only problem .
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Old 14th June 2013, 11:15 AM   #59
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It's About Time & Ultra-Linear Line Stages

The pentode has the special possibilities for Ultra Linear feedback . The cascode could be called a fake pentode . Sometimes a better pentode . One problem I had was getting the high gain triodes to make usable cascodes . For example an ECC82 makes a good casode . Problem for me was it wasn't the step forward from a single ECC 81 that I had hoped for . ECC81 wasn't a good cascode , although I accept I know very little about them and probably got it wrong . I did make a very good transistor + ECC81 cascode .

It is rare to see UL shown as if it is a natural curve of a pentode . Here below it is shown . In a very early text about UL the curves where shown in triode and pentode with the comment that they seem to be opposites . A rather wonderful curve of UL was shown as a perfect set of curves . The reality is a little different . As the author said the perfect UL point would vary between valve types . Beam Tetrodes are to all intents and purposes pentodes in this application . I must get some high voltage MOS FET's and connect them in UL cascode , I have a hunch it might work . I have ISOFET's , they might bolt to the chassis and survive for a while . EL 34's cost about $20 ( Ruby ) , FET's are not worth the bother . Just to know mostly .

A question . Did anyone ever try a triode with FET source follower as an output stage ? The idea being to use a standard SE transformer from source to ground . It should have mostly triode spectrum and a nice low impedance . I said to a friend it should be better than a 300 B by a long way ( as linear and better bass , less hum ) . 2A3 would be a nice driving triode . The amplifier I just built has a stereo PSU . If I want 1000 V it is not a problem , 500 CT 500 might suit what I describe .

You might think I eat sleep and drink valves ? Not so . I found some unfinished chassis of my late brother a few months ago . Where I am now and being a novice is a few months apart . My brother was rather conservative . What he built was not likely to please him . He was by IQ a genius . I wish I had bought him a spectrum analyzer . He would have rejected what he had built . I saw many attempts to make it work in the circuit layout . Sad thing is I gave most of the spare parts to my engineer friend John . He said with glee he will build a Mullard circuit . My brother and John , what can you do ?

Did anyone use a transistor cascode in UL ?

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Old 14th June 2013, 01:18 PM   #60
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The cascode can substitute for a pentode when reduction of feedback capacitance and high mu is the aim. It is not sufficiently similar to properly work as UL; the upper g1 is only a very rough approximation to a pentode g2. Trying to run a cascode as 'UL' is almost bound to fail whichever valve you use. The cascode makes a poor output stage, anyway, as too much voltage is lost.
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