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Old 11th December 2012, 11:24 PM   #21
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It's worth noting that triodes are the only fundamentally somewhat-constant-voltage linearizable* amplifying device. All others have a constant current characteristic: pentodes, BJTs, FETs.

*Linearizable, as opposed to devices like SCRs, which arguably are constant voltage (it switches on and stays right about 1.5V, period!), but cannot be operated in a continuous, linear fashion over their rated range (lots of hysteresis).

SCRs actually share another feature in common with vacuum tubes: they are the only two active devices which conduct no current in reverse bias. BJTs and IGBTs can be reverse biased, but (depending on base drive, for the BJT), only by Vebo (Vec for IGT) at most. FETs are either symmetrical (in which case, arguably, whichever electrode is more negative is the source; it can never be "reverse biased" by such a definition!), or have an intrinsic reverse diode characteristic (almost all MOSFETs).

This means you can't build a general-purpose, voltage source, half bridge inverter out of tubes, without adding diodes at least. SCRs require either a resonant load (resonance induces commutation) or a current source power supply (general purpose).

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Old 12th December 2012, 06:49 AM   #22
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Quote:
Originally Posted by Merlinb View Post
Actually, ultra linear mode gives higher power but *lower* distortion than triode mode. That's why its called ultra linear.

This is not quite the same as ordinary distributed loading which works on the same principle but doesn't tap the screens at the right percentage OT primary to get the lowest possible distortion.
This is not my impression. Ultralinear is a flashier term for the same thing, If it is a different thing, then in all amplifiers being sold as such, the "perfect" balance has been determined and implemented?

I d like to see some links, in any case where the "perfect" percentage of tapping is determined, thus resulting in ultralinear operation, versus ordinary distributed load.

Last edited by costis_n; 12th December 2012 at 06:51 AM.
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Old 12th December 2012, 07:09 AM   #23
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Quote:
Originally Posted by costis_n View Post
.....I d like to see some links, in any case where the "perfect" percentage of tapping is determined, thus resulting in ultralinear operation, versus ordinary distributed load.
Download this: www.tubebooks.org/Books/York_Amplifiers.pdf

And look at the page 214. There are some research findings.
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Old 12th December 2012, 10:13 AM   #24
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by costis_n View Post
This is not my impression. Ultralinear is a flashier term for the same thing, If it is a different thing, then in all amplifiers being sold as such, the "perfect" balance has been determined and implemented?
I suspect in a lot of cases they simply use the term 'ultralinear' to advertise the amp without actually knowing what it truly means,* in which case they're really just distributed-loaded amps. In other cases they will have got the tapping point from the data sheet, so the tube manufacturer should have found the optimum point (e.g., about 40% for KT88s).

*For example, I think a lot of people assume that when Hafler and Keropes patented the UL idea, they were pateneting the general idea of distributed loading. This isn't true- they knew perfectly well that it was an old idea (one of Blumleins). They were only patenting the idea of an optimum tapping- which is why they gave it a special name.
http://www.oestex.com/tubes/ul.html

Last edited by Merlinb; 12th December 2012 at 10:32 AM.
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Old 12th December 2012, 10:20 AM   #25
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The nice document artosalo posted shows 43% for KT66. I believe this was determined experimentally. If there are no formulas to calculate the "ultra" ratio, this leaves a kind of a gap , don't you think?
Same for ready made transformers that are marketed as made for a variety f different output valves.
On the other hand I only see british manufacturers use the term"distributed load", so I think its just like valve - tube, i.e. british vs american naming convention.
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Old 12th December 2012, 10:34 AM   #26
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IIRC Mullard call it distributed load and give curves for 20% and 43% taps. I also seem to recall there were some arguments across the Atlantic in the pages of audio journals on exactly what had been invented, by whom and when, and what it should be called. It may be that this was provoked by the standard US habit of patenting some minor detail of an accepted practice widely known and used by others, presumably in the hope that strictly non-infringing users will feel bullied into paying for a licence just in case the lawyers get nasty and the judge doesn't understand what he is judging.

Just noticed that Blumlein patented it in the UK, but then died long before it expired - perhaps leaving nobody to defend it?

Last edited by DF96; 12th December 2012 at 10:49 AM. Reason: extend
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Old 12th December 2012, 10:40 AM   #27
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Quote:
Originally Posted by costis_n View Post
The nice document artosalo posted shows 43% for KT66. I believe this was determined experimentally. If there are no formulas to calculate the "ultra" ratio, this leaves a kind of a gap , don't you think?
A gap for what?

There is no analytical formula for calculating the ultralinear tapping because the whole system is nonlinear. You would need to know the transfer function of the screen grid and control grid, and even if you knew that, the maths would be truly horrendous since you have multiple feedback loops.
An accurate SPICE model would do it for you though, if such models existed.
Moers appears to have attempted a graphical method, but as far as I can tell it is nonsense.
http://www.oestex.com/tubes/Moers%20UL_2.pdf

Quote:
On the other hand I only see british manufacturers use the term"distributed load", so I think its just like valve - tube, i.e. british vs american naming convention.
No, the term ultralinear is very specific- as defined by Hafler and Keroes.** That is exactly why the British use the term 'distributed loading' for the general case- to avoid confusion.* (The British tend to be more fastidious about language than the Americans!)
But in the US I don't think they ever settled on a particular term for the general case; they just talk about screen feedback or similar. Over time the issue was clouded, so now a lot of people don't realise that ultralinear is a special term that applies to one particular case of screen feedback. For example, the American website I linked to above appears confused about the distinction.

*Mullard adopted it first, but it caught on.

**Take a look at fig.2 to see how narrow the UL range is!:
http://www.aikenamps.com/UL.pdf

Last edited by Merlinb; 12th December 2012 at 10:54 AM.
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Old 12th December 2012, 12:48 PM   #28
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Thank you both for the answers, very insightful.

The question is: Considering that as input G2 follows a different law than the anode, thus introducing nonlinearity should we bother (from a theoretical view)?
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Old 12th December 2012, 01:10 PM   #29
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Quote:
Originally Posted by costis_n View Post
The question is: Considering that as input G2 follows a different law than the anode, thus introducing nonlinearity should we bother (from a theoretical view)?
Not sure what you mean by this? Feedback to the screen grid reduces distortion, and isn't that what we're trying to acheive?
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Old 12th December 2012, 02:16 PM   #30
DF96 is offline DF96  England
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The important thing is the g2 roughly follows the same shape law as g1. The anode doesn't matter, as it merely scoops up the electrons which missed g2.
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