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Old 6th September 2005, 12:47 PM   #1
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Default UL vs SE

Hi,

I'm evaluating a SE amplifier, using KT88 tubes.
I found two different, interesting schematics:

Click the image to open in full size.

Click the image to open in full size.

One uses a so-called UL schema, the other one is a simple SE (I hope I'm correct), with a local feedback on the secondary of the output transformer.

My questions are:
Which are the difference between these two schematics?
I mean, I can understand how a local feedback works (at the end, I'm an electronic), but not exactly how the UL schema does (I'm a newbie with tubes)... so, why to use a UL tap?
Which are the benefits against a standard local feedback?
The distorsion is lower?
The bandwidth is higher?

Ciao,
Giovanni
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Old 6th September 2005, 02:05 PM   #2
Sherman is offline Sherman  United States
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Hi Giovanni,

Both schematics are single-ended. The first schematic shows the tube connected in ultralinear mode. The screen is connected to a tap on the output transformer.

The second schematic gives you a choice between pentode mode and triode mode. Pentode mode connects the screen to the power supply. Triode mode connects the screen to the plate, effectively making the KT88 into a triode.

Common wisdom has it that triode mode sounds best, pentode mode gives the most power (and distortion) and ultralinear is a compromise between them. In most pentodes pentode mode will give about 2 to 2.5 times the output power of triode mode with ultralinear between them somewhere. For instance and EL84 can give you 5.7 watts in pentode mode and about 2 watts in triode mode.

In order to use ultralinear mode your output transformer must have an ultralinear tap. Without that tap you are limited to either pentode or triode.

It is possible to install a rotary switch that will let you change your amp between pentode, ultralinear and triode modes. Connect the output of the switch to the screen grid of the tube. On one position solder in a 100 ohm resistor (5 watts would work) and connect the other end of the resistor to the plate pin. On the next position make the connection directly to the ultralinear tap on your OPT. In the third position make a connection to your B+. Now turning the switch changes the mode.

Note that the best operating point for the tube changes with mode so it is unlikely that the operating point chosen for say triode mode will be the best point for pentode. However you can still get an idea of the changes in sound.

My main amp currently is a set of SE KT88 monoblocks running in ultralinear mode and I think they sound great. I am contemplating adding a switch to change modes.

I also just finished a single-ended EL84 amp and have a pentode/triode switch installed. There is no question that triode sounds much better than pentode on that amp even though the power drops from about 5.5 to 2 watts.
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Old 7th September 2005, 10:03 AM   #3
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Thanks for the answer, Sherman

I do not care about power, my "listening room" (my living room, in reality) is small, about 18 square meters...
So I think I do not need to go with power.
I love class A amplifier (up to now I realised only solid state class A amplifiers, like Pass Lab amps), so my first goal is to have the lowest distorsion and the best sounding amp (whatever it means).
I think the power of a single KT88 connected in triode or ultralinear mode will be enough for me, I ws trying to understand the different behaviour of ultra linear vs. triode, etc.

One more question: both the schematics use a feedback from the output, I've seen also sothers schematics without any feedback: do they work fine, better or worse than these two, and why?

Sorry if I'm boring you, but I'm trying to learn as much as I can before to start with building...

Ciao,
Giovanni
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Old 7th September 2005, 10:03 AM   #4
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Default Still another question

Sherman,

Could you be so gentle to send me a copy of your schematic?
I'm "collecting" them to choose the "best" one...
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Old 7th September 2005, 10:18 AM   #5
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Hi croccodillo,
I think you have to valutate also more "classical" schematics. For example a single ended of EL34(in triode mode) togheter with a triode signal tube, without FB.

Cheers.

P.s. Detta tra noi le migliori configurazioni sono a triodo, in assoluto. Ancora meglio quelle che usano come valvola finale un triodo a riscaldamento diretto, per esempio la 300B o la 2A3.
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Old 7th September 2005, 10:51 AM   #6
Giaime is offline Giaime  Italy
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Quote:
Originally posted by TheGatesOfFate

P.s. Detta tra noi le migliori configurazioni sono a triodo, in assoluto. Ancora meglio quelle che usano come valvola finale un triodo a riscaldamento diretto, per esempio la 300B o la 2A3.
Ahahaahah ma perchè lo dici in italiano? hai paura che qualche "straniero" te lo contesti?

Dai le affermazioni facciamole tutte in inglese per piacere, è diritto di tutti leggerle e replicare.

Ciao
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Old 7th September 2005, 11:42 AM   #7
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Quote:
Originally posted by Giaime


Ahahaahah ma perchè lo dici in italiano? hai paura che qualche "straniero" te lo contesti?

Dai le affermazioni facciamole tutte in inglese per piacere, è diritto di tutti leggerle e replicare.

Ciao
L'ho detto in italiano perchè ad un certo punto mi sono sentito come un cretino a dover parlare in inglese con un italiano.
Paura di contestazioni straniere? naaaaaa!

Anzi, fai pure la traduzione del tutto, così tutti capiscono.
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Old 7th September 2005, 12:07 PM   #8
Giaime is offline Giaime  Italy
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Oh good... nothing worth translating. He said in italian about the superiority of directly heated triodes over other tubes. I said if he was saying it in italian to avoid contestations from other people, and he said nope, he wasn't doing that, he felt just embarassed to speak in English with another italian.

Sorry for the OT...
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Old 7th September 2005, 12:09 PM   #9
Sherman is offline Sherman  United States
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Quote:
Originally posted by croccodillo
Thanks for the answer, Sherman

I do not care about power, my "listening room" (my living room, in reality) is small, about 18 square meters...

... so my first goal is to have the lowest distorsion and the best sounding amp (whatever it means).

I think the power of a single KT88 connected in triode or ultralinear mode will be enough for me...

One more question: both the schematics use a feedback from the output, I've seen also sothers schematics without any feedback: do they work fine, better or worse than these two, and why?
...
Giovanni,
A single-ended KT88 in ultralinear mode is capable of about 5-8 watts depending on the operating point you select. In triode mode you might get half of that.

Generally triode mode gives lower distortion than pentode or ultralinear. Feedback is used to help control distortion with the loss of some gain.

Distortion is one of those things that is hard to discuss. With tube amplifiers harmonic distortion is mostly second order and other higher even order harmonics. To the ear that type of distortion usually sounds fine. With solid state equipment harmonic distortion is usually odd order which sounds bad to the ear.

A single-ended tube amp running in triode mode with 4% distortion would probably sound great to most people depending on their preferences.

Here is the schematic I used for my KT88 amps. It was designed by Mikael Abdellah. Do a search here for that name and you can find several threads about the amp with a lot of good info.
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File Type: jpg single ended kt88.jpg (54.0 KB, 525 views)
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Old 7th September 2005, 03:40 PM   #10
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I loooove Mikael's schematic....
But I'm a little bit confused: how much power can such an amplifier produce?
It is ultralinear, so, I suppose, about 5-8 Watts, as you reported?
And, what is the "operating point" you mention?
Sorry for my stupid questions, but I'm a beginner...

Ciao,
Giovanni
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