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Why do ECC82/12AU7 have a bad reputation in hifi?
Why do ECC82/12AU7 have a bad reputation in hifi?
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Old 2nd August 2021, 08:32 AM   #91
bayermar is offline bayermar  Germany
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Back to topic:
The non-linearity of the ECC82 can be dealt with to a certain extend and this tube has useful properties. So, I would not condemn it as useless for Hifi.
However, since there are many more linear tubes available, I wouldn't know why to use it as first choice (except maybe its wide availability - but that would be more of a concern in commercial products, not DIY).
I would leave it for guitar amps - where I like this tube and don't use it at its most linear operation point on purpose.
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Old 2nd August 2021, 08:53 AM   #92
sarcastic1 is offline sarcastic1  Russian Federation
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Originally Posted by 6A3sUMMER View Post
do not use any other negative feedback.

I recently designed and built a 7591 push pull, a 6L6 push pull, and the only feedback they used were the 40% ultra linear taps.

I also designed a KT66 single ended, and the only feedback it used was the UL tap (50%).
It's like comparing oranges and apples.
Ultralinear is negative feedback.

6L6 do not react well to UL, for the simple reason the close alignment of the screen grid.
807 + versions of it are near identical.
The KT66 is a close cousin of the 6L6 so would behave the same, as it was developed originally with a top cap (KT8C).
They are always betrayed by the extremely small screen grid current under full load, so much so I use a gas discharge shunt stabiliser for the screen supply.

The 7591 is a totally different animal.
massively hi gain, and a screen grid that takes gobs of current.
You will extinguish a big shunt stabiliser the moment you start driving a pair hard.

Being as it appears the 7591 would react well to negative feedback of that sort, it's impossible to compare the 2 applications of NFB, because they are not even remotely comparable.

Gillespie says the same.
There are good and bad applications of NFB in general especially that used on screen grids in UL applications.

I guess that's why the best 6L6-oid applications run them up in the Va, 600-850V zone and apply loop feedback, so there's a 400-500V differential between screen & anode supply.
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Old 2nd August 2021, 10:14 AM   #93
artosalo is online now artosalo  Finland
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6L6 do not react well to UL
I can not say how 6L6 etc. react to UL, but based on some 6L6, 5881 and 807 pp-amplifiers I can say that they work very well.
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Old 2nd August 2021, 11:42 AM   #94
JoeAlders is offline JoeAlders  Netherlands
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May I repeat Martin's remark in his #91 response?
"Back to topic"....
Because the thread starter asked a very interesting question is MHO. And I am following this thread with great interest.
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Old 2nd August 2021, 05:13 PM   #95
tonyp063 is offline tonyp063  United States
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Originally Posted by kevinkr View Post
Many ultra-linear amplifiers historically have used global negative feedback to lower output impedance and further linearize the amplifier output.

Global NFB is not mandatory with ultra-linear operation, that said I've not personally seen or worked on any amps that use UL without global NFB, but that doesn't mean they aren't out there. I think it is safe to assume there are UL amps without global NFB. (I find it hard to believe someone hasn't recently built such an amp.)
Unless I'm reading the schematic wrong (which is entirely possible) , Tubelab_com's SSE doesn't use global feedback. Just ~6dB of cathode feedback on the output stage.
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Old 3rd August 2021, 07:41 AM   #96
6A3sUMMER is offline 6A3sUMMER  United States
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I am sorry you understood what I said.

I said: Yes, there are push pull UL amplifiers out there that do not use any other negative feedback.

Simply put, the UL amplifiers use negative feedback in the form of the Ultra Linear output stage.
But some of those same UL amplifiers do not have any negative feedback of any other kind (no additional kind, such as: global, cathode, plate to plate, plate to control grid, unity coupling, Triode wired mode, etc.

Just saying
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Old 3rd August 2021, 08:08 PM   #97
sarcastic1 is offline sarcastic1  Russian Federation
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Yes too much NFB kills the sound dead as dead can be.

Mcintosh is the perfect example of all the bells and whistles, NFB in all directions, laboratory distortion figures so low they claim it on everything they write, yet a stultifying awful sterile sound that makes you want to run away,

Mac are actually just like one of the others I auditioned, the hideous EAR yoshino V12 with the ghastly looking bulgycases, and the blingly chromy JADIS amps in general with their ginormous but largely badly made and inefficient transformers and PSUs.
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