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Old 19th October 2010, 02:09 PM   #31
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by hidnplayr View Post
Today I used the pentodes in long tailed pair.
I first tried the EF184, with 22k anode resistors etc like yvesm suggested.
It worked and has plenty of gain (a little too much even) But I was a little short on voltage to run the EF184 optimally (preamp B++ being around 300V)
So I replaced it with EF80 tubes and used 170V on screens and anode. Works like a charm now! (altough I might still raise the B++ voltage a little)
It's always a good idea to use as hi B+ as you can, even for lo level stages !
The only problem is to "clean" it specially if you have choosen a configuration having poor PSRR.
Void cascodes and use pentodes ! Just their screens should be clean.

My half penny.

Yves.
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Old 19th October 2010, 02:34 PM   #32
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Quote:
Originally Posted by waltube View Post
If you have a balanced input, why you want to use a LTP?
Common mode rejection ratio..
And the ease of the circuit. If you know a better way, teach me
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Old 19th October 2010, 03:09 PM   #33
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by waltube View Post
One question:
If you have a balanced input, why you want to use a LTP?
You mean LTP PI
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Old 19th October 2010, 03:22 PM   #34
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Quote:
Originally Posted by Yvesm View Post
Void cascodes and use pentodes ! Just their screens should be clean.
Why should cascodes be avoided by your opinion?
I think it works very well with the ECC88, and perhaps the ones with fets on bottom too (altough i still havent tried that yet)

pentodes add more (odd harmonics) distortion while a decent designed cascode with triodes adds very little distortion over the whole spectrum. (at least thats what the book says)
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Old 19th October 2010, 03:49 PM   #35
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by hidnplayr View Post
Why should cascodes be avoided by your opinion?
I think it works very well with the ECC88, and perhaps the ones with fets on bottom too (altough i still havent tried that yet)
Don't bother too much, I was joking . . . somewhat . . . cos it's true that cascode has vy lo PSRR and asks for vy clean supplies !
Quote:
pentodes add more (odd harmonics) distortion while a decent designed cascode with triodes adds very little distortion over the whole spectrum. (at least thats what the book says)
Yes, pentodes add more odd harmonics when the load line enters in the curved region near the knee, but it's not so difficult to stay away in a lo level stage where efficiency is seldomly an issue.
Using more plate current and voltage along with a lower plate load resistor may help if you accept to have'em a bit hotter and to loose some gain.
This "pushes" the load line away from the knee.

However, like triodes, they produce even harmonics also that should be cancelled in a symetrical design as the LTP is.
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Old 19th October 2010, 07:51 PM   #36
waltube is offline waltube  Italy
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You have , in theory, the maximum common mode rejecton because you are using a balanced signals. That's all.

You can use a simple two stage, one for gain and another to drive (with a little gain also or CF) the output stage.
At this link there is a simple circuit (input) with 6SL7 and 6SN7 to drive 4 pair of EL34:
http://www.multitask.it/bigamp/ingresso.bmp
the trafo in input is a 3575 from Sowter, but for use with balanced is omitted.
This is the ouput:
http://www.multitask.it/bigamp/uscita.bmp
the secondary is floating with a reference to ground with two resistors for feedback.
This project is based on the ouput trafo ( double C core); without feedback the BW is around 60 KHz, with the 20 Hz at less than 1 dB. With 16 dB of FB we reach about 120KHz (100kZ at 140 Wrms).
I test also a trafo with secondary with center tap; may be easy to get the FB signal
These are the photos for the article I wrote for Audioreview in Italy:
http://www.multitask.it/bigamp/apertura1.jpg


My suggestion is to spend time and money mainly for the output trafo instead to test different circuits where ( and this can be the case) you don't need complcations.

It is only my opinion

Ciao

Walter
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Old 21st October 2010, 08:13 AM   #37
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Quote:
Originally Posted by waltube View Post
You have , in theory, the maximum common mode rejecton because you are using a balanced signals. That's all.

You can use a simple two stage, one for gain and another to drive (with a little gain also or CF) the output stage.
At this link there is a simple circuit (input) with 6SL7 and 6SN7 to drive 4 pair of EL34:
http://www.multitask.it/bigamp/ingresso.bmp
the trafo in input is a 3575 from Sowter, but for use with balanced is omitted.
This is the ouput:
http://www.multitask.it/bigamp/uscita.bmp
the secondary is floating with a reference to ground with two resistors for feedback.
This project is based on the ouput trafo ( double C core); without feedback the BW is around 60 KHz, with the 20 Hz at less than 1 dB. With 16 dB of FB we reach about 120KHz (100kZ at 140 Wrms).
I test also a trafo with secondary with center tap; may be easy to get the FB signal
These are the photos for the article I wrote for Audioreview in Italy:
http://www.multitask.it/bigamp/apertura1.jpg


My suggestion is to spend time and money mainly for the output trafo instead to test different circuits where ( and this can be the case) you don't need complcations.

It is only my opinion

Ciao

Walter
I myself dont like the use of the input transformer
I do like to experiment with different circuits. The building of the amp and knowledge gained while doing so is more important to me right now then the date when it's finished or how it looks when it is. The sound offcourse also matters.

My initial question has been answered so I guess the mission of this thread has been accomplished
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Old 21st October 2010, 08:52 AM   #38
waltube is offline waltube  Italy
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The input transformer is only for unbalanced input.

I understand what you mean but to reach a good results you must have the right test equipmets so you can check the different circuit you are developping.
I hope you have these stuff, the job will be easier.

Ciao

Walter
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