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Old 30th June 2011, 12:36 PM   #1
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Default How to reduce gain in mu-follower?

Dear fellows,
Sorry to ask again a newbie question; too many things to learn...

How to design a mu-follower in such a way to get a dedicated gain?
I simulate the circuit with an overall load ob ~ 8K using two different tubes. Using the 6N14P I got an gain of ~26db at cathode ot the top triode and using an ECC82 ~ 24.5. The reality is 2 dB less than simulated.
I need only a gain of ~ 14dB because the stub R8 will be replaced by an 5:1 line out transformer. Unfortunately Iīm not able to use a transformer model ( again a leak of knowledge.. )

May be itīs a total wrong way to use a mu-follower in this case, but nevertheless I will learned a bit more ...

Thanks a lot for anny hint or suggestion.

Regards
Karsten
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File Type: gif Gain.gif (21.3 KB, 264 views)
File Type: gif Workline-ECC82.gif (31.3 KB, 255 views)
File Type: gif Workline-6N14P.gif (27.6 KB, 253 views)
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Old 30th June 2011, 12:56 PM   #2
DF96 is offline DF96  England
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Use a different valve. The voltage gain of a mu follower is a bit below mu of the bottom valve, whether cathode degeneration is used or not, because the upper valve presents a high impedance to the lower valve anode. Cathode degeneration may change the output impedance.

Alternatively, use feedback to create a hybrid of mu follower and anode follower. This will have a low input impedance, which might not be what you want.

You could try a genuine SRPP, which is a particular mu follower with R2 set to zero. Gain is then about mu/2.
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Old 30th June 2011, 01:04 PM   #3
TheGimp is online now TheGimp  United States
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The main way to change the gain of a Mu Follower is to change the lower tube to a lower mu tube. The circuit gets its name in part from the fact that the gain is equal to Mu of the lower tube with the upper tube acting as a current source. Current source anode loads allow the tube to achieve a gain of near Mu as it is working into a very high impedance in the form of a current source.

good sources of info:

http://www.freewebs.com/valvewizard1/mufollower.html

http://audio.fam-gelder.nl/index.php...-mu-stage.html
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Old 30th June 2011, 01:21 PM   #4
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Thanks a lot for your advise.

I tried a SRPP ( optimized ) before but found out it has to high output impedance using unbypassed Rk ( well, for "better" sounding ).Especial if the load is not constant. So I tried the mu-follower.
If the lower tubeīs mu define the gain I need an tube with mu ~ 5 which seemed to me unusal. The idea to use a local feedback sound good but I would not "loss" the 2nd harmonics of the lower tube if the feedback is to strong. How to do this local feedback in this case?
Otherwise I will go back to an ordinate gainstage with an CF...

Best regards
Karsten
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Old 30th June 2011, 03:44 PM   #5
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Well, for low gain (but superb fidelity) use let's say a 71A - or perhaps a 27 would do with a gain of 8?
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Old 30th June 2011, 04:02 PM   #6
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Another way to do it is to use negative feedback from a later stage to reduce the gain.
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Old 30th June 2011, 04:21 PM   #7
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I would use either different tube, or different line out transformer.
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Old 30th June 2011, 09:41 PM   #8
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Well, a 71A is unfortunately not an option. Itīs to old and need to much effort. To use an other transformer will lead me also in trouble. I know only three good manufactors: Sowter ( my favourite ), Lundahl and Jensen. None of them sells a 16:1 transformer which I need to come back to unity gain on output. The maximum of transfer ratio I found is 10:1 but this transformer has a low maximum input level.
Reading your expertise, the mu-follower seemed not to be the right choice. So I must come back to an odinary gain stage with an attentuation to bring down the level followed by an AC coupled CF to drive low loads.
Thank you all for your patience and suggestions. This is the best forum for tubes with the gentlest members. Thanks a lot!
Karsten
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Old 30th June 2011, 09:51 PM   #9
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You could take output from top of R5 ?
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Old 30th June 2011, 10:31 PM   #10
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by karsten21 View Post
Dear fellows,
Sorry to ask again a newbie question; too many things to learn...

How to design a mu-follower in such a way to get a dedicated gain?
Use negative feedback. A feedback amplifier is a lot less sensitive to variations between tubes and tube aging and will have a constant gain even as the tube ages or is replaced. The other thing that happens is that NFB will dramatically reduce distortion. Because distortion is caused by non-linear gain.

In your case I'd add enough NFB until you get the gain you need.

The only problem with using NFB around a stage like this is if you like to "roll" tubes and hear how they sound you will be disappointed, they will all sound alike
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