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Old 10th February 2011, 01:11 AM   #1
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Default Lower gain 12AX7?

idly wondering... has anyone ever come up with some simple feedback circuitry that could be tacked on to a 12ax7 to give it the characteristics of a 12au7 or 12at7?
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Old 10th February 2011, 01:31 AM   #2
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You can add voltage shunt feedback which will affect (drop) your input impedance but will reduce voltage gain. Is that what you were looking for ?

Implementation will be typically an input resistor in the grid ( where the grid stopper usually is) and a resistor from the grid to the output past the output cap to prevent dc from going back to the input.
It isn't a trivial circuit and component values have to be chosen carefully.

While gain might be reduced it can hardly be expected to 'sound' like other tubes with similar lower gain ! Electrically too it can never be made to perform like 'other' tubes as their characteristics will be completely different.
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Old 10th February 2011, 01:33 AM   #3
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It's been done: Anode Follower.

Of course, this won't help with the limited current sourcing capability, but will get the gain down to the 12AU7, 'AT7 level.
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Old 10th February 2011, 02:02 AM   #4
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That's exactly what I described in my earlier post , without the maths !
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Old 10th February 2011, 05:46 AM   #5
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Just to have some numbers I did a quick simulation. Might give you an idea of what you will get with NFB.
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File Type: doc 12AX7 with NFB.doc (26.5 KB, 57 views)
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Old 10th February 2011, 01:59 PM   #6
kevinkr is offline kevinkr  United States
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I have no idea what the end application is, but there is another way to do it.

See attachment for details. Note also that this was not intended for applications where the required voltage swing is more than a few volts rms. (It's a low cost line stage circuit.) Also the approach rather significantly increases the already high rp of the 12AX7A and the cathode follower shown as part of the implementation is not window dressing for that reason. Information provided is for personal use only...
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File Type: pdf line_stage.pdf (66.6 KB, 104 views)
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Last edited by kevinkr; 10th February 2011 at 02:06 PM.
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Old 10th February 2011, 02:03 PM   #7
zobsky is offline zobsky  India
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12ay7perhaps?
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Old 10th February 2011, 03:09 PM   #8
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Always remember that what sounds good to you is what you should keep/use.....no matter what the rest of the world thinks of your implementation. In this matter your ears are your God !

When somebody says that XYZ produces the most accurate sound on planet Earth, it doesn't mean too much to you if you don't know what the other persons references / preferences are . In all likelyhood you might not like the other persons 'Best system'! But that's perfectly OK as long as you understand it.

So try all things that are possible for you and build your system. Spend 80% of your time listening to "music" ! The main reason why you are meddling with all these things.

So , try out all the circuits that you can try and pick your choice. Then put up your feet and listen to the best music you have irrespective of what others feel about your choice of equipment/schemes. You do have the final say in such matters !

Cheers.

Edit: Just a pointer. Check out the threads on the 12B4A preamps. I've tried it. I LIKE it ! You might too. Key reason for using this tube is that you can use it in standard configuration and it has very low gain without resorting to negative feedback. Lots of other reasons you will see in those threads. There must be other NOS tubes that can do this also.
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Last edited by ashok; 10th February 2011 at 03:16 PM.
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Old 10th February 2011, 08:55 PM   #9
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re: 'no idea what the end application is' - well, I did say it was idle speculation :-)... actually I was thinking that I had a shedfull of AX7s, and would maybe useful to be able to substitute them for others in the same family, in any application (although I was thinking of the driver in amps like the Red Light District).

As Miles says however, the limited current sourcing capability is likely to be the killer.
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Old 10th February 2011, 09:29 PM   #10
limono is offline limono  United States
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Well since it's a "speculative" post I dare to hijack it a little since I also have reducing gain related Q. Years ago I've got Audio Note kit4 6V6 PP stuffed board and some time ago a pair of old Dyna Z- 565 transformers. I heard kit and always liked it althoug my tube savy friends call it a "bad design". I put Z-565 instead of original AN OTO (8k primary) transformers and I have way too much gain with original 10dB feedback RC 6.8k/560pF network ,and overblown bass . My friend suggested getting rid of 6SN7 stage and driving 6V6 from paraphase splitter.What are my options with circuit intact .Increasing feedback (I'd like to lover it actually, loosing bypass caps on 6SN7 driver (counter productive since it'll inrease driver impedance and make this redundant stage according to my friend even more redundant). The circuit sounds quite natural and pleasing with good balance of resolution and natural detail. It doesn't image or project very well (little monophonic) but I can live with that.
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