low gain 12ax7 schematic

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#10
search for 5687 line preamp schematics here and on net ;

I abandoned these search in last several years because probably I see them all and I usually make my own things from scratch .

in any case - stage with lower mu tube ( without or with moderate global feedback) will sound much better than any stage with drekload of global feedback

YMMV , as always ......
 
#16
bakpaosiomay said:
can any one suggest me a schematic?
i search for low gain 12ax7 preamp.
but i dont find any.
i wanted gain under 10
because my amp already have plenty of gain.
i really appreciate if anyone can give me the schematic.
thanks
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If you need low gain from an AX type tube, maybe the best way, after using a reasonably low valued plate load resistor, is to use local feedback. You will get flatter frequency response and low distortion using feedback I can't draw a schematic but the feedback is just a resistor from plate to grid.

Theory (in simple terms) and an example with gain of 8.5 using the 7AX12 is here
http://www.freewebs.com/valvewizard/localfeedback.html

If you need a gain of 1 try a cathode follower.
 
#18
12AX7 belongs to the big family of nonlinear tubes
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Have to disagree. It takes some effort, but the linearity of a 12AX7 is astonishingly good IF one accounts for the high mu and plate resistance. With a CCS plate load, LED cathode bias, and a follower to keep the plate load high, THD at 19VRMS out is better than 0.04%, almost totally 2nd. That's pretty linear.

The problem is that the rp is very high, so that the plate load MUST be high to run the tube linearly. There are a lot of circuits out there with 100-150k plate loads, and that's much of the source of the ECC83's bad reputation.
 
#19
bakpaosiomay said:


because i already have the tubes.
beside 12ax i have 5687 and 1pc e88cc too.
any suggestion?
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Consider using your E88CC or 5687 as the preamplifier. Each has two triodes in the valve tube, so you only need one tube to do stereo.

Looking at the E88CC datasheet suggest a reasonable operating point is...
-2V bias
7.5 mA
80V anode voltage

(Actually the 5687 will work at about those settings too).

Some feedback from anode to grid (via a capacitor please) will reduce the gain. The output impedance is going to be a little high, so maybe you would need to follow it with a cathode follower stage. So maybe the E88CC as the amplifier part and the 5687 as the cathode follower?

http://www.mif.pg.gda.pl/homepages/frank/sheets/009/e/E88CC.pdf

http://www.r-type.org/pdfs/5687.pdf

http://www.tubezone.net/pdf/12ax7ecc83.pdf

One final thing – if your amp has plenty of gain then maybe you do not need a preamplifier at all, in which case consider just using a potentiometer of about 5k to 10k between the source and the amplifier.
 
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