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12AX7 gain test
12AX7 gain test
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Old 14th February 2018, 03:25 AM   #1
rubli is offline rubli  Mexico
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Default 12AX7 gain test

Hello
I am wondering if there is a commercial tube tester that specifically test for gain of 12AX7 tubes

I guess not all 12AX7 tubes are equal, and some has more gain than others, not sure if this is true.

Does it makes sense to build a test rig with a simple amplifier circuit ? or will all test equal ?

thanks in advance for your opinion

regards

Alexander
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Old 14th February 2018, 06:16 AM   #2
petertub is offline petertub  Sweden
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Why care ? Tubes differ, maybe +-20% but an amp that depends on a specific value
of gain ( or Gm or Ra ) is broken by design.
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Old 14th February 2018, 09:06 AM   #3
artosalo is offline artosalo  Finland
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With the below circuit you can sort 12AX7 tubes by gain.
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File Type: jpg ECC83_basic.jpg (49.3 KB, 243 views)
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Old 14th February 2018, 09:58 AM   #4
Koonw is offline Koonw
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The Valve Wizard

I can use it to determine the Mu of tube model, real triode or pentode strapped triode when in doubt. Together with Gm mode, it should a very handy tool to own without having spend much money.
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File Type: png Tube Mu tester-1.png (56.5 KB, 236 views)
File Type: png Tube Mu tester-2.png (54.9 KB, 227 views)
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Old 14th February 2018, 10:25 AM   #5
DF96 is offline DF96  England
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Tubes do not have "gain"; circuits have gain. Tubes have amplification factor and transconductance. Which parameter are you interested in?
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Old 14th February 2018, 08:30 PM   #6
audiowize is offline audiowize  United States
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Quote:
Originally Posted by petertub View Post
Why care ? Tubes differ, maybe +-20% but an amp that depends on a specific value
of gain ( or Gm or Ra ) is broken by design.
So all phono stages that don't use feedback are flawed by design?
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Old 14th February 2018, 08:37 PM   #7
rayma is offline rayma  United States
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Quote:
Originally Posted by rubli View Post
I am wondering if there is a commercial tube tester that specifically test for gain of 12AX7 tubes.
You want a tester that can measure transconductance. The application circuit
produces the voltage gain.
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Old 15th February 2018, 05:54 PM   #8
hpeter is offline hpeter  Slovakia
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Quote:
Originally Posted by rubli View Post
Hello
I am wondering if there is a commercial tube tester that specifically test for gain of 12AX7 tubes

I guess not all 12AX7 tubes are equal, and some has more gain than others, not sure if this is true.

Does it makes sense to build a test rig with a simple amplifier circuit ? or will all test equal ?

thanks in advance for your opinion

regards

Alexander
this tube has much internal resistance.
that means, even biggest possible Ra and next stage Rg will significantly drop the gain.
also the miller capacitance of big tubes loads the 12ax7 at treble..
to maximize gain&minimize distortion, you must put this tube under mosfet gyrator. it sounds much better.
Click the image to open in full size.
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Last edited by hpeter; 15th February 2018 at 05:56 PM.
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Old 15th February 2018, 06:22 PM   #9
petertub is offline petertub  Sweden
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Quote:
Originally Posted by audiowize View Post
So all phono stages that don't use feedback are flawed by design?
Depends on your expectations. If you want it to follow riaa curve you must
have a design that is not depending on tube parameters.
Same with the output level, here a simple adjustment pot could be used.

A good design ( imho) would accept any valid ECC83 and still work within
narrow limits.
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Old 16th February 2018, 08:09 PM   #10
petertub is offline petertub  Sweden
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I do like rubli's delay circuits !
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