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Old 9th April 2011, 08:17 PM   #1
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Default Substituting a 12AX7 for a 12DW7

I have attached a schematic for a Fisher preamp. It uses a 12DW7 / 7274 in the final stage. The high mu section is the first triode and the low mu is used as a cathode follower.

As I understand it, the gain is developed in the first triode and the cathode follower is close to, but just under, unity.

If my understanding is correct, then that begs the question, why not just use a 12AX7, since the mu of the second stage is not really coming into play?

I'm sure there is a good reason and I'd like to be let in on it.

Thanks
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File Type: jpg Fisher 400-CX Schematic page 2.jpg (458.0 KB, 104 views)

Last edited by Captn Dave; 9th April 2011 at 08:20 PM. Reason: Commas are half price today, so I thought I'd throw in a few extras.
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Old 9th April 2011, 08:20 PM   #2
SY is offline SY  United States
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Source impedance (which is inversely proportional to gm) and current swing capability, especially driving any kind of capacitance. There may be a worse CF tube than a 12AX7, but not many.
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Old 9th April 2011, 08:28 PM   #3
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The 12DW7 is 1/2 'X7 and 1/2 'U7. The 'U7 section makes for a tolerable cathode follower.

If you are willing to work hard at reworking the layout, installing 'X7s and 'U7s may be feasible.
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Old 9th April 2011, 08:33 PM   #4
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That's interesting. I did't know that source impedance is inversely proportional to GM. I'd like to learn more about that. I have a copy of Morgan Jones 3rd ed; I'll do some reading.

If anyone can link me to some good reading on the subject, I'd be grateful.
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Old 9th April 2011, 08:34 PM   #5
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Quote:
Originally Posted by Eli Duttman View Post
The 12DW7 is 1/2 'X7 and 1/2 'U7. The 'U7 section makes for a tolerable cathode follower.

If you are willing to work hard at reworking the layout, installing 'X7s and 'U7s may be feasible.
No, a 12DW7 is not that hard to come by. It's the understanding I'm after.
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Old 9th April 2011, 08:40 PM   #6
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Jones explains it pretty thoroughly. Basically, the looking-in impedance is rp/(1+mu) because the plate is at AC ground. If mu >>1, then that reduces to 1/gm.
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Old 9th April 2011, 09:15 PM   #7
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OK, I get it. I'm re-reading the chapter on the common cathode triode amplifier and it is more clear now. Thanks SY.
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