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Old 9th June 2011, 04:38 PM   #1
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Default Preamp with single 12ax7 Ecc83 for stereo?

I have a few single 12AX7/ECC83 tubes from various manufacturers, so I was wondering if there are any good designs than use only one single tube for a stereo preamp. I don't need a lot of gain, if any, just a simple circuit with one triode system per channel.
A search didn't bring up much, and the only circuit I know of that does use a single 12AX7/ECC83 for stereo is the Grundig CS200 tuner's output stage. There must be more. I attached a pic of said output stage and the pdf with the full circuit.

Can anyone point me to other schematics using a single 12AX7/ECC83 for a stereo preamp?
Attached Images
File Type: png ecc83_ell80_CS200.png (78.6 KB, 636 views)
Attached Files
File Type: pdf Grundig CS200 .pdf (104.6 KB, 131 views)
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Old 9th June 2011, 04:41 PM   #2
kevinkr is offline kevinkr  United States
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I did post a design using a pair of 12AX7A/ECC83 for a line stage here sometime in the not too distant past. Might try searching for it. (Line stage thread or similar)
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Old 9th June 2011, 04:57 PM   #3
DF96 is offline DF96  England
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Low gain and single ECC83 do not go together too well, as it is a high gain valve. There are basically two ways to get low gain from this valve:
1. use a low anode load, and accept the distortion you get - maybe OK for a guitar effects box.
2. use lots of negative feedback, so you will get low distortion until you get near the clipping threshold - a carefully designed anode follower might work quite well.
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Old 9th June 2011, 06:01 PM   #4
Ar4 is offline Ar4  Latvia
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It also depends of your amplifier.. If it have low input impedance, you may need buffer, so it means one double-triode per channel.
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Old 9th June 2011, 07:34 PM   #5
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I don't think that the amp's input impedance will be a problem, my mini Aleph has around 220k I'd guess.
Distortion I can live with, since I'm simply looking for a fun project for these unpaired 12AX7 tubes.
I'll dig out some tube literature, maybe I can cook up something...
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Old 9th June 2011, 08:10 PM   #6
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To reduce gain and output impedance, you could use an output transformer.
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Old 9th June 2011, 08:45 PM   #7
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by DF96 View Post
a carefully designed anode follower might work quite well.
Tube CAD Journal: Tube regulators part 2, July 1999
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Old 9th June 2011, 09:16 PM   #8
Ar4 is offline Ar4  Latvia
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Sure, if impedance is so high, you can use anode follower, with feedback to reduce gain.
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Old 18th April 2012, 09:26 AM   #9
tipetu is offline tipetu  Norway
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I followed this schematic from a K-5 tube preamp.

Click the image to open in full size.

Swap some resistors and stuff since this is a 5670 tube. Also, remember C5=22uF otherwise you`ll get no stereo. Dont ask me why, it`s just the way it is.
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Old 18th April 2012, 10:27 AM   #10
DF96 is offline DF96  England
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With high output impedance this is a good example of how not to design a line stage. Simply changing the cathode resistor to change gain is rather crude, as it will also significantly change the bias point - with a range from 100 to 8.2k they can't all be near optimum. Omitting C5 gives a little cross-talk between channels.
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