Preamp with single 12ax7 Ecc83 for stereo?

Rodeodave · 9th June 2011, 04:38 PM

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?

kevinkr · 9th June 2011, 04:41 PM

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)

DF96 · 9th June 2011, 04:57 PM

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.

Ar4 · 9th June 2011, 06:01 PM

It also depends of your amplifier.. If it have low input impedance, you may need buffer, so it means one double-triode per channel.

Rodeodave · 9th June 2011, 07:34 PM

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...

hidnplayr · 9th June 2011, 08:10 PM

To reduce gain and output impedance, you could use an output transformer.

Sheldon · 9th June 2011, 08:45 PM

Quote:

Originally Posted by DF96

a carefully designed anode follower might work quite well.

Tube CAD Journal: Tube regulators part 2, July 1999

Ar4 · 9th June 2011, 09:16 PM

Sure, if impedance is so high, you can use anode follower, with feedback to reduce gain.

tipetu · 18th April 2012, 09:26 AM

I followed this schematic from a K-5 tube preamp.

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.

DF96 · 18th April 2012, 10:27 AM

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.

9th June 2011, 04:41 PM	#2
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	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) __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan

9th June 2011, 07:34 PM	#5
Rodeodave diyAudio Member Join Date: Dec 2004 Location: in the alps Blog Entries: 1	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... __________________ Gravity - Making the G since 13.7 billion B.C.

9th June 2011, 08:10 PM	#6
hidnplayr diyAudio Member Join Date: Sep 2008	To reduce gain and output impedance, you could use an output transformer. __________________ Real tubes have top-caps

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9th June 2011, 04:57 PM	#3
DF96 diyAudio Member Join Date: May 2007	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.

9th June 2011, 06:01 PM	#4
Ar4 diyAudio Member Join Date: Apr 2010 Location: Latvia	It also depends of your amplifier.. If it have low input impedance, you may need buffer, so it means one double-triode per channel.

9th June 2011, 09:16 PM	#8
Ar4 diyAudio Member Join Date: Apr 2010 Location: Latvia	Sure, if impedance is so high, you can use anode follower, with feedback to reduce gain.

18th April 2012, 09:26 AM	#9
tipetu diyAudio Member Join Date: Jan 2010	I followed this schematic from a K-5 tube preamp. 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.

18th April 2012, 10:27 AM	#10
DF96 diyAudio Member Join Date: May 2007	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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