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Any 4S Universal Preamp 12a*7 builds here
Any 4S Universal Preamp 12a*7 builds here
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Old 16th January 2019, 10:19 AM   #1
eldarvanyar is offline eldarvanyar  United Kingdom
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Default Any 4S Universal Preamp 12a*7 builds here

Hi all,

I am about to order the parts for my first valve build which is a 4S Preamp which can use any 12*7 tube.

Click the image to open in full size.

Click the image to open in full size.

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4S Universal Preamplifier for 12A*7 Tubes

I am going to order Hammond 369EX transformer as I am 8n the U.K. and so need 240v

https://www.mouser.co.uk/datasheet/2/177/300-951115.pdf

Further details of the design development and specs of the 4S are found here

The “Universal” Preamp | Cascade Tubes
Has any one built or used this preamp, or any views on the circuit design, or any advice?

Thanks

Last edited by eldarvanyar; 16th January 2019 at 10:40 AM. Reason: Further info
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Old 16th January 2019, 10:56 AM   #2
Eli Duttman is offline Eli Duttman  United States
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Quote:
Has any one built or used this preamp, or any views on the circuit design, or any advice?
Yes, save your money! That is a poor design. Truly universal is not possible. A volume control in the O/P circuitry is very bad. I could go on & on.

What are your goals?
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Old 16th January 2019, 11:19 AM   #3
Ketje is online now Ketje  Belgium
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Quote:
Originally Posted by Eli Duttman View Post
Yes, save your money! That is a poor design. Truly universal is not possible. A volume control in the O/P circuitry is very bad. I could go on & on.
Perhaps, but it's a nice simple thing to start with tubes
The only objection is the much to heavy rectifier tube.
You can use a EZ90/6X4 , much cheaper.But it has a 7pin miniature base, must replace the 9pin noval.
Mona
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Old 16th January 2019, 11:35 AM   #4
DF96 is offline DF96  England
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How is it that a circuit which has no purpose can be considered "nice"? That is, I cannot think of a purpose for this circuit in any modern audio system. It is not even a good learning exercise, except as an example of how not to design a circuit and how not to build it.
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Old 16th January 2019, 12:31 PM   #5
Ketje is online now Ketje  Belgium
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Well, by playing with it you find out how it works (or not )
It's by making mistakes that learn, no ?
Mona
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Old 16th January 2019, 01:03 PM   #6
eldarvanyar is offline eldarvanyar  United Kingdom
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Quote:
Originally Posted by Eli Duttman View Post
Yes, save your money! That is a poor design. Truly universal is not possible. A volume control in the O/P circuitry is very bad. I could go on & on.

What are your goals?
I had thought of simply purchasing a Marantz 7 clone preamp from eBay as a valve preamp

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12AX7B Tube Pre-Amp Vacuum tube Bluetooth preamplifier Based on the Marantz M7 | eBay

But I thought the money I could spend on that and not know about the parts being genuine or not, reliability, import duty etc I could attempt a simple build. I have always understood that the less components the better for sound etc

It appealed to me being able to change the 12*7 tubes without hassle, although people on the forum think that the 12au7 is the way to go.

What would you say is inherently wrong with the circuit?

I would be interested in building a simple preamp with something like the
630p eh as the preamp tube.

So over to you to suggest something simple and superior

Thanks
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Old 16th January 2019, 01:28 PM   #7
Dinolobe is offline Dinolobe  Canada
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I have built one of these, but with a few major differences.

First, the goal of the circuit was to be able to tube roll pretty much all the 12a_7 familly without any adjustments needed between tube changes. It helps to find wich tube sounds you like the most.

The volume Is on the output to avoid amplifying the noises the the pot creates. But yes, it changes the output impedence when you adjust the volume. if built this way, the 4s might not be able to drive all power amps, especially solid states ones.

My build is quite different than the original 4s preamp. A cathode follower buffer stage was added after the amplification stage in order to lower the output impedence. It allows the 4s to drive pretty much any amp out there. Mine also has a switch that allows me to choose between a low and a high output impedence depending on the amp that the 4s is driving. The low frequency roll off is than approximately correct for most solid states and valve power amp.

With the cathode follower added to the circuit, the volume pot was installed in the input, so the output impedence does not varry with each volume adjustments.

I donít think that this preamp was ever meant to be part of a main stereo system. Itís a toy for the tube geek to play around with. Itís also quite easy and fun to build in a point to point layout.
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Old 16th January 2019, 01:28 PM   #8
Eli Duttman is offline Eli Duttman  United States
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Quote:
It appealed to me being able to change the 12*7 tubes without hassle, although people on the forum think that the 12au7 is the way to go.
As previously stated, the claim of universality is false. The 12AU7/ECC82 is non-linear. The more linear 6FQ7/6CG7 and 12BH7 usually work in 'U7 circuits, providing the requisite doubled heater current is available.

Quote:
So over to you to suggest something simple and superior
Again, I ask what your goals are. What level of drive signal does your power amplifier require? Many amps need zero gain in the "line" stage. Either a passive or buffered control center is an appropriate mate for such an amp. Amplifying the signal and then attenuating it, to adjust overall gain structure, has an adverse impact on S/N performance.
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Old 16th January 2019, 01:30 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by Ketje
It's by making mistakes that learn, no ?
Making mistakes while trying to do something sensible is one way of learning. There are others too.

Making mistakes by choosing to do something which is not sensible will merely teach someone not to choose daft things to do.

Quote:
What would you say is inherently wrong with the circuit?
Almost any circuit which tries to accomodate 12AX7 and 12AU7 without other changes is making a big mistake. Their characteristics and biasing are so different that one circuit cannot cope with both. The circuit is only "universal" in the sense that it will probably behave somewhat badly with almost any valve and will be inappropriate for almost any audio system.

The circuit has too much gain.

The circuit has very high output impedance.

The circuit may have significant distortion.

The volume control is in the wrong place.

The rectifier is too big for the job.
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Old 16th January 2019, 01:36 PM   #10
Dinolobe is offline Dinolobe  Canada
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I strongly recommend this circuit if you ever decide to build this preamp
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