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4S Universal Tube Preamp Project (12A*7)
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Author:  diyAudioProjects.com [ 15 Dec 2013, 11:34 ]
Post subject:  4S Universal Tube Preamp Project (12A*7)

Please post your comments and questions of the 4S Universal Tube Preamp Project (12A*7) in this thread.

Image

For alternate power supply ideas see Mark's 4S Universal Valve Preamplifier builds.

Project Link - 4S Universal Tube Preamp Project (12A*7)

FAQ
  • Load lines for the various 12A_7 tubes with the Universal Preamplifier circuit are attached in this forum post.

Author:  gofar99 [ 15 Dec 2013, 15:33 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi, Nicely done. I have always wondered if the very early tube designers planed on being able to use the same value of cathode resistors for biasing most of the tube with the 12A_7 base. How did you determine the distortion levels?

Good listening
Bruce

Author:  Suncalc [ 15 Dec 2013, 16:34 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Distortion levels are theoretical and determined in the usual way via ratios of plate current values at specified conditions. I really should post the load line designs for the various tubes; in the interest of education.

Author:  Suncalc [ 15 Dec 2013, 17:19 ]
Post subject:  Load Lines - 4S Universal 12A_7 Tube Preamp

As promised, here are the load line designs for the 5 different tubes in order of increasing gain.
Attachment:
12AU7 4SU.jpg

Attachment:
12AV7 4SU.jpg

Attachment:
12AY7 4SU.jpg

Attachment:
12AT7 4SU.jpg

Attachment:
12AX7 4SU.jpg

As always, question and/or comments are more than welcome.

Author:  RossD [ 16 Dec 2013, 09:33 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Thanks to Matt and everyone for this community and the body of work that is developed by it. It truly is a joy coming here, learning and building!

My last couple of components will be arriving today, weather permitting, for my 4S Universal preamp. I just need to find a chassis...

I will be using Matt's tube power supply as opposed to Mark's solid state design. Hopefully I'll have time before Christmas to finish it. (Then on to my 2A3 EH Gold build...)

Author:  ckleung17 [ 26 Dec 2013, 08:20 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I built a unit using Matt's PSU today. I am a tube amp beginner and would like learn more charactistics of valves, this schemetic and Matt's testing result is very helpful to me.

Attachment:
SAM_1205.jpg


Attachment:
SAM_1206.jpg

Author:  Suncalc [ 26 Dec 2013, 12:40 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Beautiful Build! :thumbsup:

How does it sound?

Author:  ckleung17 [ 26 Dec 2013, 22:36 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

The sound is clear and dynamic :P . I would like to reproduce your results with my unit, but I don't know what equipments should I need? I have a multi-test meter, an audio generator and an oscilloscope.

Author:  Suncalc [ 27 Dec 2013, 20:19 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

ckleung17 wrote:
I would like to reproduce your results with my unit, but I don't know what equipments should I need? I have a multi-test meter, an audio generator and an oscilloscope.
If you mean the gain and phase plots, what you really need is a signal generator, an oscilloscope, and, if possible, a good AC voltmeter. Your multi-test meter will not really help because they will generally only produce accurate AC readings at 50Hz to 60Hz. Even the True RMS volt meters like the Fluke 179 are only specified to measure AC voltages from 45Hz to 500Hz. If you look at the picture of the unit under test on the project page, you'll see the Leader LMV-185A Dual AC voltmeter that I use. This unit is specified to produce readings accurate to 2% from 20Hz to 200kHz

The best way to take the gain measurements if you don't have a wide-band AC voltmeter is to read voltages directly off of the oscilloscope. Just make sure to periodically zero the inputs to make sure that the traces are still baselined to zero volts. The phase measurements are slightly more problematic. The way I did it was to use a plotting compass and take measurements of the relative zero crossings between the input and output waveforms accounting for the 180deg mid-band phase shift. If you take a measurement, divide it by the length of one full cycle, and multiply by 360, you get the relative phase shift in degrees. Then just repeat both procedures at a number of frequencies to generate the response plots.

Author:  mwhouston [ 30 Dec 2013, 18:01 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

ckleung17 wrote:
I built a unit using Matt's PSU today. I am a tube amp beginner and would like learn more charactistics of valves, this schemetic and Matt's testing result is very helpful to me.

Clean and neat build, very smart.

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