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4S Universal Tube Preamp Project (12A*7)
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Author:  mwhouston [ 31 Dec 2013, 02:33 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I test for maximum swing in volts before clipping and +\- 3db frequency points. Also noise level. 3db points at least 20hz to 20khz swing, anything??? and noise under 2.5mV. At low output voltage levels distortion is generally around 0.6% for these preamps. A two stage tube preamp with feed back I built some time back never got over 0.05% distortion even driving into heavy loads at large voltage swings. But these sound better.

Author:  paulb [ 17 Jan 2014, 17:38 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

About how much in parts would this cost to build?

Being a noob, a parts list would be helpful. At least for suggested caps, resistors, and transformer.

Thanks!

Author:  mwhouston [ 17 Jan 2014, 19:21 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Depending on your enclosure you should be able to builds the preamp for a few hundred dollars. This is based on a very cheap or free enclosure (use something at hand) and a wall wart AC supply. I use a bride rec for HT and generally supply DC to heaters. Use a small step down tranni backwards to generate 240V of AC from a 24V wall wart.

I always make list of parts direct from the schematic. Then draw my own PS and list parts for that.

Author:  paulb [ 20 Jan 2014, 23:28 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Some dumb questions. Ignore if they are too dumb.

1) Using a 24V wall wart, what should its current rating be if used in your circuit (the mark III)? example: 2000mA, 1.6A, 830mA, etc.

2) In your PS schematic what is the symbol to the right of the PT1, that looks like 4 capacitors in a circle? Is it 4 capacitors in a circle? What is that thing?

Author:  mwhouston [ 21 Jan 2014, 01:59 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Make the plug pack 1A to drive the heaters. Rectify the plug pack and regulate it to 12V to run the heaters in series. The ring of caps is a 0.01uf poly on each leg of the rec. bridge for HT only. You have to isolate the AC plug pack from the enclosure if metal. I use a short lead with a 6.5mm mono phone plug on the end. If the AC touches any earthed connection the tranni will die.

Author:  paulb [ 21 Jan 2014, 18:14 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Once I delved deeper into the forum labyrinth I discovered that you have answered these questions before. Thanks for answering them again.

Have you guys made, or thought of modifying this preamp to equalize an RIAA signal? Or is that an entirely different beast?

I really want to build a glowy phono preamp, point to point, to go with my audiosector chipamp. Any ideas?

Author:  RossD [ 21 Jan 2014, 18:25 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

There is the RIAA tube circuit in the RCA RC-30 manual. If you look, there are pdfs of the manual.

It uses two stages however. I think it listed under the tube schematics page, here:

http://diyaudioprojects.com/Schematics/ ... ematic.htm

Although, I'd like to combine the two designs into the same chassis and use some of the componets for both design. I am just not quite sure if it worth the effort.

Author:  jani_m [ 26 Jan 2014, 13:24 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi to all!
I'm new to the DIY audio and I found this thread while looking for a simple (headphone) preamp design for my first project.
I have an old German radio (tubes: ECC85 ECH81 EF89 EABC80 EM84 EL84 EZ80) and I was wandering if I could use the ECC85 (6AQ8) and EZ81 (6V4) tubes with this design? I believe that the gain with this tube will be smaller but (if I understand right) can be increased with the bypass capacitor.
I was also looking for a way to use the 'magic eye' (EM84). Can you give me some indications of how to implement it in this design?
Thanks for everything!

Author:  Suncalc [ 26 Jan 2014, 22:31 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

jani_m wrote:
I was wandering if I could use the ECC85 (6AQ8) and EZ81 (6V4) tubes with this design?
The short answer on the ECC85 is yes but the wiring will be different, of course. Gain will be relatively high. I would omit the cathode bypass caps if using this tube. Here is the load line design for the 4S using this tube.
Attachment:
ECC85 Load Line.png
The EZ81 should also work in this circuit as well.

Author:  mwhouston [ 27 Jan 2014, 02:39 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Suncalc: thanks for the input and I'm not even building.

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