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4S Universal Tube Preamp Project (12A*7)
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4628
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Author:  mwhouston [ 28 Apr 2014, 19:30 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Suncalc wrote:
Ross;

Looks nice. :up:

One question... where's the fuse?

Yep looks nice. You could run an inline fuse in the base.

Author:  denny3249 [ 28 Apr 2014, 20:26 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

mwhouston wrote:
denny3249 wrote:
Would there be any problem with adding a selector switch to add two or three more inputs?

I always build mine with at least two selectable inputs and lately three selectable inputs.


Thanks.... Onward to start the build...

Author:  mwhouston [ 28 Apr 2014, 22:56 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

For God's sake try a quality 12AU7. In all my 4S builds I consistently found it sounded the best.

I just received 4 x matched Genalex Gold Lion gold pins for $168AU. Locally I would pay $52 each plus a little postage.

Author:  RossD [ 29 Apr 2014, 07:23 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I am going to use a plastic fuse holder that will get screwed to the inside. When I used the tube style with the compression cap, I realized I didn't have a drill bit big enough, so I used an alternate design.

I thought the Tung-Sol black glass 12AU7 was a quality tube... The NOS ones are expensive enough for me to consider them as such ;)

Author:  mwhouston [ 29 Apr 2014, 07:46 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

RossD wrote:
I am going to use a plastic fuse holder that will get screwed to the inside. When I used the tube style with the compression cap, I realized I didn't have a drill bit big enough, so I used an alternate design.

I thought the Tung-Sol black glass 12AU7 was a quality tube... The NOS ones are expensive enough for me to consider them as such ;)


The Tung-Sol sounds great to me.

Author:  whaam68 [ 03 May 2014, 06:52 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I've just started building this project. Are there any issues using an octal socket for 6sl7 etc? Then using adapters for the 9 pin dual triodes? I've got a few of all these in the spares box including a 6F8G which I can also get an adapter for.

Thanks

Mike

Author:  Suncalc [ 03 May 2014, 10:41 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

whaam68 wrote:
Are there any issues using an octal socket for 6sl7 etc? Then using adapters for the 9 pin dual triodes? I've got a few of all these in the spares box including a 6F8G which I can also get an adapter for.
Technically you can do this. However you need to understand that this design is only "Universal" from the standpoint of the 12A*7 dual triode family. The load resistance and bias points were chosen to get good performance from these tubes.

As far as the dual octals go there are some limitations. For example, the 100kΩ load used in this design is really too low for good performance from a 6SL7. With this tube the design will bias at about 1.3v,1.1mA. But the distortion will be unacceptably high unless the drive level is kept very low (~200mV). It would be a better design for the 6SN7 albeit a bit on the cool side. However, If you look at the 4S preamp thread here, you'll see that I have provided a much better solution for that tube as well. And I haven't even looked at the 6F8G.

Bottom line is that anything is possible. You just need to realize that with these other tubes you are in uncharted waters; and as they say "Here there be Monsters".

Author:  whaam68 [ 03 May 2014, 11:42 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Cheers Matt

I will build this one stock & review the other thread re the octals. 6F8G just looks good!

Mike

Author:  RossD [ 05 May 2014, 08:05 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I finished my 4S Build:

Here it is between my thrift store SS radio and receiver:
Image

It sounded great and even with my not so clean P2P wiring I couldn't detect any hum, but my basement was kind of noisy with the dehumidifier and freezer running.

On a different note, I've heard that 12BH7 is another good sounding tube. It just so happened that I have one that came in my tube case that my in-laws gave me. My power transformer can handle the increase current on the heaters, any idea if it will sound any good? I suppose I can just toss it in there and see how it works... :headphones:

Author:  mwhouston [ 05 May 2014, 08:53 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

P2P is the way to go. With careful layout you shouldn't even need shielded wire. Put up an image of the back.

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