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PostPosted: 05 Sep 2011, 21:52 
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Matt: Ginger Beer (Ozz for engineer) or no Ginger Beer every stage added, every component in the sig. path, every correction for sins committed against the original sound will only bugger the sound more. Keep your hand in your pockets and your ears open. The only change, completely left of center, I would like to see, is a transformer output stage. NO caps in the preamp sig. path. That should keep you off the streets for a while.

For those who wish to build the Universal at this point I would not say it is the best when compared to it's ancestors. I'm still listening to the MKII (The Black) which I feel is the best to date. Until I construct a Universal and compare it "The Black" with a GD 12AU7 is my axe of choice.

I finally have the Oatley K303 phono preamp kit up and going (sounding "sick as" ) so time to move on. The Universal V1.0b coming up.

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PostPosted: 09 Sep 2011, 21:53 
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I am over half way through building the Universal. This time on vera board (line board). This has to be the easiest build so far. I am going to include the bypass cap for the cathode resistor to make this a distinctive sounding build compared to the black and MKII.

No special solder used here, a very standard build. I have some 12AX7 and 12AU7 GD tubes coming for this one. Plus I have a handfull of second hand tubes to roll. This build is in a Hammand case as I used with BoZ and the B1. Should look smart.

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PostPosted: 09 Sep 2011, 23:51 
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nice one mark.
i've built the power supply, stopped by the frequent flyer electronics store this morning and grabbed a case to throw it in
also managed to score a wire wrap tool from protogear for 22 bucks which is a bargain.
will build the circuit tomorrow

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PostPosted: 10 Sep 2011, 00:04 
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Here is the cct. brd. very close to finished. The heater PS and HT PS run either side of the brd, similar to my other builds. The strip board has made this an easy build. The amp section is at opposite end to the tranni. This layout, keeping the AC bits from the amp bits, makes this a very quiet preamp with only well filtered DC for both HT and heaters reaching the amp section. The heater voltage again is regulated.

Because the K303 phono preamp sounded so good using the MKP caps I decided to use them here. I happen to have a pair of 0.47uf/250V on hand. I was to use some 0.68uf but Matt thought 470nf for output caps would be better. And I used two 33uf electros to bypass the cathode Rs. They are very small so dropped in easy. Again no hook-up wire in the amp section.

Now I can't wait to here it. Time to get off the keyboard.

Attachment:
MKIVCctBrdSS.jpg


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PostPosted: 10 Sep 2011, 00:07 
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Grega wrote:
nice one mark.
i've built the power supply, stopped by the frequent flyer electronics store this morning and grabbed a case to throw it in
also managed to score a wire wrap tool from protogear for 22 bucks which is a bargain.
will build the circuit tomorrow

Are you going to wire wrap the PS and amp??

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PostPosted: 10 Sep 2011, 07:16 
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mwhouston wrote:
Are you going to wire wrap the PS and amp??
When I was about 14, I wire wrapped a high gain ss preamp. It sounded ok, but when it wasn't in an aluminum enclosure, it generated so much RFI that it killed all UHF TV reception in a two block radius. :o

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PostPosted: 10 Sep 2011, 07:54 
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Matt, not trying to top your wire wrap story, when I was doing my electronics/microprocessing course I wire wrapped a whole 8085 micropressor trainer, ran it on batteries so I could use it in class. I added to the trainer a stack more RAM and an A2D and a D2A so I could interface it to control externals. This was possibly the first truely portable computer!!

I still have it.

I have finsihed the Universal preamp. Apart from a short on the HT after the 300ohm filtering resistor in the PS it works and sounds great. A friend built a Shuguang 300B amp but it has a slight hum in one channel he cannot find. I said I would have a look at it. He happen to have a box of 12AT7s and has leant me four or five different ones.

I put in one of the GE tubes and fired up the Universal. I did not have a 250K pot so I used a 100K dual gang. This preamp/tube combo has a huge amount of gain and goes from nothing to loud on less than a 1/4 of a turn. Still controlable but it comes up fast. The GE tubes glows like a Xmas tree dowsed in kerosene and set alight. Not only is the tube bright but so is the overall sound. I would say it is tilted up. Mids are bright where The Black with the 12AU7 GD tube is rich through the mid range and a solid bass. This sounds thinner through the mid. I always wanted a tube preamp that sounded like a solid state preamp (bull!).

The sound stage is wide bit not deep. The preamp with the 12AT7 would suit a system that leans to the dark side, one Luke Skywalker might have, or his dad with emphysema. I have only had a brief listen with one tube so a way to go yet. Based on this empty evaluation The Black is still the one to beat. But I'm open to change.
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MKIV in caseSS.jpg


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PostPosted: 10 Sep 2011, 11:12 
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I was actually very surprised when you described the 12AT7 performance until I realized that you said that you had substituted a 100KΩ output pot for the 250kΩ in the design. This lowers the AC load from 71.4kΩ to 50kΩ. The rotation on the 12AT7 load line is a little extreme.

Here is the load line diagram showing the DC load in blue, the original AC load in red, and the 50kΩ load in green.
Attachment:
12AT7_Loads.jpg
The asymmetry in the load line at this level is obviously extreme enough to let some more significant 3rd order harmonics into the signal.

Remember when I said back on the original universal post that...
Suncalc wrote:
I had to go to a 250kΩ volume pot to control distortion
This is why. I think you'll get much better performance from all the tubes if you replace that 100kΩ output pot with a nice conductive plastic 250kΩ.


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PostPosted: 10 Sep 2011, 18:30 
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Matt: Ok I'll attempt to get a 250K pot. I just can't get one locally so I went for the largest I had on hand.

What about if I move the pot to the input and just had a 250K resistor across the output?

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PostPosted: 10 Sep 2011, 19:37 
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mwhouston wrote:
What about if I move the pot to the input and just had a 250K resistor across the output?
That is the simplest solution. It will raise the noise figure of the stage significantly however. This may be noticeable or it may not.

There is another rather unorthodox solution. Leave the grid circuit alone and use the 100kΩ pot as the plate load resistor, then feed the wiper into the output coupling cap and hang the 250kΩ on the output. This keeps the pot in the plate circuit so the grid circuit is not affected (i.e. it preserves the low noise figure of the original), and it let you use your 100kΩ pot. Just make sure you wire it the proper direction. Remember that at AC, the B+ terminal is ground. Clockwise rotation of the wiper should move it from the B+ terminal to the plate terminal.

And make doubly sure that the pot shaft is electrically isolated from the resistive elements and wiper. Some are not. We don't want any high voltage accidents. :hot:

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