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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 05 Sep 2011, 12:41 
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I have a slight update to the "Universal" design I posted a while back. When I looked again at the max output impedance numbers they, seemed a little "off". As it turns out, I failed to account for the effect of the unbypassed cathode resistor in the numbers. The changes aren't really that big, but I thought I'd post them anyway just to keep myself honest. I also tweaked the value of the coupling cap. The 0.68µf value used originally might make the stage just a little too susceptible to blocking. So I lowered it to 0.47µf. This raises the low frequency cut off frequencies to just over 1Hz for all stages. Although you could probably go all the way to 0.1µf without any significant effect on the 20Hz roll off numbers.

Also, while I was at it, I calculated a reasonable value for a bypass cap that would work for all stages (it could even be switched in and out with a DPDT switch for both channels). It tends to push up the stage distortion just a bit. The highest value is for the 12AT7/12AZ7. It gets almost to 6% for a 1v peak input signal. However, the bypassed gain for the 12AT7/12AZ7 is about 33 so an input of less then 1/2 a volt would be more typical for this tube. This means that the "real world" distortion would be more like 3% (much more reasonable). This will definitely be the "warmest" of the 12Axxx series tubes in this circuit.

Here is the modified schematic:
Attachment:
%22Universal%22 Preamp Stage.jpg

And here are the calculated performance numbers:
Attachment:
Universal_performance.png

The circuit values are at the top, the performance for the unbypassed cathode stage for each tube are next, and the performance for the bypassed cathode stage for each tube are at the bottom.

As usual, questions and comments are more than welcome.


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PostPosted: 05 Sep 2011, 16:47 
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Matt: Thanks for the update. I have all the parts for the Universal but are currently stuck on an RIAA phono preamp kit from Oatley Electronics, the K303. Firstly they packed two of the wrong jFETs and on assembly in a timbre box I get a 50hz hum.

Today I'll put the kit in an Al cheap case and see if the hum goes. It is a two stage jFET afair but had too much gain at the front end. With all the knowledge I have gained from working with the 4S preamp I figured if I cut away the bypass cap on the first jFET I would drop the gain and its sensitivity to hum. It worked. The preamp also has a sibilance issue (maybe high frequency distortion) which by increasing the local current feed back of the front jFET did help. I'll keep more of this story for a new thread.

Back to the Universal. It was my intentions to be able to switch in and out a bypass cap so with the new schematic I can incorperate this.

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Last edited by mwhouston on 05 Sep 2011, 18:01, edited 1 time in total.

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PostPosted: 05 Sep 2011, 16:52 
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Hi Matt, I have both 12AT7 and 12AX7 tubes available and am about to build the above - are there any real world 'tests' that I can help with?

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PostPosted: 05 Sep 2011, 18:13 
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Quote:
... - are there any real world 'tests' that I can help with?
Not really, just build one and tell us what you think.

Actually, the design is fairly solid. I prototyped it and then measured performance with a 12AU7, 12AT7, and 12AX7. All the numbers seem to be close to the design points.

I'm considering a mod to attack the design's only real Achilles' heel, the output impedance. It requires another triode to get a constant low output impedance and variable gain, but I think I can do it without any overload conditions and without adding any more frequency dependent phase shift than it already has. But that will have to wait for me to finish the design.

Keep us posted on your progress. :up:

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PostPosted: 05 Sep 2011, 18:21 
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If you add another stage we move right away from the whole core of this now long and well traveled project - Single Stage!

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PostPosted: 05 Sep 2011, 18:28 
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Quote:
If you add another stage we move right away from the whole core of this now long and well traveled project - Single Stage!
Yes you are quite correct. I'll probably have to start another thread for that one; "The Not So Simple Two Stage Tube Preamp". ;)

It's a character flaw of all Engineers that we can't seem to let anything be without "improving" it. :blush:

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PostPosted: 05 Sep 2011, 20:26 
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Quote:
It's a character flaw of all Engineers that we can't seem to let anything be without "improving" it. :blush:


tell me about it..........i just drew your diagram in visio..... ;)
oh and created a BOM for the mrs to go to jaycar ... again. pity they don't do frequent flyer there.

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PostPosted: 05 Sep 2011, 20:41 
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Attached as .JPG - can't attach the .vsd :(


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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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Projects:"Illusion" - 6SN7 SRPP preamp in a plastic jiffy box | “Salt Cellar” - Mono 807 triode integrated amp |
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