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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: 14 Jul 2011, 19:16 
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mwhouston wrote:
Matt: If you have some fresh batteries in the slide rule how about a 6SN7 version of the 4S?
How about the same basic circuit but with a higher load (for a better bias position for the 6SN7) and a slightly different Rk? Given the way the 6SN7 usually stacks up against the 12AU7 this should be a little crisper and have a lower noise floor.

Here's the circuit...
Attachment:
4S-6SN7 Circuit.jpg

Gain is -10.3 (just a little higher than before), current is slightly lower (3.3mA) so you shouldn't have to change the power supply (except for the heater), distortion is similar (small). The stage should be able to swallow a 4.9vpeak (3.5vrms) input with no problem.

Here's the full specs and load line...
Attachment:
4S-6SN7 Load Line.jpg

You could also use a 6CG7 if you wanted to keep the same 9-pin tube form factor as the 12AU7.

Let me know what you think.


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PostPosted: 14 Jul 2011, 22:39 
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Matt: If you think the values are right that is good enought for me. Here's a thought. What about a preamp with both tubes inplace and a switch to go from 12AU7 to 6SN7. Might need a few switches or a miniature relay system.

Thanks for the quick design. I'll think about it.

Pop night at my place tonight for 6 or club members. The 4S MKIII is on show and its naked! We'll see what the club members think.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 14 Jul 2011, 22:49 
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mwhouston wrote:
The trumpeter is Chris Botti. The CD is just called "Italian". If you don't buy it at least steal a listen at you local CD shop. Through the new preamp and the rest of my tube gear it sounds excellent.
I just finished listening to the first three tracks of the Chris Botti "Italia" CD (the US release is titled "Italia" instead of "Italian") through the "Lacewood" 6V6 amp. The only word I can use to describe the sound is exquisite.

Mark; Thank you for the excellent recommendation!

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PostPosted: 14 Jul 2011, 23:12 
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Suncalc wrote:
mwhouston wrote:
The trumpeter is Chris Botti. The CD is just called "Italian". If you don't buy it at least steal a listen at you local CD shop. Through the new preamp and the rest of my tube gear it sounds excellent.
I just finished listening to the first three tracks of the Chris Botti "Italia" CD (the US release is titled "Italia" instead of "Italian") through the "Lacewood" 6V6 amp. The only word I can use to describe the sound is exquisite.

Mark; Thank you for the excellent recommendation!

I listened to it twice in row. Great recording. The classical stuff is just a little weak but the rest makes-up for it.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 15 Jul 2011, 10:23 
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The 4S MKIII on pop night? A Big success. T'morrow I case it.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 16 Jul 2011, 09:25 
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Suncalc wrote:
mwhouston wrote:
The trumpeter is Chris Botti. The CD is just called "Italian". If you don't buy it at least steal a listen at you local CD shop. Through the new preamp and the rest of my tube gear it sounds excellent.
I just finished listening to the first three tracks of the Chris Botti "Italia" CD (the US release is titled "Italia" instead of "Italian") through the "Lacewood" 6V6 amp. The only word I can use to describe the sound is exquisite.

Mark; Thank you for the excellent recommendation!

First a correction, my CD is called Italia also. I mis-read the cover. I am not going to turn CD critic but I like classical trumpet and for a bit of Baroque Trumpet, if you like that style, the Naxos CD 8.553531: The Art of the Baroque Trumpet, Vol1 is very nice. One trumpeter; Nilas Eklund, covers half a dozen well known composers.

I picked up this CD second hand for $5. Nice.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 18 Jul 2011, 17:00 
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I tried the Conn Tube in the MkII (with NFB etc.) to see if it produced the full mid range of the MKI. It did to a point. These Conn tubes seem to have a very rich mid range. Then, after listening for a while, I dropped the Golden Dragon (GD) back in. The treble brightened up and the sound stage tightened up. I would say over-all, in the MKII build the GD is a better tube. But it would get down to taste: mellow and fluid verses bright and detailed. When the latest cct. brd. build is cased (MKIII) I will try the tube swap again.

Matt: What if I dropped a 12AX7 in the MKII (NFB, 820ohm Rc and 28K Rp, 100K blocking R). I doubt it would damage the tube but what would I expect from the preamp from a performance point? Is it worth a try or are the values all wrong?

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 18 Jul 2011, 20:59 
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mwhouston wrote:
Matt: What if I dropped a 12AX7 in the MKII (NFB, 820ohm Rc and 28K Rp, 100K blocking R). I doubt it would damage the tube but what would I expect from the preamp from a performance point? Is it worth a try or are the values all wrong?
Keeping in mind that this may technically qualify as tube abuse (I recall a quote from the SET thread about you building with a soldering iron in one hand a whip in the other), no don't think it will damage the tube and the results might be "interesting".

Where as I would never recommend running a 12AX7 at this current level, my calculations show a bias point at 198v and 1.56mA, the grid bias is about 1.25v, unbypassed gain is about -16.2 (24dBv), with a distortion number of 2.2%. This is all based on the static bias point. However, the AC (as well as the DC) load line is VERY steep. This can easily lead to oscillatory conditions, especially in a tube with as high a µ as the 12AX7 family. Here is the load line.
Attachment:
Abusive 12AX7 Stage.jpg

So, you can go ahead and give it a try. I think it's even money you'll get either free running oscillation or audible ringing on anything with a steep attack. :xfingers:

Let us know what happens.


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PostPosted: 19 Jul 2011, 00:49 
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Matt: Even I'm not that silly. That's "off the chart". Do you have some values for 12AX7? Please?

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 19 Jul 2011, 07:58 
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I sprayed the case for the 4S MKIII this morning. It should be dry enough to assemble Friday night. I am holding an Opera day on Saturday and it would be nice to have the MKIII going for then. Do-able.

For some reason I sprayed this one Gloss Black. Of all the colours (and tones) I went for black. I could nick name this build "Black" after the Mercedes http://www.mercedes-amg.com/webspecial/clk63blackseries/ of the same name. But I stuffed up. I miss-drilled the tube port. So I had to drill another hole. A big hole. I'm going to get either a timber of metal piece (styled of course) to cover the hole.

Images and test results to come. In a future build (if Matt is not sick of charting my course) I may do a build which can switch from 12AU7 to 12AX7. It depends on how much has to be changed.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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