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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: 06 Jul 2011, 09:15 
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Take a look at the original cct.at the begining of this thread. Values are very different to what I used. I know it is a different tube but half the feed back????

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PostPosted: 11 Jul 2011, 22:23 
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I have decided to build Matt's version of the preamp. Half the PS is completed to this point.

Matt: I have 1uf poly caps for the output but they are rated 100V. Are these OK or do I need higher voltage rating?

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PostPosted: 12 Jul 2011, 09:39 
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You would like to think that if you build three of the samething one after another you would improve or tweak the build. Hopefully this would lead to a better sounding device. I found I had some very nice Sonel 680nf 630V caps and I used them for output caps. The Sonels I used have pure silver connecting wires. Also I fully snubbed the rec. bridge on the HT, employed a ceramic 9 pin tube socket and really neatened up the connecting wiring. Even the amp wiring improved on this build, shorter component lengths still with component-2-component wiring. WBT 5% Silver solder again.

This is the Matt Special: Rp=27K, Rc= 820ohms, Rg=10K. No cathode bypass cap, no output R (just the 100K pot) Cout=680nf (Solen). No feed back R. Let's see if it flys Matt!

I would like to offer this as a kit with all the components required (including tranni), a drilled chassis sprayed to the colour of desire.You supply the 24VAC wall wart and your own version of the free-hand wiring. This would suit the experienced DIYer not the beginer. I'd even throw in Golden Dragon tube.

Take a look at the cct. brd. This time I have bigger components on the same real-estate. All on the one cct. brd. Can't wait to hear this one. Also I have found another CONN 12AU7 made in the USA and tests 85%. These tubes are out of a very, very old Conn Organ. The last lot souned excellent and I gave them away with the preamp. I have about 6 of them. err... somewhere??
Attachment:
MattSpecial_Iss.jpg
Attachment:
MattSpecial_IIss.jpg
Attachment:
MattSpecial_IIIss.jpg
Attachment:
MattSpecial_IVss.jpg


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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: 12 Jul 2011, 18:22 
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Mark;

You did mean an Rgs=10kΩ and Rg=470kΩ right?
Suncalc wrote:
This will leave you with a much simpler stage that should have excellent performance and the tight sound of the original. You have just a 470kΩ grid resistor, a 10kΩ grid stopper, an 800Ω cathode resistor, and a 28kΩ plate load. Leave the output cap at 0.47µf and replace the 470k bleeder with a 100kΩ pot as an output volume control. Gain will be ~19dBv (-8.9), distortion 1.1% worst case, and low end rolloff will be set exclusively by the output capacitor.
A 10kΩ Rg would leave you with a pretty small input impedance. This is the circuit I had in mind...
Attachment:
pic704.jpg
I also realized that this stage will have a variable output impedance ranging from 6.45kΩ at full volume increasing to 26.7kΩ at close to the resistance midpoint of the 100kΩ potentiometer and reducing to 0Ω at zero volume. Not bad really, just something of which to be aware.


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PostPosted: 12 Jul 2011, 19:38 
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One more idea...

(I know, I'm suggesting things faster than you can build. ;) )

I've been experimenting with 12AU7 designs and if when you get this put together it doesn't sound quite "tight" enough for you, you could up the plate load resistor to 50kΩ without any other changes. This will increase the gain to about 11.4 and reduce the 2nd harmonic distortion to about 36% of the original stage.

In fact, you could just make the change on one channel and then compare real time.

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PostPosted: 12 Jul 2011, 22:12 
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Matt: I built it exactly like you have drawn it but I increased the output cap to 680nf @ 630V (Solen). The input impedance into my tube amps (no input caps no pot) is 470K so this should not load the preamp at all.

I am yet to fire up the preamp. Tonight I will add the pot and use clips to "hook" it up to my power amp and give it a go. Because of the way I component-2-component wire amps, preamps etc its hard to make small changes. So increase the plate load resistor may no tbe an easy job.

I'll see how it sounds as is and if I need more gain (doubt it) I'll increase the load resistors.

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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: 12 Jul 2011, 22:52 
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With a following stage 470kΩ input impedance and the 0.68µf coupling cap, the low end corner frequency should always be below 3Hz at all potentiometer settings. I'm interested to hear how it sounds. :sing:

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PostPosted: 13 Jul 2011, 07:09 
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I completed a rough hook-up to the pot and RCAs: The 4S MKIII worked first time. Sweet.

Voltages are good:

HT = 270V
Vplate to ground = 125 and 135V
Bias = 3.7 and 3.8V
Vfilament =12.5V

The tube (12AU7) I am using is a CONN Organ company tube pulled from the organ oscillators by a DIY friend in the States. It tests 85% on emissions and reminds me of the sweet tones which came out of the 4S MKI. The tubes have green writing on them "CONN USA". Currently it is running upside down. Can't see this as a problem this is how they run in a lot of guitar amps.

The schematic is as Matt drew it a few posts back but with a 680nf (630V) Solen as an output cap. The pot (on the output) is a 100K Alps blue velvet. No bypass cap, 27K load and 800ohm cathode R. 10K blocker and 470K on the input. Pure ans simple.

The CD I am listen to is DECCA recording of an Italian trumpeter. I'd say the music style is laid-back jazz (easy listening). Typical DECCA to produce such an exquisitely recorded CD. 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'm yet to start the case for the MKIII which I should get a chance to do this weekend. So far so good. Vinyl is the big test of gain as my BHL phono pre has only a low output. I'll let you know and do some tests when the components stop burning (lel - laugh even louder).

If I made this up as a kit do you think there maybe some takers? Case drilled and sprayed, Supply your own 24VAC wall wart.

We now have a "Matt Audio Technology Trends" (aka MATT) design we can call our own.

P.S. Matt: I just took a quick look at the schematic your pencilled. Your guess was 3.7V bias - spot on good buddy!

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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, 08:01 
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Matt: If you have some fresh batteries in the slide rule how about a 6SN7 version of the 4S? I will need to know current requirements. I may have to beef-up the PS.

By the way your design I think is the absolute winner. Played it for about 5 hours tonight. Good bass and clean through the rest of the audible frequency range. Stacks of gain even from my phono preamp. Very good. Pleased. Yet to be cased.

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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, 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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