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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: 20 Oct 2018, 00:25 
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All finished with external PS.
Attachment:
image.jpeg
Attachment:
image.jpeg


Uses a 24VAC wall wart and external PS. Sounds excellent possibly better than my 4S builds. I plan an another.


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PostPosted: 20 Oct 2018, 01:18 
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M.W. it looks really nice. Those tubes are so unique. I like the way you have presented them. Very elegant. Excellent job. Out of my budget, but if I could afford them, I would not hesitate getting a pair.


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PostPosted: 21 Oct 2018, 01:12 
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laurie54 wrote:
M.W. it looks really nice. Those tubes are so unique. I like the way you have presented them. Very elegant. Excellent job. Out of my budget, but if I could afford them, I would not hesitate getting a pair.


Well its gets worse. Just paid A$557 for the low loise pair and again to A$136 for more Jensen Copper PIO caps. The EIZZ 24 step stepper attenuators are A$80 also. Yes all very expensive but when I think a lot of comercial preamps are $6K etc. I think I'm getting out of it cheaply with premium parts.

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PostPosted: 21 Oct 2018, 03:20 
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Have you tried erse pulse x capacitors, maybe you can save money next time. IMO even if better caps than pulse x exist I doubt it can be totally diffrent.


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PostPosted: 21 Oct 2018, 04:08 
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So glad to see you didn't put a switch or anything else on the front or top. Either
would have diminished the final result. I would expect, also, the simplicity and
choice selection of the innards signal path that the resulting sound would be
equally better than the average pre-amp and par with ones costing much more.


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PostPosted: 21 Oct 2018, 06:02 
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In each channel there is two resistors and one output cap. When every component buggers the sound the simpler the cct. the less sound degradation.

Thanks for the nice comments.

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PostPosted: 22 Oct 2018, 15:19 
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Hi Mark, really cool looking preamp. I would expect the tubes to give a gain of about 10 (20db) un-bypassed and a few db more when they are bypassed. Performance is similar to a 12AU7 in that circuit and probably 90% or more of stuff I use and learned in the Forewatts would apply. The gain can be a problem. If you put the volume control at the output side you run the risk of overdriving the stage or running it into a non-linear region. If you put at the input (my personal preference) then that will not happen, but you will have all the stage noise in the output signal. If carefully designed and assembled this really is not an issue. I can easily get the S/N at 1 volt output to about -90dbv. At that level the contibution of the noise is insignificant in nearly any system designed to use 1-2 volts of drive on the power amps. In either location it will affect the impedance a bit. It may not matter though. It mostly boils down to what is feeding it and what it in turn feeds.

EDIT: I find with both the 12AU7 and 6SN7 that there is a sweet range in that configuration with about 6-8ma of current and a B+ of between 215 and 275. I measure the lowest distortion and best linearity there (output signal of +/- 15 to 20 VRMS best is naturally in the 0-3 range)

Good listening
Bruce

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PostPosted: 22 Oct 2018, 16:15 
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Bruce, mate, you definitely know this topology well as I would expect you would. My B+ is spot on at 215V and current is 6.5mA. I did leave the pot (24 step stepper - 100K) on the output. And yes I get a little over driving though the bias voltage is 2.4V. But only on one LP from my tube phono system. The rest of the sources, Pi server and DAC and CD all OK.

I can back the phono output down by replacing the 12AX7s with 12AU7s. Very happy with the preamp and love totem pole for its simplicity and lack of interstage cap. Next build will be similar but with some higher end components again.

Thanks for the info.

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PostPosted: 23 Oct 2018, 17:34 
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Hi Mark, It comes from a lot of trial and error over a period of nearly 10 years now. Sims only get you close IMO. I use some, but refinment is usually done the hard way.

Good listening
Bruce

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PostPosted: 23 Oct 2018, 18:35 
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Bruce, sonicaly what do you think of this cct.

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