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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: 03 Mar 2011, 21:11 
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Location: Lisbon, Portugal
Hi!

I´m about to start my KT88 project, and it´ll have a phono-stage for my vintage turntable. I would like to use only Octal tubes. There are lots of good schematics over the forum, but I also found this one from a well known audiophile, though I have no feedback from this. It offers RIAA, AES and Columbia switchable EQ´s and it uses 6SL7 plus 6SN7 tubes!

What is your opinion? Who is / was JE Labs?

Main Schematic: http://members.myactv.net/~je205d/varEQph-1.jpg
Image

Optional Line Stage: http://members.myactv.net/~je2a3/JELsc-linepre1.jpg
Image

It looks good to me! It´s easy to see that this fellow really has a passion for Hi-Fi Tube / Vintage audio stuff.

Cheers,
Miguel


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PostPosted: 03 Mar 2011, 23:14 
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Joined: 23 Jul 2010, 17:03
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Hi Miguel,
I've heard of him.He's either a concert violinist or celloist.He's also a BIG tube guy.Octals or bigger.I built his 45/2A3 SE Amp last year and plan on building another this summer.Don't know what happened to him,his web sight hasnt been updated in a long time.If you really want to know you could ask the guy at http://angela.com/I think he might know him.
David

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PostPosted: 04 Mar 2011, 00:05 
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Hmmm, I think there's an intentional schematic error in the mic. pre - there's a cap missing between the wiper of the 250K log and the output follower grid (as shown, it will cutoff)

Cheers!

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PostPosted: 04 Mar 2011, 03:59 
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Joined: 26 Feb 2011, 15:18
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Location: Norh Europe
The circuit looks fully OK to me. I have also used hi-mu triodes, but 6N2P-EV tubes with practically similar topology and the results are good.I simulated the RIAA-corrector and it's response is within + 0,3 dB compared to standard RIAA curve.
I quess you will only build the fixed RIAA, not all those switchable alternatives ?
Such a switch would be located in very sensitive place with respect to hum.
You can expect the total gain of some 42 dB (@ 1 kHz) including the output cathode follower.

Ofcourse some higher gm-triode would have lower noise, but in practise the final layout mainly determines the overal noise and hum performance of this kind of low level amplifier. And use DC heater supply voltage.

There is a mistake - the missing dc-blocking capacitor - as Geek already mentioned.


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PostPosted: 04 Mar 2011, 14:32 
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Missing capacitor? Do you mean at the 6SN7 grid? Is it really necessary even if you don´t use / connect the HiZ line stage?

@ART: Yes, I only plan to build the fixed RIAA version. :)

Cheers,
Miguel


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PostPosted: 04 Mar 2011, 15:45 
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Location: Arizona, USA
Hi Miguel, How's things over there? The circuit looks fairly straight forward to me. I like the 6SL7s. I use NOS RCA 12SL7s in my reference amps and they have a really nice sound. :soapbox: After spending the last 6 months on RIAA phono preamp designs I have a few general comments that you might want to consider. The design you have is one with passive RIAA equalization, this has some pluses an some negatives. One it is easy to tweak if needed (good). It is fully a lossy process (not good). IMHO anything that reduces a signal level introduces problems. The circuit requires a lot of gain (neutral) but in doing so is rather sensitive to noise pick up (not good). Layout is critical (not good in a diy project). On the other side, an active RIAA arrangement is harder to design (not good). harder to tweak as well (not good). But is more tolerant of tube variations (good). Tends to reduce distortion (yes it uses NFB) and noise levels (very good here). Layout is more relaxed (good). One thing both will have in common is that the tubes may require shields. Either that or placemement inside a metal enclosure. I found that particularly in the passive version that I could pick up hum from an unshielded transformer from as far away as 2 feet. Flourescent lamps from twice that. :?

When I compared both types side by side, the passive version seemed to have an edge on speed and clarity. The actively equalized one was quieter and warmer. I believe you could spend many years perfecting either type and still have more to do.

Just my thoughts on it, let us know how it comes out when you get it built. Now jumping off the soabox. :)

Good listening
Bruce

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PostPosted: 04 Mar 2011, 16:28 
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Quote:
Missing capacitor? Do you mean at the 6SN7 grid? Is it really necessary even if you don´t use / connect the HiZ line stage?

Yes, think about how the bias voltage forms at the grid of 6SN7 when the 250 k potentiometer is adjusted compared to the case there is the capacitor blocking the bias from the pot.


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PostPosted: 04 Mar 2011, 16:42 
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The decoupling/blocking capictor is there. It precedes the 250K-log pot instead of following it. This amp has some similarity to this one:
http://diyaudioprojects.com/Schematics/ ... ematic.htm

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PostPosted: 04 Mar 2011, 16:58 
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The cap I mean is because the cathode is lifted off ground by the 2.7K ressitor. This creates extra bias beyound the normal bias created by the 5.6K resistor. This dials the grid quite negative.

(I'm not saying it doesn't work or sound good, I've just never seen it before without cutoff risks)

Cheers!

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 04 Mar 2011, 20:14 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
Thank you all for all the opinions and suggestions! :) I really enjoy this forum!

I will give a try on the circuit and measure the voltages at the 6SN7 while playing with the potentiometer.

@ Geek: Now I understand your point of view. Maybe it can really need a 0.22uF decoupling capacitor...

I have 12SL7 NOS WestingHouse and 6SN7 NOS Sovtek tubes for this project. I have big expectations for these! :)

Regards from Lisbon, Portugal!
Miguel


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