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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: 09 Apr 2019, 09:13 
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Joined: 09 Aug 2018, 03:15
Posts: 21
Location: Forsbacka Sweden
gofar99 wrote:
Hi, I agree. It can not provide any isolation from the mains and IMO is dangerous to use. It is not suitable for the project.

Good listening
Bruce

Yes you are right. I have ordered fully isolated transformer now. But strange it did work (not good but it did) at upstart and i acually blowed output at a REGA DAC :down: :( before I found out it was not suitable. So now I starded to fix final stage at the Rega..
Happy fixing and building all

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PostPosted: 16 Apr 2019, 03:05 
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Joined: 09 Aug 2018, 03:15
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Location: Forsbacka Sweden
I changed the input transformer for one with 2*55V in series. Also in a external box with line filter. Now everything is working like a charm. No humm and better responce than the Edcor output transformer. Better music punsh. And some pictures.. this will be used at my office with headphones B&W P7. Tey are not as sensitive as my Sennheiser HD600 but now they sound really good too :headphones: :D


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PostPosted: 16 Apr 2019, 13:16 
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Joined: 15 Nov 2018, 19:43
Posts: 13
Location: Belgium
My old Russian 6n1ps have arrived and they are very much noise free! On top of that, I now have some neat USSR era boxes and a specsheet to hang on to. I might experiment with some more of these old russian tubes, seeing as you can get 4-5 matched pairs for the price of one set of Western NOS ones.
Image
I'll just have to keep an eye on the power trans, and maybe add some more ventilation to the case in case things get a bit toasty.


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PostPosted: 29 Jun 2019, 13:27 
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Joined: 29 Jun 2019, 13:20
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Hello, I will go for it, it is so elegant design!

I wonder the the value of the input impedance. I want to use it after a very simple RIIA pre and skip its cathode follower if possible.

Bes regards,
Adrian


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PostPosted: 30 Jun 2019, 17:45 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, It should work fine. The gain of the headphone amp is low, but you really don't need much for phones. Input impedance is about the same as the control (100K).

Good listening
Bruce

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PostPosted: 16 Dec 2019, 22:01 
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Joined: 16 Dec 2019, 21:45
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I hate to post this ask, when I am sure if I keep sifting through things, the answer may lie within (or if I just reread the main schematics a few more times!!).

That said, I wanted to confirm the parts list, primarily the caps, I have most of the other parts, and have ordered the Edcor Transformers, the the LM317T, and have some 4700uf 25V Electrolytics, however, before I order caps, I want to make sure I am in the right ballpark. I have Attached are the remainder of what I believe I need, plus few oddities I want to just have in my supply box. I know I am missing some things, so assistance is appreciated!! Sorry for the silly ask!

Attachment:
2019-12-16_20-57-14.png


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PostPosted: 17 Dec 2019, 08:47 
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Location: Arizona, USA
Hi, I'm not sure what other parts you have. Some variation is fine, particularly in capacitor values as you can increase the filter capacitors in value but do not go lower in voltage ratings as they will fail. Sometimes this can be rather dramatic as like a mini-explosion. BTW the need for 350 volt or higher capacitors in the B+ string is to keep them from failing during the start up period when the tubes are not yet conducting. With the shown transformer the B+ voltage will get to about 280 before the tubes conduct. The parts list seems fine.

Good listening
Bruce

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PostPosted: 17 Dec 2019, 12:46 
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Joined: 05 Feb 2018, 11:49
Posts: 17
I'm excited to build the JBAA. I didn't realize it was already posted, figured someone would mention it here. Going to use some Thomson 6005s I picked up awhile back. I have a couple suitable chassis and almost everything else except the trannies. Debating 12AX7 or maybe going wild with Western Electric 408A or similar.

I'm thinking about making PCBs. Is there JBAA thread to discuss? I didn't see one started. Bruce, if you'd like to do the honors...

Thanks, and Happy Holidays, everyone.


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PostPosted: 06 Jan 2020, 02:19 
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Joined: 19 Mar 2014, 14:34
Posts: 26
Location: San Francisco, CA
Hey all,

Revisiting this thread after many years :) now with much more experience

Isn't the 25 mA bias current a little hot for the tubes? That's about 135*0.025 = 3.375 watts for the tube as a whole. I revisited this matter after getting a lot of noise running some vintage 6DJ8's, and I know from some experience that tubes get noisy when they're running hot.

With 6DJ8 / 6922's I believe 2W is the datasheet max when using both tubes together. That would suggest a more appropriate bias current of ~12-15 mA (assuming the plate voltage will rise as a result). Thus, each tube would be biased at 6-7.5 mA. This gives us a range of 83 to 104 ohms for the CCS series resistor, or 100ohms for a good compromise.

Thoughts? Think we may get excessive crossover distortion biasing the tubes this cool? I will try it soon and report back regardless!

-Ben


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PostPosted: 06 Jan 2020, 16:11 
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Joined: 19 Mar 2014, 14:34
Posts: 26
Location: San Francisco, CA
Well I lowered the bias current to ~6.5mA per tube and the NOS tube I have is still quite noisy :( so it looks like it's just a bad tube. The JJ's I have still work great :D

Any thoughts on keeping the bias current low though? Seems like good practice if there's no ill effects. Maybe reduced gain? Bias voltage is at about -7 V for the curious


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