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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: 24 Aug 2020, 10:49 
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Joined: 04 Jan 2020, 08:22
Posts: 18
Hi

Im now reducing the input volyme pot on the 300B amp I use as power amp, so that acts as an Forewatt output restriction, and then the hiss isn’t noticeably, and I have no problem reaching the sound level I want. So I keep the fore watt as it is, and maybe ad an input attenuator switch on the 45 tub amp I’m planning to build.
Still haven’t decided what tubes I like best, compared to passive mode, the amp its more engaging to listen to active, but also a bit more sharp s-sounds (don’t know the right English word here for the sound) on singing that has that type of sound recorded, that is a bit annoying on some recordings. I think I will reduce my tweeter one dB or so (lifted it up 1 db with my NAD pre amp previously), to slightly reduce the higher frequencies. Maybe with an L-pad so its easier to seek the right level.

Br Svante


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PostPosted: 29 Aug 2020, 07:20 
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Joined: 04 Jan 2020, 08:22
Posts: 18
Have now had time to listen to some different tubes, and in the end, I liked the JJ ECC82 best of the ones I tested
Sylvania 12AU7 from 53, First I thought this was the winner, similar sound as JJ ECC802S, but I preferred the Sylvania slightly better, both of them was extremely god in separating details in the music, but after some days using the Sylvania, I notice that I rather quickly got listening fatigue when listening with medium/ high volume.
Ecc82 slightly less detailed than the two above, but also much less fatigue in the long run.
Nos Telefunken (don’t know the year, but marked Western Germany), these tubes was extremely microphonic, which destroyed the sound in high listening’s volume
I also had a pair of Golden dragon, but one was broken, so I tried to judge the working one with an JJ in the other channel, so maybe not a good judging, but I thought is sounded boring VS the other tubes.
So, for me its JJ ECC82 that stays in the amp

Br Svante


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PostPosted: 29 Aug 2020, 07:24 
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Joined: 04 Jan 2020, 08:22
Posts: 18
I forgot to say, al tubes had 5-15 h on time before the judging, so its maybe a bit short time for them to stabilize fully.


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PostPosted: 17 Sep 2020, 11:36 
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Joined: 13 May 2012, 17:47
Posts: 4
Hello Bruce. First of all, I dont know veeery much about tubes, just a little.
I am curious on why you chose that 470R cathode resistor in your design.

Did you by chance take a look at Merlin Blencowe paper on SRPP?
There it explains what he says is the optimum value for Rk.
Thanks!!


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PostPosted: 17 Sep 2020, 21:58 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, I have his books. I chose 470 based on testing using a distortion analyzer and two different PC based scopes. It gave the best combination of distortion and output. I respect MB, but he is not really fond of SRPP stages. (he has lots of company as well) The ones I use are the symmetrical variety and when properly tweaked work really well. Most folks don't seem to take the time to get them right. The results can be rather dismal. After all the original design was for use as vertical deflection amplifiers in tube based TVs. Linearity was not a big deal as other circuitry was used to correct the problems. They wanted a very large voltage swing with a sort of triangular wave shape. I use them rather differently. At the typical voltage signal levels they perform rather well. You can get as much as 20 volts output and still have really nice distortion levels and linearity. BTW I tried Rk values from 150R to 2.2K ohms and anode voltages from 150 (top anode) to 350V. It seems that there is a sweet spot with approximately 200-215 V B+ and 470 ohms. As a plus it is very tube friendly and nearly any brand of 12AU7 or varient (like ECC82. ECC802,and so on) will work with nearly identical levels of quality. Just pick the one you best like the sound of.

Good listening
Bruce

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PostPosted: 18 Sep 2020, 07:57 
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Joined: 13 May 2012, 17:47
Posts: 4
gofar99 wrote:
Hi, I have his books. I chose 470 based on testing using a distortion analyzer and two different PC based scopes. It gave the best combination of distortion and output. I respect MB, but he is not really fond of SRPP stages. (he has lots of company as well) The ones I use are the symmetrical variety and when properly tweaked work really well. Most folks don't seem to take the time to get them right. The results can be rather dismal. After all the original design was for use as vertical deflection amplifiers in tube based TVs. Linearity was not a big deal as other circuitry was used to correct the problems. They wanted a very large voltage swing with a sort of triangular wave shape. I use them rather differently. At the typical voltage signal levels they perform rather well. You can get as much as 20 volts output and still have really nice distortion levels and linearity. BTW I tried Rk values from 150R to 2.2K ohms and anode voltages from 150 (top anode) to 350V. It seems that there is a sweet spot with approximately 200-215 V B+ and 470 ohms. As a plus it is very tube friendly and nearly any brand of 12AU7 or varient (like ECC82. ECC802,and so on) will work with nearly identical levels of quality. Just pick the one you best like the sound of.

Good listening
Bruce


Thanks for the explanation Bruce!


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