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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: 26 Sep 2010, 13:50 
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Any progress as yet on the new amp? I'm eager to hear any news.

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PostPosted: 26 Sep 2010, 16:42 
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Suncalc wrote:
Any progress as yet on the new amp? I'm eager to hear any news.

I have been trying to contact you. I have all the parts to start the 807. BUT a friend from the audio club heard my 6AS7 SET and wanted one. Happy to pay well for a super version. After listing to my SET on his gear he has changed his mind and now wants a 300B for a little extra power.

I purchased the TJ Full Music 300B/SE carbon plate tubes and ShuGuang 6SN7 last night so now I'm locked in. I was wondering would you please design a 6SN7/300B SE amp for us. I have one 6sn7 / SE 300b design from Sampleaccurate (which is three stage) but would like to see what you could come up with. I would like to make it two stage if possible. That way I only need three tubes and one rec tube.

Are you interested. I full intend to build the 807 amp but as this is a paying job it must come first. I would be happy to show my appreciation in some small way for your efforts.

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Projects:“Salt Cellar” - Mono 807 triode integrated amp |
"Rosso" - E88CC SRPP preamp | ”Salt Cellar II” - 6EM7 mono wine box triode amp | “MWH” - 1920s UX-171A preamp |


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PostPosted: 25 May 2011, 08:56 
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Matt: Happy with all your design to this point. If there is anything you would like to change speak now forever hold your peace. And as you are aware there is another Silver Dragon in the wings.

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Projects:“Salt Cellar” - Mono 807 triode integrated amp |
"Rosso" - E88CC SRPP preamp | ”Salt Cellar II” - 6EM7 mono wine box triode amp | “MWH” - 1920s UX-171A preamp |


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PostPosted: 25 May 2011, 09:30 
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Please give me 24 hours to review the design. I just want to make sure I didn't do anything too silly the first time through.

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PostPosted: 25 May 2011, 17:15 
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Take your time. How are you off for interconnects. What types/brands do you have. I was going to make you some of hy Houston Specials as a thank you.

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Projects:“Salt Cellar” - Mono 807 triode integrated amp |
"Rosso" - E88CC SRPP preamp | ”Salt Cellar II” - 6EM7 mono wine box triode amp | “MWH” - 1920s UX-171A preamp |


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PostPosted: 26 May 2011, 09:15 
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Three aspects of the design I am unsure about :

Why have the volume pot at the 807 grid ? Also why such large grid stoppers ?

There is no feedback . In my experience having a tube with Rp>Rload results in one-note bass

BDA


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PostPosted: 26 May 2011, 13:48 
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bda wrote:
Why have the volume pot at the 807 grid ?
This is because the volume pot will most likely be hot formed carbon or cermet which means that it has the highest noise figure of any part of the amp. If placed at the input to the 6SN7, it's noise figure will dominate the amplifier noise figure. However, by placing it after the first amplifier, it's noise figure contribution to the amp consists of it's noise figure divided by the first stage gain. This means that it's contribution to amplifier noise is reduced by over 22dBv. As such, the amplifier will be significantly quieter.
bda wrote:
Also why such large grid stoppers ?
The grid stoppers are sized to set the high frequency roll off of the amplifier marginally above the audio frequency range. Their size is dictated by the miller capacitance of the 807 at this bias point. They also significantly improve bias excursion recovery time by limiting grid current during overdrive conditions. The 807 is an RF transmitter tube. This design practice will help prevent high frequency oscillation and instability in the output tubes.
bda wrote:
There is no feedback.
Yup. My stereo amps only contain feedback when absolutely necessary. A point of design philosophy: NFB is like canards on an airplane, a sign of a serious design flaw trying to be fixed with a band aid. ;)
bda wrote:
In my experience having a tube with Rp>Rload results in one-note bass
Not sure I understand your third comment. Rp > Zload is almost unavoidable in most tetrode/pentode amps. Take the 6V6 for example: Rp is typically 50k to 80k. Perhaps you could expand a little on you comment about "one note bass"?

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PostPosted: 26 May 2011, 16:37 
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Suncalc wrote:
This is because the volume pot will most likely be hot formed carbon or cermet which means that it has the highest noise figure of any part of the amp. If placed at the input to the 6SN7, it's noise figure will dominate the amplifier noise figure. However, by placing it after the first amplifier, it's noise figure contribution to the amp consists of it's noise figure divided by the first stage gain. This means that it's contribution to amplifier noise is reduced by over 22dBv. As such, the amplifier will be significantly quieter.


Each to their own . It's not how I'd do it , saying that I wouldn't use a carbon pot ;)

Quote:
The grid stoppers are sized to set the high frequency roll off of the amplifier marginally above the audio frequency range. Their size is dictated by the miller capacitance of the 807 at this bias point. They also significantly improve bias excursion recovery time by limiting grid current during overdrive conditions. The 807 is an RF transmitter tube. This design practice will help prevent high frequency oscillation and instability in the output tubes.


I go the other way . I scale back the stoppers as low as feasibly possible . This is from listening rather than going by technical theory . I'm not too familiar with UL amps but with DHT amps I usually don't use any at all but when I tried 1624 UL I used 470 ohms . Even with a high mhos D3A I typically use a maximum of 330ohms

Quote:
Yup. My stereo amps only contain feedback when absolutely necessary. A point of design philosophy: NFB is like canards on an airplane, a sign of a serious design flaw trying to be fixed with a band aid. ;)


I'm not too keen on feedback either : for me it's usually placing the output transformer secondary in the cathode circuit

Quote:
Not sure I understand your third comment. Rp > Zload is almost unavoidable in most tetrode/pentode amps. Take the 6V6 for example: Rp is typically 50k to 80k. Perhaps you could expand a little on you comment about "one note bass"?


One note bass is what you get when trying to drive 8 ohm bass reflex speakers from an amp with a low damping factor ;) I suppose if you use horns or t-lines things may be different but I wouldn't want to go higher than an output impedence of 4 ohms


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PostPosted: 26 May 2011, 16:40 
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Next amp I build will have canards. That should level out the frequency response. I have found better dynamics and mid range detail in amps I have built with 0NFB. But not all amps will work best this way. Bruces Oddwatts have a small amount of NFB and if I know Bruce it would improve the overall performance of his push-pull amps. I am not against a littler local FB on a driver stage, just global.If you look at most mid range commercial amps they have FB which spans many stages. It makes the amp sound like what most people would expect a HiFi amp to sound. But not always the best sound.

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Projects:“Salt Cellar” - Mono 807 triode integrated amp |
"Rosso" - E88CC SRPP preamp | ”Salt Cellar II” - 6EM7 mono wine box triode amp | “MWH” - 1920s UX-171A preamp |


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PostPosted: 27 May 2011, 10:46 
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I don't like FB. With FB all amplitudes are linear in diapason Hi-Fi, but for small signal from freq. generator.
It like that all is OK, but.....
In dynamic moments sound is pressed. All my amplifiers are without FB and with linear Freq. band.
For ampl. in markets is very important that all is OK in 20-20000 Hz and hummmm = 0, if someone will measure freq. band, but how they will sound....it is question number 2 or 3.....

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