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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: 27 Feb 2015, 17:59 
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Todd;

Building this amp again today would surely involve a few different design decisions. And there are some things which I would definitely change. I'll try to outline them as best I can.

1. I would definitely replace the Hammond 270FX with an Edcor XPWR163 and the Hammond 193J choke with an Edcor CXC100-7H-150mA or CXC125-10H-200mA. This would give me an "all Edcor" build above the plate. Since completing this amp in 2011, I've come around to using all Edcor power transformers in most instances. In general this is because they have a lower temperature rise per VA which makes then run much cooler. In general, I don't have anything against hammond chokes, and the large selection of open frame values is nice, but I'd like every thing to match above the plate.

2: I would replace the JJ can caps with discrete components and move them underneath the top plate. This would be solely to clean up the look of the top plate a little bit.

3: I would replace the volume pot with a switched attenuator and raise the value to at least 200kΩ. This is just to relax loading on the driver stage and thus better control the AC load line rotation.

4: I would probably split the power supply filter both in the driver and power stages. This is a SET amp and, as such, it can share power supply rails and still achieve relatively good channel separation. I can't remember for sure but I think I got about 70dB of channel to channel separation on this build. However, these days I almost always go for much more. As such, I would redesign the PS filter to supply isolated power to the driver channels and use an additional LC filter stage for each power channel off the output of the common LC stage.

As for the 5U4GB, you need to understand that the choice of rectifier actually has very little impact on the sound of a SET amp with a properly designed power supply. It basically drives the B+ voltage and transformer loading. As such, changing the the rectifier is really an exercise in adjusting the B+ voltage. I chose the 5U4GB initially because it gave me very close to the 272v I wanted to be able to use the old style "S-Shaped" 6V6G tubes (which have a maximum B+ of 250V). It also allows me to insert another of the standard pin out 5V octal rectifiers and vary the B+ voltage. By going all the way to a 5AR4/GZ-34 rectifier I can increase the B+ by approximately 34v. So if I go to NOS 6V6GTs or the new JJ 6V6S, I can push the tubes a little harder without changing anything in the amp. So rectifier choice really depends on what I'm doing with the amp at the moment.

I don't think I would change anything else in this design. The amp was conceived with specific goals in mind, some of which are still applicable, some of which are not. Some of these goals are what drove the sonic design decisions for bias points, loads, frequency response, etc. and these I would not change.

I hope this answers your questions. But definitely chime in if it doesn't.

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PostPosted: 01 Mar 2015, 16:44 
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Matt,
Thanks for the response.
Good information.
Based on the size, my build experience, and your comments on the split power supply, I am considering dual mono blocks.

I am also thinking about a 6L6 build, using the information from Mark's build with el34's (pimp my amp thread).

Fortunately I am not in a rush.


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PostPosted: 17 Sep 2015, 07:49 
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How would you describe the sound of this amp compared to the 6EM7 amp. Collecting amps is not the end desire, building is the fun part.


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PostPosted: 17 Sep 2015, 07:55 
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wdecho wrote:
How would you describe the sound of this amp compared to the 6EM7 amp. Collecting amps is not the end desire, building is the fun part.

Having built two I can tell you with complete confidence that you would be shocked by just how good a 6EM7 amp sounds. The 6V6 is a different animal and I still have to sort some issues with mine. But what i have heard is very relaxed and smooth. Cheap and easy to build.

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PostPosted: 17 Sep 2015, 12:18 
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I've built the 6EM7 and the sound is gorgeous and this is coming from someone that has 3 Firstwatt Class A SS amps the last build being the F6. There are differences of course with rock probably favoring the F6 but I am thoroughly enjoying this first tube amp of mine. Everything sounds very musical and a great soundstage. This is an amp I could live with. I am constantly wanting to build and having never heard any other tube amp being curious how this 6EM7 compares to a PP or more watt SE. Right now I am on hold building a SIT Pass amp. Nelson is working on publishing the latest project he has promised. The SIT transistor has the same type of curves as a triode. I would build another tube amp if I thought there would be an advantage but also do not want to build something that is not better.


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PostPosted: 19 Apr 2016, 07:42 
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So, given the information I've recently uncovered on biasing the 6V6 in SE-UL mode (documented here: http://diyaudioprojects.com/Technical/6V6-SE-UL-Bias-Optimization/) I've been considering rebiasing the power stage in this amp. I would go to 390kΩ bias resistors to push up the power a little and to reduce plate dissipation in the 6V6s. I am also considering more heavily loading the driver tube for a little more end-to-end sensitivity.

What I want to know is how do people feel about rebuilding amps like this. I really like how the amp performs now, but I think it could be better. And I can't see building an entirely new 6V6 amp just to alter bias resistors. What do people think? Rewire or not rewire?

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PostPosted: 19 Apr 2016, 14:21 
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If you want to preserve the "magic" make lots of notes and take pictures or build another and leave the original alone.


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PostPosted: 19 Apr 2016, 14:51 
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Suncalc wrote:
So, given the information I've recently uncovered on biasing the 6V6 in SE-UL mode (documented here: http://diyaudioprojects.com/Technical/6V6-SE-UL-Bias-Optimization/) I've been considering rebiasing the power stage in this amp. I would go to 390kΩ bias resistors to push up the power a little and to reduce plate dissipation in the 6V6s. I am also considering more heavily loading the driver tube for a little more end-to-end sensitivity.

What I want to know is how do people feel about rebuilding amps like this. I really like how the amp performs now, but I think it could be better. And I can't see building an entirely new 6V6 amp just to alter bias resistors. What do people think? Rewire or not rewire?


There are always compromises. So what may you lose?

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PostPosted: 19 Apr 2016, 15:15 
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mwhouston wrote:
There are always compromises. So what may you lose?
See that's the thing, I can't find any.

Now the paper I wrote was technical, and based on hard cold data. That's the way I like it because there is no subjective "my year tells me" stuff. That being said, what I didn't include in my write up was the fact that I also listened to the 6V6 at a lot of different bias points. And subjectively I think the optimum bias points just sound better. Not only in distortion, but in transient response as well. I can't find the thing to measure to show it analytically, but I think it was there. Now maybe it's just an old man's foolishness, but I can't be sure.

So that's my conundrum. I've got nothing hard to go on, but I think I can do better.

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PostPosted: 19 Apr 2016, 15:48 
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I'm a little rough when changing components. I would probably cut away the old resistor and solder on top of the stubs the new. I hook components in place and sometimes it's hard to get them out.

I'm in Europe for five weeks but when home will evaluate the minor change. Maybe a lot easier than I think.

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