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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: 13 Oct 2020, 06:19 
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Joined: 16 Feb 2016, 19:45
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Location: Boston, USA
Oh, no! Now I have to build another to join my poddwatt, oddblocks, forewatt and groovewatt family :)

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PostPosted: 20 Oct 2020, 16:25 
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uauuuuuuu...very nice!!! i would love see inside photos....


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PostPosted: 20 Oct 2020, 19:34 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
gofar99 wrote:
Hi, New addition to the amp family. A stereo integrated power amp with volume and balance controls and sufficient gain not to need a separate preamp. A more complete write up to follow later. It sounds just like the other Oddblocks (no surprise there). The extra stage has a gain of about 9 as shown with about a 50% loss in the balance control. Note also that a capacitive multiplier filter is used in place of the LR8 regulator. Only one channel of the filter section is shown so you need two of the CMs. The input stage of the amp is run full gain and can handle up to about 4 VRMS inputs without attenuation. It ought to be fine for most uses. If not then series resistors can be added to form a voltage divider there. The reason for not having the volume control there is to minimize hum and noise. As show it is really quiet. Under 1 millivolt of hum and noise at the output with full volume inputs shorted. I used a 6NO60 delay tube for warm up delay, feel free to do something else. I recommend a delay as the solid state rectifiers will hit the filters with over 525 VDC if the tubes are cold. Operating voltage is 475.

Good listening
Bruce
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Stereo 20 OCT 2020.jpg
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Oddwatt Integrated Stereo 10-12-2020.jpg
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Oddwatt StereoPS 10-12-2020.jpg


Hi Bruce!

It looks awesome!

I wonder what are your opinions on the power transformer, because i´m running the same setup but with the

https://edcorusa.com/xpwr173

"Power transformer for a 120V or 240V, 50/60Hz. line to 700V (350-0-350) at 400mA center tapped, 100V at 10mA, 12V at 1.5A and 6.3V (3.15-0-3.15) at 7A center tapped."


- Does it buzz? (mine a little bit but acceptable)

- Does it get really hot? (mine goes up to 65C)

Please let me know.

Stay healthy!

Cheers,
Miguel


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PostPosted: 20 Oct 2020, 21:34 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Miguel. It should be fine. If the trannie buzzes then it probably has loose laminations. On Edcors this does occur from time to time. What I do is run them a while to get warm and then tighten the bolts one the bells. Usually it works. Typically with SS rectification and relatively large first filter caps the transformer will get rather warm. I have never had one fail though if run within the ratings. They usually are designed with a relatively large safety factor.

For those of you all that want to see inside....not at the moment. This amp was built and taken apart and reconfigured about 5 times and shows it. Works fine but is a lot uglier than I usually do.

Good listening
Bruce

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PostPosted: 25 Oct 2020, 13:24 
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Location: Romania
Has anyone used Ogonowski transformers in Oddwatt family amps? It's a Polish manufacturer and they seem very well built. One that would electrically fit is LO PP40-2, Za=8Kohm, Pmax=40W, Imax=200ma, 15Hz-68kHz @-3db see attached. I wonder how they sound.


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PostPosted: 25 Oct 2020, 21:28 
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Hi, It would probably be OK. I have concerns about the imax. All Oddwatts are class A designs and use the maximum current all the time. So with a pair of KT88s that would be about 180ma. KT77s, EL34s would use about 125. The use of KT88s would be a bit iffy IMO. I can use the 25 watt Edcors with them as they really can handle nearly 300ma (even though they spec them for less). Edcor usually factors in a fairly large overhead just to be sure they will work as they say they will. I have to go to the 70 watt ones on the KT120 amps to handle the continuous 300 ma they draw. I am not familiar with the ones you listed....they may be fine. Also the designs are taking into account transformer resonances. If you use the ones you indicated I would use a sweep generator and o'scope to look for above audio band resonances. The Edcors peak in the 60-75K HZ range. Thus the small amount of frequency tailored NFB. You might have to adjust the values to be sure of stability with odd signals and speaker loads. Modeling indicated (but I have never seen it happen) that with largely capacitive loads (like my Martin Logan ESLs) and a poorly filtered DAC that you could excite the resonances and bung up the sound and not know why. Cost wise, I am sure the Edcors would be much more (shipping is a killer).

Good listening
Bruce

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PostPosted: 30 Oct 2020, 11:27 
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Location: Romania
Thanks Bruce. I'll use lower power tubes, like 6P3S. Or, maybe even KT88, but with less idle current than normal, around 60ma.


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PostPosted: 30 Oct 2020, 12:18 
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Hi, Oddly, one of the best combinations is using KT120s at about 65 ma each and 8K U/L output trannie. I use that in my system. Power output is about 18 watts and the sound is really excellent. I have thought as well as others that have done this that it has to do with the physical construction of the tubes. I don't know, but it has the best sound. Just be aware of the heater current needed.

Good listening
Bruce

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PostPosted: 03 Nov 2020, 11:51 
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Location: Boston, USA
Hi Bruce- I've been running my dual-oddwatt (http://ohm.bu.edu/~hazen/TubeAmp/#oddoctal) for about 3 years now and it sounds great! I gave up on Shuguang KT88s though since they would never stay in bias. I've been running JJ KT77s and it sounds good but I miss the punch of the KT88s.

Since KT120 are similar in cost I'm thinking about upgrading to them (I think my XPWR117 heater winding should be fine, as it is rated 6A).
What brand(s) of KT120 have you had good luck with?

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PostPosted: 03 Nov 2020, 12:02 
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Hi, Yes the KT88s you had have difficulties in this sort of circuit. I suspect it has to do wit thermal stability but I don't know for sure. I use Tungsol KT120s at around 65 ma each. Really lovely sound IMO. The balance will fluctuate a bit but not enough to matter...1-3 ma typically over time. Just set them, let them heat up a while and reset as needed. Then check a few days later and adjust if needed. After that they seem to behave well and only need to be checked about every six months. Beware there was a bad batch of KT120s floating around about 3 years ago or so. They could not handle the dissipation and would red plate quite easily. If you are sold some of them, make them exchange them until you get good ones. Even reputable dealers got stuck on this one.

Good listening
Bruce

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