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 Post subject: Re: KT120 Oddblocks
PostPosted: 27 May 2011, 21:26 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, As I mentioned in another thread, I retired early this month and did a bit of traveling. 8-) (see the EL84 thread). I just got back to the KT120 oddblocks this week. Good news of sorts. You can parallel LM317s to get more current. They do each need a separate resistor though. I set a pair a 150 ma each and turned it on... no smoke or fire and everyone was happy. :) The odd waveform was definitely related to the protective circuitry in the LM317. They are surprisingly fast to act and when the event (too much dissipation) doesn't take place un-react. I still need to do some work on the amps as the power supply is not up to the job. I'm only getting 365 volts on the plates. I really designed for 450-475. This restricts the output power. I'm using the CXPP70-MS-3.5K Edcor outputs as the smaller ones can't handle the B+ current. I have some higher voltage power trannies on order as well. However, as it stands now I get a clean 36 watts RMS before the power supply causes compression. The compression is symmetrical. I have tried alternate driver tubes and it seems that the 5751s are better than the 12AX7s I planned in the original design. (more linear and lower distortion). I don't want to put out figures for that yet as it will be influenced by the B+ available. I see an easy 40 to 45 watts with the correct voltages. I used the prototypes at an audio fest three weeks ago and everyone really liked how they sounded (powerful and in control of the speakers). Really good rockers.

Good Listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 30 Jun 2011, 20:49 
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Location: Arizona, USA
Hi All, An update. I got the bigger power trannies yesterday. I now have plenty of B+. 440 volts and up to 400ma. The initial results are quite good. I'm using a pair of LM317s with a single sense resistor for 310ma (per pair of KT120s). Edcor CXPP70-MS-3.5K output trannies as well. I am temperarily handicapped in making all the normal measurements as I smoked my PC based DSO (see posting under diy on that :eek: the KT120 amp was undaunted by the event). So my back up stuff doesn't give me as accurate results as I would like. Indications are 45 watts rms class A U/L. I spent a goodly amount of time listening to the amps. They give an authoritarian sound. Powerful, accurate, excellent sound stage and dynamics. Not for the faint of heart. The KT120s are still low miles so it may change, but if the final results hold up, this is a winner. BTW, it also generates a lot of heat. Each mono block consumes close to 200 watts so in class A mode you know where it goes. Great for someone like Gio in the winter. Here in Arizona in the summer, it is not quite as welcome.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 02 Aug 2011, 03:15 
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Joined: 25 May 2011, 15:02
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Hi Bruce
Just wondering how the project is coming along over the past month, any further thoughts on how its settling down, or is just to hot to listen in Arizona at the moment? over here in the UK its miserable (at least around Manchester).

Tony


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 Post subject: Re: KT120 Oddblocks
PostPosted: 02 Aug 2011, 14:25 
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Location: Arizona, USA
Hi Tony, It is quite uncomfortable here as well. :hot: It's the rainy season and about 90 + F every day and high humidity. Fortunately it will end in about 6 weeks. There has been progress on the KT120s. I'm still not completely happy with the distortion levels at high power. It may be an artifact of the test method. Not too many amps have to deliver 45 watt sine waves. I am also concerned with the actual tubes. The KT120s may not be real clean to start with. :? They remind me in construction of the Sovtek 6L6WXT. They are not great in this design (I'm being kind there). I would love to have something to compare the Tungsols with. Global NFB in the modest amounts that I consider OK is not enough to clean them up to the level I would like. I am working on local NFB now on the individual power tubes. That's all the bad.... The good is they are formidable performers. The sound is a lot cleaner than the numbers would indicate (reaches 2% at 20-25 watts). Full range response like all its sibling Oddwatts. Below 10 HZ to past 25K. S/N excellent as well. I figure I'll be done with the design this month and will publish the changes from the original postings. My sense is this is not the most refined amp in the series (the EL34 version is) but it certainly sounds a lot like the others and has a huge amount of power for a class A U/L amp.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 Aug 2011, 04:08 
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Bruce, thanks for the update keep up the good work.

Tony


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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 Aug 2011, 07:00 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
mmmmm... Paralleled push pull -> 90W!!

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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 Aug 2011, 12:26 
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Hi Mark, You can only run them in the winter (I suspect it is that now down there so you need to build them fast) as your AC will get overloaded. :hot:

Good Listening

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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 Aug 2011, 16:45 
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I would have thought that PP would have been more efficient. Paralleled SE now that's another story. I'm use to the heat from tubes. I still run the Be Bamp KT120s in listening area number "2". I can even feel the heat when I walk past. I did drop the current through them as the power tranni start to melt after an hour or two of play. Much better now.

I understand you get it real hot where you are.

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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 Aug 2011, 21:54 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Mark, The class A U/L PP version is more efficient than SE or parallel, but the difference between 25% efficient and say 40% when you are pumping in 155ma per tube at 450 B+ is kind of academic from a house warming point of view. Locally we do get rather warmish. My conversion chart says it gets up to about 41 C from time to time. This week only 35 C, but the humidity is up to about 60%. Good weather to stay inside and listen to music. Back to the KT120 Oddblocks. I expect to finish them up this month. There are some choices that may make it a better or at least simpler project for many diyers. One I use AC heaters. It simplifies the build quit a bit. Getting enough DC for the heaters on these monsters is not a simple process. The difference in S/N is measurable, but not so much a problem with such a monstrous output. Also I'm sure it has to do with how tubes perform close to overload vs SS gear. But they sound more like 100 watts than 45. Second choice was to use the 12AX7 in place of the 5751 or 6SL7. Lots of opportunity for tube rolling here. BTW the best sounding ones so far are (gasp) the new Tungsols. Less distortion than the more commonly available ones. I have a bunch of other 12AX7s coming to try so their leadership may be short lived. The transformers I used (which will not be easy to get in your area) are inexpensive and work great. Stay tuned for further developments.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 04 Aug 2011, 03:54 
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Bruce: Bloody good work. I think you suggested earlier that the Tung Sol KT120 may not be the best. What other tube have you thought about? A 12AX7 driver sure makes the amp attractive to tube amp builders. And as you said lots of rolling to be had at the front end. I'll stand by the Golden Dragons for the price I think they are good value. They make a gold pin version too. Some where in the future I must ask Matt to take a look at a SE version (KT120 SE) which exceeds the Be Bamps slapped-together-and-take-a-listen approach. I have a few of Matts other designs to knock-up first. For now the Be Bamp get lots of use and the big tubes give me a hint of what could be gained with better SE designe.

Once more mate, good work.

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