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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 24 Jul 2011, 12:45 
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Joined: 26 May 2009, 18:51
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Location: Lilburn, Georgia USA
I just now got out the Edcor Transformers I bought for the KT 120 Oddblocks and really looked at them. They are huge. Easily twice as big as the ones for the KT88 amps I already have built and by the way have done well for 2 years. I guess I will have to build a base from some 3/16 " aluminum plate I have on hand. Then inlet some wooden sides. These will be some large and heavy amps no doubt. I just need to know what the different resistance values will be for the cathode resistors and such. I am ready to start!


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PostPosted: 25 Jul 2011, 09:49 
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WOW! :shock: 3/16" Aluminum plate! You do realize that that's thicker that the skin on the space shuttle main fuel tank right?

I build all my base plates from 80mil 6061-T6 and it very sturdy stuff. I would think that if the transformers are really as heavy as you say, you could go to 100mil 6061 T-6. That should support about 100 pounds of transformers on a good size chassis. But 3/16" is about 187mils. That's insanely thick stuff.

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PostPosted: 25 Jul 2011, 16:42 
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I agree, you don't need to go that thick. I think 50 mils would be ok, if you mount the heavy iron near folded corners.

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PostPosted: 25 Jul 2011, 18:22 
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Location: Lilburn, Georgia USA
I know it is overkill but I don`t think the Hammond 12 X 10 X 2 inch chassis are large enough size wise. The metal I have is from rear inside shelves out of parcel delivery trucks that were being outserviced and the shelves were going into the scrap aluminum dumpster so I asked for some of them and they were given to me. I don`t know if my Greenlee hole punching dies will go through 3/16" aluminum or not. I did notice that Harbor Freight now has a hydraulic hole punch tool that punches up to 3 and a half inch holes for $99. I paid $160 for the Greenlee set. The transformers I am going to use are the Edcor CXPP70-MS-3.5K and the XPWR117-120/240 360v 500ma and 12v 6amps. I plan to run the heaters on the AC on this one. These transformers weigh 10 pounds each. If Bruce put these same trannies on one of his KT88 Oddblock kit chassis I bet it was a tight squeeze what with 2 LM317HV heatsinks and 2 KT120s . I like to spread all this heat producing stuff out a little bit further apart. I am still waiting for a final schematic on this one. Thanks, Robert Parsh


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PostPosted: 26 Jul 2011, 12:34 
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Hi Robert.

The 16 x 8 chassis may be a better size. If you got them for free, go for it. I'm not sure about punching 3/16", but I can't imagine you will have any problems drilling the aluminum with a step drill.

Cheers

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PostPosted: 26 Jul 2011, 19:57 
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Gio wrote:
I'm not sure about punching 3/16"...


I've gone that thick for an 845 build and I could punch it still with a 3/8" ratchet and Greenlee :D

Cheers!

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PostPosted: 26 Jul 2011, 20:31 
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Location: North Carolina,USA
Ya know you could thread all your mounting holes ... in that thick of material!Give it that profesional touch and make it easier to work on.Technically a girl,Ha Ha!
David

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PostPosted: 27 Jul 2011, 19:04 
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Location: Lilburn, Georgia USA
Yes I will thread the mounting holes. I remember somebody else here did that too. I`m glad to hear you can cut 3/16" with the Greenlee punches. You know, with this thick aluminum I could go ahead and mount both channels on one platform. There would be room for a Forewatt on there too. I got my order of K40Y9 caps, 20 of the 400volt and 20 of the 1kv, all .22uf. the 1kv aren`t really that big . I got 18 6N1P tubes also. Half of them theEB suffix. Thanks, Robert P.


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PostPosted: 27 Jul 2011, 22:26 
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I should mention that I used the ball bearing bolt thingy on the punch (I only use the ancient square head drives for thin stuff)

Cheers!

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PostPosted: 29 Jul 2011, 11:28 
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Hi, Sounds like fun. Yes the CXPP70s are big. I managed to get them and the XPWR102s on a 12X8 chassis (used to be KT88 monos) but it was tight and I only did it because the bulk of the support wiring was already in place. The 12X10 is OK but I would opt for something like a 14 or 16 X10. Extra room is good especially with things that give off so much heat. Putting the transformers in corners away from each other and at right angles to each other is best for hum reduction. Building in a Forewatt is a viable option as well. The extra gain may be useful with sources that don't deliver sufficient output. A hint now on the selection of driver tubes. I have found a wide variation in the gain and some of the distortion levels (may be gain related as the lower gain tubes will not have enough gain to use the minimal NFB properly). I have more tubes coming to try, but so far the Tungsol 12AX7s were the best ( :confused: ). JJ ECC803s, ECC83s, and some NOS ones I had around all had either higher distortion or less gain (or both). In this amp (not in the other Oddwatts though) the driver is critical. To keep the design simple, it is working a lot closer to its practical limits than in the other builds. It can do the job, and do it well, but inferior tubes will limit the performance of the amps. The amps will also work with the 5751 without change, but require more input to achieve full output and I would reduce the value of the NFB resistor by about 25%. I have not tried 6SL7s but they ought to work like the 5751s. :thumbsup:

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