Help a dummy out please:

for Dynaco Mark II/III/IV and DIY PP monoblocks

Help a dummy out please:

Postby Blair » Mon Jun 04, 2007 8:57 pm

How close to a toroid can a 6550 or KT88 be? Here is what I have so far:

http://img.villagephotos.com/p/2005-12/ ... /AMP_5.JPG [img]

Here are a few more pics:

http://www.villagephotos.com/pubbrowse. ... id=1535206



Hope the image works. I've never posted an image before here just links, but I originally intended for the tranny to bottom mount. I can still pull this off, but it will get crowded. Any suggestions for how to cover this up if I do top mount it?

Thanks in advance for any advice, and you can laugh as you look at the picture. I'm young and dumb;)

Blair[/img]Image
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Postby mesherm » Mon Jun 04, 2007 10:47 pm

You could mount it vertically on a larger angle plate than I used.
http://webpages.charter.net/mesherm/mm-3.jpg
http://webpages.charter.net/mesherm/mm-4.jpg
I can tell you from experience that a toroid PT will get hot.
I eventually had to install a stainless steel mesh screen behind the black filter cap and the mounting plate.
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Postby EWBrown » Tue Jun 05, 2007 5:18 am

Very nice looking projects! I'm green with envy of your workmanship. My projects usually end up looking like Russian WWII surplus Yellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_02

If there is room under the chassis, mount the toroid there, it would be away from the radiated heat of the tubes, and there would be no need to pass the leads through the chassis plate. Besides, bare naked toroids are spugly Yellow_Light_Colorz_PDT_01 If you want to top mount it, look for a nice looking metal cover, one place to look is the local Ikea or other "fancy" kitchen supply store, they usually have all sorts of stainless steel containers for dry foodstuffs.

One major caveat, when using a metal cover with a toroid: Don't create a shorted turn by running the bolt through the top cover, through the toroid and to the chassis plate. This will create a very low resistance shorted torn, and all sorts of evil and nasty things will happen :o
The mounting bolt will have to be isolated from the cover, with an insulating shoulder washer or some other method.

It will need ventilation and airflow, which is good practice with any (tube or SS) power amp, if you use a bottom plate, you would need to make it of perforated material, and stand the base off the surface with some feet, at least a half inch. The chassis plate will act somewhat as a heatsink for this trannie.

For the two OPTs mounted on the wood, they should have individual wires, connecting their frames to a chassis safety ground. "Floating" trannie frames / cores can cause all sorts of nettlesome problems.

HTH

/ed B in NH
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Postby Blair » Tue Jun 05, 2007 7:24 am

Thanks Ed.

Well, I was going to mount the toroid to the bottom, but it is a bit too tall. Not that I can't add feet. I was planning on that anyway. Maybe I should just use an angle grinder and grind the toroid down a bit Yellow_Light_Colorz_PDT_08 .

What I was thinking to gain a bit of clearance was cut a hole in the top plate about the size of the toroid, and then cut a larger diameter piece of Maple to match the rest of the frame. This way, the toroid gets countersunk a bit into the top plate. The top plate is not metal by the way, it is masonite. Yes, I know this is not optimal, but I'm going to adhere a copper plate to the bottom to block RFI. I just worry about the tranny getting really hot down there and burning whatever the amp is sitting on.

Any thoughts?

Thanks again,

Blair
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Postby kheper » Fri Jun 08, 2007 6:39 am

The company below sells toroid transformers
with covers and covers for toroids. Maybe
they can make you one that fits.

http://www.engineeringvista.com/trafomatic/index.htm
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Postby EWBrown » Fri Jun 08, 2007 6:52 am

I've got a couple of their ST35 toroid tranny sets, through E-bay.
Boris Sasic is a good guy, and is located in upstate NY, though the trannies are custom ordered from Trafomatic in Serbia (former Yugoslavia). Originally I ordered one set of two OPTS and 1 PT, but they goofed and sent me two sets of OPTs at the 3-tranny price, and wwith a 10% additional discount. Then I bought two power trannies at a generous discount, and in the end, came out ahead on the total cost All said and done, I got about $75 off the total cost. So their "bad" was my "good" and I can't complain Yellow_Light_Colorz_PDT_01 Yellow_Light_Colorz_PDT_03

How do they sound? I dunno, I haven't gotten around to building up an amp yet.. Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_02

/ed B in NH
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Postby Blair » Fri Jun 08, 2007 7:04 am

Here is where I'm at. I managed to find something to help the clearance issue a bit. I founf a bunt cake pan with a removeable bottom, the cone shaped center fits perfectly inside the toroid. I recessed a washer inside the cone for support and to countersink the bolt head. I assembled the PS and one channel to see if I had everything I needed to make sound, and turned it on last night. I did not run a signal through it yet, but all tubes look happy. I'm still a bit torn as to whether I want to try to fit that tranny on top between the tubes. I'm not afraid of the heat as much as the looks. I also fear RFI if the tranny is bottom mounted. I guess I would really need to run power to see if the amp is quiet or not before jumping in and worrying. I just spent so much time on this chassis. Looking hindsite, I should have clamped the chasis to a piece of mdf and routed a semi circle in the top of the chassis in between the two opts thus leaving muc more room for my tranny up top, but I ain't that smart and this is only my second tube amp build. Thanks for all the advice guys.

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Postby Blair » Fri Jun 08, 2007 7:36 am

One more dumb question. When biasing the tubes. Is it an inverse tuning. i.e. When I move the pot screw out, one tube goes up in voltage and the other goes down? It was late, and I did not have time to tinker with the bias much.

Thanks
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