Missing 6.3V CT/'Cockeyed Bridge' problem - diyAudio
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Old 15th January 2009, 06:35 PM   #1
pjanda1 is offline pjanda1  United States
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Default Missing 6.3V CT/'Cockeyed Bridge' problem

I'm working on a power supply for a version of Eli's El Cheapo that uses the iron from a Sherwood S-5000. I got a good distance through the construction before realizing that I don't see the 6.3V winding's CT. There is one on the schematic. (the trafo has center tapped HV, Bias and 6.3V secondaries and a 5V winding) I removed this thing from the S-5000 myself. I haven't opened it up, as the caps appear to be attached with an adhesive and I don't want to pry on it. No CT wouldn't be a big deal, but here's my problem.

I measure .4/.5 ohms resistance from the HV (360V) secondary's CT to either leg of the 6.3 winding (which measures .9 ohms from leg to leg). It is indeed the 360V winding's secondary too, as the colors match and I measure 56/61 ohms to either 360V leg and 116 ohms between them. I don't know much, but it seems to me that the CT's of the 6.3V and 360V secondaries are internally tied. Is that possible/likely?

If that is the case, then surely I can't use both the "cockeyed bridge" and the 6.3V winding, as the cockeyed bridge takes the B+ from the CT, right?

pj
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Old 16th January 2009, 04:00 PM   #2
pjanda1 is offline pjanda1  United States
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I carefully pried the end bell off. As is suspected, it was filled with some sort of damping/potting material that cracked up bit. I loved how mechanically quiet this transformer was on my line (which often gives trafo's fits). I hope it remains so.

I think I get it now. I never checked the resistance from the 360V legs to the 6.3 legs before. The readings are nearly the same as from the 360V to the CT. It appears that the 6.3V isn't it's own winding, but taps on the 360V secondary. Is that possible? For all I know it's common.

I'd really appreciate some help to figuring out whether this transformer will work in this circuit. Also, when I reassemble this thing, do I just try and fit all the pieces of crusty stuff back in? Is there something I should be adding to hold it together?

pj
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Old 17th January 2009, 01:13 AM   #3
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Dude,

FWIW, I'd forget about the Sherwood power trafo's 6.3 VAC capability and get something separate. The AnTek AN-0506 looks like a possibility.
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Old 17th January 2009, 01:41 AM   #4
pjanda1 is offline pjanda1  United States
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That was my assumption. Is that thing 6V or 6.3V? Can't beat the price or the size. If you say it'll work, that would be a perfect solution. The Triad's look like economical options too. 3A should be enough right? Of course, if I get antsy by next week, I might just order a Hammond as Digikey gets here quick.

Does it make sense to locate the filament transformer in the amp chassis? I know folks do it, but I'm nervous about running a long noisy filament pair in the umbilical. I think I'll have room in the amp. I guess I can't just hook up the filaments to load down the transformer and check the B+ voltage. I hope the Sherwood trafo goes back together easily.

pj
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Old 17th January 2009, 03:50 AM   #5
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Quote:
Is that thing 6V or 6.3V?

It's 6 V., with 115 V. on the primary side. A 4 A. draw is no problem. I can't see why that toroid would fail to to the job. Another advantage of toroids is a minimal magnetic interaction with the surroundings. Put it inside the chassis. Make darned sure to put ferrite beads on the primary connections, right next to the trafo. Toroids have a much larger bandwidth than EI and you want to keep crud out of the heaters.
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