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Old 14th October 2012, 02:51 PM   #11
kevinkr is offline kevinkr  United States
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Originally Posted by M Gregg View Post
Regards the windings,

I am thinking of putting two 60V bias windings on the heater Toroid which is 120VA. So they are low current windings.

M. Gregg
Besides the tedium of winding several hundred turns on the existing core there is also the distortion and rectifier harmonics from the filament windings if you are using dc for filament heating.

You might consider a small EI or toroid, were you here in the U.S. I would recommend one of the Antek line, some of them even have electro-static shielding. In terms of dealing with garbage (dust ) from the line you can always use a common/diff mode choke on the primary side.
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Old 14th October 2012, 03:06 PM   #12
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Yes, 60V will require a few turns but if its fairly thin wire that's not a big problem.
As post 8 suggests, and many before it, make yourself a little bobbin that will easily pass through the centre of the toroid (allow for your extra turns). Calculate how long the winding will be and add a good few metres (its sickening if you are a few turns short). You don't need to be exact about spacing the turns around the toroid, close enough is good enough, just try to space them around the core.

HF "hash" passing through the core. Yes, it's true but I've never had any problem with it. A bit of filtering will usually get rid of it. Hell, you've got a toroid in there aleady.
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Old 15th October 2012, 12:57 AM   #13
tomchr is offline tomchr  Canada
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Been there, done that: Turning a 6.3 V secondary into a 5 V secondary on an Antek 2T350

It probably took me about an hour to add two 12 V windings to a toroid. I'm not sure I'd attempt it with a bias winding - definitely not without a bobbin.

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Old 15th October 2012, 03:22 AM   #14
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Originally Posted by M Gregg View Post
Regards the windings,

I am thinking of putting two 60V bias windings on the heater Toroid which is 120VA. So they are low current windings.

M. Gregg

#30 awg magnet wire is ample for bias winding....virtually no grid current flows, only in the voltage deviders...

you can wind say 10 turns to get your actual turns per volt....then you will get your actual turns for 60 volts...when winding, always add 1 turn at the end....

this is very doable....and a lot of fun...
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Old 15th October 2012, 10:30 PM   #15
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For bias applications, using a simple voltage doubling rectifier circuit will cut the turns count in half. I get -15vdc bias from just 24 turns of #30 wire around the power toroid using this technique. It is close wound and covered with 1 turn of 1/2 in strapping tape.
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Old 3rd May 2015, 09:52 PM   #16
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I just added a winding to a tube preamp toroidal power transformer so that I could bump up the B+ by about 50 volts. It was incredibly easy, the trick is definitely all in making a really nice bobbin shuttle. Mine could have been a bit thicker, as I wound the wire around it lengthwise. I used a 26.5 inch long piece of aluminum. I notched the ends for the wire, and then rounded all corners and polished smooth. If I did it again I would use thicker aluminum so that the wire bends around a larger radius at the ends. Perhaps most important, I covered the ends with a shiny smooth liquid rubber that's sold for sealing scratches in vinyl-coated dishwasher racks. It goes on like fingernail polish, but is slightly rubbery, and not high-friction grippy. I should coat all the corners, perhaps the whole thing!
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Old 3rd May 2015, 10:34 PM   #17
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