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Old 8th January 2009, 10:55 AM   #1
tmhajw is offline tmhajw  United Kingdom
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Default transformer unused secondary noobie question

Hi There
Further to my 76/6sn7 pre build I'm going to get a second filament transformer to reference voltages for the cathode follower.
I'm looking at an rs components 2x 0-6.3v.
I only need one of the 0-6.3 secondaries. Can I leave the second unused ?
Sorry if this is a dumb question- bit of a newbie.
Thanks
Thomas
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Old 8th January 2009, 11:32 AM   #2
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Yes, you can leave it unused, but I would recommend grounding one of the leads to reference the floating winding to something. Another option would be to parallel both windings for a more stiff 6.3V. That way you load the transformer evenly, and get better regulation. However, you might end up with 6.5V or something at the tubes instead of 6.3; depends on the size of the transformer.
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Old 8th January 2009, 11:47 AM   #3
tmhajw is offline tmhajw  United Kingdom
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Thanks for your quick reply!
Just to clarify, when you say connect one of the leads of the floating winding to earth, I presume you are referring to the unused winding?
If I parallel the two windings do I have to worry about correct 'phase'?
I've got some dropper resistors at the ready as my mains supply is rather high (247vac) & I've used them elswhere....
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Old 8th January 2009, 01:32 PM   #4
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Correct, the unused winding should get some kind of reference. The assumption is that your used winding already has a correct reference based on its use in the circuit.

If you parallel windings, phase is critical. Easiest way to do this is identify the 6.3V windings. They will be the lowest resistance windings as measured by an ohmmeter. Call them W1 and W2 windings, each winding producing 6.3V between its leads. Connect any ONE of the leads of W1 to any ONE of the leads of W2. Then, measure the voltage between the unterminated leads. The correct phasing will give you less than a volt. Incorrect phasing will give you around 12V.
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Old 8th January 2009, 01:50 PM   #5
tmhajw is offline tmhajw  United Kingdom
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Ah, enlightenment..I think I understand...
Thanks very much for your help.
(Yes btw the used winding will be referenced to ground via a voltage divider...)
Transformer now ordered.
Thomas
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