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Old 9th October 2009, 01:37 PM   #1
bst is offline bst  United States
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Default Best way to terminate unused OPT winding?

I recently picked up several NOS Thordarson T-17S12 output transformers at an estate sale. These were designed for two 6L6s running push-pull, with a 4300 ohm primary, and secondaries tapped at 4, 8, 15, 250 and 500 ohms. They also have a tertiary winding intended for 10% feedback.

Even though still boxed and unused, these are very old transformers (approximately 70 years!), so I removed the end bells to inspect for deterioration. Everything looks amazingly good, but I began to wonder if I shouldn't simply clip off the leads for the unused taps before buttoning them up again.

I plan to assemble two simple push-pull amps with 12W6s as output tubes. I'll be running these transformers well below their maximum ratings of 95mA per side and 25 watts.

I've seen recommendations to ground one end of a completely unused winding, and to just tape off the leads for unused taps. Is this the best practice?

Thanks for helping out a relatively inexperienced builder; I'm finally getting a basic understanding of theory, but have little experience in practical new construction.
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Old 9th October 2009, 03:15 PM   #2
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IMO grounding one side of an unused winding serves no useful purpose. Just isolate all unused leads by taping them up. I usually slip a piece of heatshrink tubing over the ends first.
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Old 9th October 2009, 03:21 PM   #3
pjp is offline pjp  India
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Wouldn't grounding one end of an unused winding effectively create a capacitor to ground ?
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Old 9th October 2009, 03:42 PM   #4
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Extra windings in an output transformer create added distributed capacitance to the core just by being there. I don't thing grounding one side affects that very much at all. But I'm not 100% sure.
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Old 9th October 2009, 04:43 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
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Wouldn't grounding one end of an unused winding effectively create a capacitor to ground ?

YES! PJP is dead on with that comment.

And I would not cut off the unused leads either.. Nothing is forever and you or the next guy might have a brilliant idea circumvented by the cut off leads
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Old 9th October 2009, 04:47 PM   #6
kevinkr is offline kevinkr  United States
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Extra windings in an output transformer create added distributed capacitance to the core just by being there. I don't thing grounding one side affects that very much at all. But I'm not 100% sure.
In my limited experience this does have an effect, even though you are quite right about the distributed capacitance.

I have noted significant differences in resonances and overall HF extension in some transformers depending on which end of a secondary is grounded, and also true in the case of grounding one end of an unused or infrequently used winding.

For safety I always ground the secondary and live with the sometimes measurable consequences on frequency response. A friend who has extensive transformer winding experience has actually rewound diy transformers upon discovering that grounding the secondary seriously altered out of band resonances in terms of frequency, Q, and amplitude peaking.
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Old 9th October 2009, 04:50 PM   #7
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I'd crank a square wave though the amp and look what happens as you terminate the unused winding to different places. Depending on the % and winding location, it may or may not have an observable effect -- but it may help kill a HF resonance, for example, by tying to to another winding or ground (or it may also make things worse). I'd also agree with the other fellas and say don't cut the leads.
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