Rewinding Power transformer

[ThSpeakerDude88]

I wasn't sure where to post this question, but since it is for a tube amplifier I guess it works in this area.

I have a 115v:120vct transformer that I need to make 115v:115v. This would effectivly give me 124v on the secondary, as my wall voltage is 124v. Even better, I would like to make it a little lower , say around 117v or so, doesn't have to be completly accurate.

It also has a center tap at 60v. I would still like to use this tap, and I am thinking that if I unwind a few turns that it will give me about 50v, which is more like what I need.

I have never rewound a transformer before, but it seems simple. Am I ok just unwinding a few turns? I might not even need to dissasemble the laminates, as there is enough space I believe to remove a few windings on the secondary without problems. If not I will just dissasemble the laminates and then re-varnish them when finished. If I do this, however, what should I use to varnish the laminates? Would ordinary polyurethane work ok?

[SY]

Lazy fellow that I am, I'd just add a 6-12V transformer with the same or higher current rating and use its secondary as a buck winding.

[ThSpeakerDude88]

Too big. This transformer is oversized as it is for my projects, at 120v 500ma.

I decided to try it after all, and I removed around 50 turns or so repeatedly checking with my dmm untill I reached 122v with a 123v input. This means It should be fine with a given input of anywhere between 120vac and 124vac. When using the center tap , I get on the upper leg, 67v ( unloaded) and on the lower leg I get 54v ( unloaded).

[Tom Bavis]

Your center tap is no longer centered, but I don't think that matters - you're not using it for a full-wave half-bridge, are you?

If it's wound on a bobbin, no need to varnish, just tape over it. If it's paper insulated, varnish and tape.

[ThSpeakerDude88]

Yeah, its a new hammond with a plastic bobbin. And no I don't need the center tap for fwhb, I'll only be using it for heaters and the like. The wires were actually covered with plain old plastic tape, so I'm thinking some good sticky masking tape with some poly over it will be just fine.

Btw, I was able to unwind it without having to take the lams apart. It only took about 15 minuites too!

[Ty_Bower]

Originally posted by SY
Lazy fellow that I am, I'd just add a 6-12V transformer with the same or higher current rating and use its secondary as a buck winding.

I wish someone could explain to me exactly how this is done. I've read more than one reference to using filament transformers to buck line voltage, but no one goes into any kind of detail. It's kind of assumed that everyone just naturally knows how this works. I'm sure it's real obvious once someone shows you how to do it... but it hasn't clicked for me yet. Maybe a picture would help...

[HollowState]

Ok Ty,
A small 6v transformer can be used to boost or buck another transformer. This can be done either in the main transformer's primary or secondary. But it's most often done in the primary. Please refer to my attached hand drawn diagram.

The idea is to use proper phasing of the smaller transformer to either add or oppose the main transformer. When the small transformer is in phase with the main one, it's output is boosted. When it's 180º out of phase, it is bucked.

By reversing either the primary or the secondary of the smaller 6v transformer, you will boost or buck the output of the main transformer.

With standard American levels of 117v, a 6v transformer will provide about a 5½% change. A 12V transformer, about 11%.
Hope this makes sense.

Btw, this works well up to about 15%. Beyond about that level there is heavier current flow and heating in the buck mode.

Victor

[chrish]

Probably posted too late, but if you have a 115:120 and your wall voltage is 124 and you want 117 volts, couldn't you just wire it backwards? that would give you about 118v...

[Ty_Bower]

Originally posted by HollowState
A small 6v transformer can be used to boost or buck another transformer...

The idea is to use proper phasing of the smaller transformer to either add or oppose the main transformer. When the small transformer is in phase with the main one, it's output is boosted. When it's 180º out of phase, it is bucked.

Of course. That makes sense. The picture helped a lot, thanks.

[ThSpeakerDude88]

I could have... I probably should have now that I think of it, because the primary winding would be running at less than normal voltage which would probably equate to it running cooler. Oh well! It works fine the way it is now.