Mystery Output transformer

[Buddha's Guitar Tech]

First time ever trying to decode a suspect OT.
Selmer 30SV – someone modded it from EL506 to EL34 and swapped in a replacement OT. Had a hunch it was wired incorrectly... Secondary.

3 taps. Four wires are Black, Brown, Orange, Green.
Black was wired common.

FADED HAND-WRITING on the casing, I can just about see that someone wrote BROWN is common. So I'm scratching my head.
I have a Fluke 179 meter; doesn't read low resistances very well, so I can't trust simple DCR measurements across the taps.

With BLACK as common, turn ratios looked like this:
GR–8:1
BR–11:1
OR–38:1

Using ORANGE as common:
GR–11:1
BR–17:1
BK–38:1

Using GREEN as common:
BK–8:1
OR–11:1
BR–31:1

Using BROWN as common:
BK–11:1
OR–17:1
GR–31:1

I got these results by sticking a fixed AC source across the primary legs and measuring voltage on the other side. Divided primary V by secondary V.

My question is: does that jump up to the 30s seem out of whack? It happens between Black & Orange, and Green & Brown... I'm still scratching my head, drawing it out...

Am I losing my mind?

[Phil_S]

Remember, the impedance ratio is the square of the turns ratio. That means there will be a jump. For example, a 4K OT with 4-8-16 ohm taps will have turns ratios of 31.6, 22.4, and 15.8.

[Buddha's Guitar Tech]

Check what I have...

The chart on the right is a known OT. Primary impedances in a similar range are grouped by colour.

I can't find any such pattern in the chart on the left, my OT.

[martin manning]

Which could mean it is defective.

[Buddha's Guitar Tech]

That's my gut feeling.
The amp actually sounds ok – pretty loud. The single output was wired black & orange – 37.6 : 1 into 8 ohms.

[R.G.]

People play tricks with transformers.

If I were faced with this problem, I would put a voltage on the secondaries some way and then use an oscilloscope to tell what voltage ratios *and* phase the secondary wires put out.

One of the tricky things that can happen is that for 16, 8 and 4 ohm taps, it is possible to wind the sections all separately, which is what we expect for "taps". However, it is also possible to wind the 16 and 4 ohms windings simultaneously with non-identical wire, as a 4-ohm winding needs half the turns of a 16, so from one end of the winding to the other, the CT is the 4-ohm output, the 16 is the full winding. The 8 ohm winding has turns equal to 0.707 times the full 16 ohms winding and 1.4 times the 4-ohm winding. This could be done as a tap on either the full-16 side or the end-to-CT side and 41% into the 16. That lets you change the common between CT and one end of the full 4/16 winding.

**WHY** anyone would want to do that is a mystery, but it is possible if someone saw a way to save a buck.

Watching the voltages and phases on the various windings tells you if you have a wire in the middle of other windings as a "common" as the voltage is in one phaseon one side, the opposite on the other side.

[katopan]

Phasing is important when measuring to really determine which wires are the two ends of the winding. Or of course you can work that out with a meter that measures low ohms OK.

Is this single ended or push-pull? Just having trouble finding solid info on the SV30.

[Buddha's Guitar Tech]

Katopan, this is a replacement OT. Push-pull. Someone stuck it in there years ago.

[Buddha's Guitar Tech]

RG, I'm afraid my brain can't keep up.

Let me get something straight. **If** the secondaries were wired separately, we wouldn't see continuity through all four wires, right?
I'm getting readings through all four, suggesting to me that this is a tapped OT. Am I getting the lingo right?

Did you mean, stick some AC on the primary, and check phase & volts on secondary? Did that, but didn't check for phase. (I do have a scope.)

BEFORE THAT THOUGH...

This is getting complicated; my experience tells me when that happens, to go back to principles.

I measured DCR on the secondaries again. I used shorter leads and got some results that at least showed a difference between wires. I even used a second meter, not quite as accurate, but the readings from that supported the Fluke's.

Both meters measured ohms to one decimal place, so when they danced between .1 and .2, I used an arbitrary .18.

And along with all the data I collected yesterday, I've drawn a plausible conclusion.

The big '30' turns ratio support the idea that green & brown are close to each other, and that orange & black are close to each other.

Primary impedance: I'm still scratching my head. With green tap @ 16 ohms, and brown tap @ 8 ohms, you get results close to each other. (around 1,110) Orange @ 4 ohms is way off. If it was for a 1-ohm speaker... Now I'm talking crazy.

[Buddha's Guitar Tech]

Yeah, so uhhh... THIS might explain a few things...

[martin manning]

I was going to suggest a test for shorted turns. This has been described by R.G. himself, and I wondered why he hadn't suggested it. You could still try it and see what you get.

[Buddha's Guitar Tech]

I was already leaning toward the idea of shorted turns, based on what I perceived as wild jumps in turns ratio. But is there a rock-solid method?

[martin manning]

Described here: http://www.premierguitar.com/articles/t ... r-tester-1

[katopan]

Yeah, that'd do it.

Other test that can show a lower reading for shorted turns is measuring inductance. But you need a meter that can measure it.

Or measure current when you hook it up to an AC supply. A short will draw more than the typically small amount of leakage current.

[Buddha's Guitar Tech]

Interesting, thanks Martin.