Mystery Output transformer
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- Buddha's Guitar Tech
- Posts: 178
- Joined: Sun Dec 06, 2009 6:21 pm
- Location: Larne, N. Ireland
Mystery Output transformer
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?
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?
None More Black – guitar & amp repair – https://www.facebook.com/nonemoreblack
Re: Mystery Output transformer
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
- Posts: 178
- Joined: Sun Dec 06, 2009 6:21 pm
- Location: Larne, N. Ireland
Re: Mystery Output transformer
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.
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.
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None More Black – guitar & amp repair – https://www.facebook.com/nonemoreblack
- martin manning
- Posts: 13547
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Mystery Output transformer
Which could mean it is defective.
- Buddha's Guitar Tech
- Posts: 178
- Joined: Sun Dec 06, 2009 6:21 pm
- Location: Larne, N. Ireland
Re: Mystery Output transformer
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.
The amp actually sounds ok – pretty loud. The single output was wired black & orange – 37.6 : 1 into 8 ohms.
None More Black – guitar & amp repair – https://www.facebook.com/nonemoreblack
Re: Mystery Output transformer
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.
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.
Re: Mystery Output transformer
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.
Is this single ended or push-pull? Just having trouble finding solid info on the SV30.
- Buddha's Guitar Tech
- Posts: 178
- Joined: Sun Dec 06, 2009 6:21 pm
- Location: Larne, N. Ireland
Re: Mystery Output transformer
Katopan, this is a replacement OT. Push-pull. Someone stuck it in there years ago.
None More Black – guitar & amp repair – https://www.facebook.com/nonemoreblack
- Buddha's Guitar Tech
- Posts: 178
- Joined: Sun Dec 06, 2009 6:21 pm
- Location: Larne, N. Ireland
Re: Mystery Output transformer
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.
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.
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None More Black – guitar & amp repair – https://www.facebook.com/nonemoreblack
- Buddha's Guitar Tech
- Posts: 178
- Joined: Sun Dec 06, 2009 6:21 pm
- Location: Larne, N. Ireland
Re: Mystery Output transformer
Yeah, so uhhh... THIS might explain a few things...
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- martin manning
- Posts: 13547
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Mystery Output transformer
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
- Posts: 178
- Joined: Sun Dec 06, 2009 6:21 pm
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Re: Mystery Output transformer
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?
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- martin manning
- Posts: 13547
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Mystery Output transformer
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.
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
- Posts: 178
- Joined: Sun Dec 06, 2009 6:21 pm
- Location: Larne, N. Ireland
Re: Mystery Output transformer
Interesting, thanks Martin.
None More Black – guitar & amp repair – https://www.facebook.com/nonemoreblack