Testing Turns Ratio On Output Transformer

[RickyLee]

This is on a 50W Marshall style output transformer. I used my VariAC and had/measured 9VAC feeding the supposed 8 ohm tap on the secondary. This gave me 151VAC on the primary winding.

151/9 = 16.77 ratio.

16.77 squared = 281

281 X 8 = 2248 load resistance

Is this close enough regarding these specs? I was expecting to see something closer to a 20 to 1 ratio.

 

[Alabama Thunderpussy]

Dude go the other way. Feeding from the secondary side is dangerous. First time I did it absent minded, my 15am variac vibrated and sucked in a screwdriver from a foot away. God knows how much voltage I generated on the primary.

 

[RickyLee]

Dude go the other way. Feeding from the secondary side is dangerous. First time I did it absent minded, my 15am variac vibrated and sucked in a screwdriver from a foot away. God knows how much voltage I generated on the primary.

I wanted an accurate reading. Did not want to mess with mV in the translation. If I went 120VAC into primary side, that would have given a good reading above 1VAC on the secondary as well. I used coated clips and such and monitored the high side while I turned the VariAC up SLOWLY from 0 to 10VAC.

Trust me, I was thinking about that possible high voltage as something similar happened to me on an unknown Xfmr a few years back where I had over 800VAC and messed up a cheap meter.

 

[RickyLee]

@Bama: What's your take on the voltage ratio I had?

That ratio I had works out to a 12 ohm load for two EL34's. So Would this OT be a good candidate for running a higher load/15 ohm cab to knock down a slight bit of output/volume?

 

[VanR]

[ame=http://www.youtube.com/watch?v=nNlPgirHP48&feature=c4-overview&list=UUuR4hQTXkG_KxozLxwPzEjQ]Output Transformers: A Discussion of Basic Principles - YouTube[/ame]

Start about 12 minutes in.

 

[mickeydg5]

Ricky
Can you post some pictures? How do you know it is a Marshall clone OT?
Describe the taps, give length-width-height and weight.

 

[RickyLee]

I will see if I can get a picture later.

I did find out it is a Weber Marshall style 50W. But there is a low headroom and high headroom version.

There's no model numbers on this thing at all.

 

[RickyLee]

So the winding is that way for either reason. Seems to me that this OT would be slightly louder having the windings this way? Or am I looking at it backwards? Less resistance in the windings would have less voltage drop across them giving slightly less output?

My questions go either way. My first question sounds good related to the secondary. But then the second part of my question sounds logical regarding the primary winding.

 

[VanR]

This should be the same OT in this thread:
http://www.marshallforum.com/workbench/66860-ot-what-tubes.html

 

[mickeydg5]

Ah, you finally gave a clue.
If it is Weber at about 50 watts with a little extra voltage I would guess WOT45LHR.

Now it makes sense. That second tap is truely a 16 ohm with primary of 3400 ohm. This will get you to around a 16 turn ratio for that tap. If the tap is used as an 8 ohm tap then the primary presents itself as 1700 ohm.

Do you see it?

Add
VanR got the thread with the Weber specification sheet in it.

 

[RickyLee]

Ah, you finally gave a clue.
If it is Weber at about 50 watts with a little extra voltage I would guess WOT45LHR.

Now it makes sense. That second tap is truely a 16 ohm with primary of 3400 ohm. That will get you to around a 16 turn ratio for that tap. If the tap is used as an 8 ohm tap then the primary presents itself as 1700 ohm.

Do you see it?

Add
VanR got the thread with the specification sheet in it.

Yes, I understand that and did look at that spec sheet. Was figuring the math should work out closer to 3400 ohm instead of the lower 2248 load resistance I measured. I will have a lower load resistance then the aprox. 3400 ohm reflected back in a EL34 spec. So a higher headroom design would possibly see a higher turns ratio or a higher load resistance calculated out than the 3400 ohm?

I also just got very bummed as I measured the ohms on the primary. 94 ohms total and 60 ohm on one side and 34 ohms on the other.



I was ready to scrap this OT as I figured that is a defect. Never heard of a center tap being off by that much. But then as I thought about it, maybe that is part of the design to generate distortion?

 

[VanR]

I wouldn't worry too much about the difference in ohms measured. If it works and sounds good that is all that matters. Every OT is off some because of the way they are wound.

 

[RickyLee]

Yeah, I have never seen more than a 20% mismatch.

Hopefully this OT has some MadgiK MoJo TON3 going on.

 

[RickyLee]

Another question: How close can you have the output xfmr to the power xfmr? I have them both standup and the OT is 90 degrees in relation to the PT. But I am looking at the Studio 15 right now to see if I can fit this 50W OT in there. It would be even closer to the speaker frame as well. The Artist 30W OT fit in there quite well, but this 50W is a tighter fit for sure.

Is it one of those wire it in and move it around and check for noise and interference type experiments?

 

[VanR]

Frankly I always turn them 90 degrees and as close as they have to be. Never had any problems and I sure haven't used the headphone listening thingy either. I got better things to do.

You may be thinking about this too much. Just hook it up and play and see what's what.

 

[RickyLee]

Frankly I always turn them 90 degrees and as close as they have to be. Never had any problems and I sure haven't used the headphone listening thingy either. I got better things to do.

You may be thinking about this too much. Just hook it up and play and see what's what.

Are you saying you have installed PT and OT extremely close to each other with no noise issues?

Yeah, I am trying not to over think any of this. Just trying to learn as much as I can. Trying to fish info out of the theory experts on this Forum. I am sure there's a few others that might be interested as well . . . .

 

[mickeydg5]

Ricky

I am sorry about forgetting to address that issue. I would think that one side of the primary or maybe even that and the secondary may have heat damage. That is putting it off on one side. That is not normal. The ratio should have been around 14.6 for the 8 ohm tap at 1700 primary.

If the sides of the primaries measured 37 and 34 or something like that it would be normal. 60 and 34 is bad. I believe over power and heat damage caused this situation.

It is best to keep the transformer plates at 90° and as far apart as possible.

 

[VanR]

Are you saying you have installed PT and OT extremely close to each other with no noise issues?

Yeah, I am trying not to over think any of this. Just trying to learn as much as I can. Trying to fish info out of the theory experts on this Forum. I am sure there's a few others that might be interested as well . . . .

I'm not suggesting they be close together unless you have no choice. If you have plenty of top side chassis room put them on either side of the chassis. But you got to put them somewhere and sometimes that is closer than you would like.

 

[VanR]

Ricky

I am sorry about forgetting to address that issue. I would think that one side of the primary or maybe even that and the secondary may have heat damage. That is putting it off on one side. That is not normal. The ratio should have been around 14.6 for the 8 ohm tap at 1700 primary.

If the sides of the primaries measured 37 and 34 or something like that it would be normal. 60 and 34 is bad. I believe over power and heat damage caused this situation.

It is best to keep the transformer plates at 90° and as far apart as possible.

So this is a used PT? Then I might agree with Mickey.

 

[RickyLee]

Supposed to be new unused. I am hoping I will hear back from Weber with a reply on the specs. But I am not holding my breath . . . .