Inside an Output Transformer

[Gregg]

Hi,

Finally I got a MOP50-T which is a taiwanese copy of MOP50 both currently sold by New Sensor at the lowest wholesale price for such a product. They claim to be built as per Marshall specs. According to users these offer descent quality. Since there is great reluctance to publish/share winding data for OTs in the net I decided to sacrifice one of those in order to learn what's inside it?
So do you want to know what's inside this babe and how it's wound?

[Alex R]

Certainly do.

[Tiago]

+1!

[defaced]

I'm interested.

[Gregg]

Here's some preliminary data:

1/ Dimensions - 96x80x41mm, which means EI96 laminations 41mm across. This translates into approx. 13 sq.cm. core.

2/ Resistance of the primary - 138 Ohms. One half measured 62 Ohms the other one 76 Ohms.

3/ Primary inductance. Two types of measurements were performed:

a/ using the method from the link below:

http://engr.nmsu.edu/~etti/fall96/el...ct/induct.html

The results using 1Volt signal and 1kOhm resistor are:

Hz L /Henry/

50 - 85,9
60 - 75,7
70 - 68,8
80 - 62,1
90 - 57,0
100 - 54,8
200 - 31,8
400 - 23,4
1000 - 28,9

b/ using a semi professional LCR meter /measures only at 1kHz/:

22.9H

In both cases there's plenty of it. The leaflet that came with the transformer states 55Hz-20kHz frequency range. It looks like it's not exaggerated.

[slowfingers]

What about a de-gutted shot

[Gregg]

What about a de-gutted shot

I'm not there yet. Coming up soon!

[Old Tele man]

b/ using a semi professional LCR meter /measures only at 1kHz/:

22.9H

In both cases there's plenty of it. The leaflet that came with the transformer states 55Hz-20kHz frequency range. It looks like it's not exaggerated.

...as a double-check, simply backsolve that 55Hz low-frequency value to see what value Lpri is needed to produce that 55Hz (-3dB) value.

[R.G.]

Here's some preliminary data:
In both cases there's plenty of it. The leaflet that came with the transformer states 55Hz-20kHz frequency range. It looks like it's not exaggerated.

Don't forget the crucial high frequency measurement - the leakage inductances.

You need to measure the plate-to-plate inductance with the secondary shorted. This gives the bulk high end cutoff leakage.

Then measure inductance of half-primary to shorted secondary for each half-primary to get the asymmetry caused by non-symmetrical coupling of half-primaries.

Finally measure the inductance of one half-primary with the other half-primary shorted. This leakage will give give you a number for the inductance that keeps the primaries from smoothly crossing over between tubes.

You gotta do the short circuit tests before you de-wind, because shorted tests tell you the details of the high frequency end.

[Gregg]

Don't forget the crucial high frequency measurement - the leakage inductances.

You need to measure the plate-to-plate inductance with the secondary shorted. This gives the bulk high end cutoff leakage.

Then measure inductance of half-primary to shorted secondary for each half-primary to get the asymmetry caused by non-symmetrical coupling of half-primaries.

Finally measure the inductance of one half-primary with the other half-primary shorted. This leakage will give give you a number for the inductance that keeps the primaries from smoothly crossing over between tubes.

You gotta do the short circuit tests before you de-wind, because shorted tests tell you the details of the high frequency end.

Will do. Unfortunately I can't use the LCR meter /which I borrowed/ but I suppose I can use the other method from the link this time at higher frequencies?

[Gregg]

I did some measurements using the same method. The results are in the table below.
The light blue section is leakage inductance of the whole primary.
The second table is one half of primary, the shaded section is with the other end of primary shorted to ground. The same applies for table 3.
I'm not very much into theory/math so if the calculations below are wrong please correct me.

I know this formula for calculating the reactance for a given leakage inductance:

ZLL=Lx2pixF,

where L is leakage inductance in Henrys and F is the high frequency in this case 20kHz.

If the result is lower than OT's impedance it's OK. In this case OT's impedance is 3800 Ohms.

ZLL=0.0267x6,28x20000=3353.52 which is less than 3800.

Please feel free to make any comments.

[Bruce / Mission Amps]

What does an amp sound like with this output transformer installed?

[Gregg]

I haven't performed listening tests yet but I have that in mind although they will be more or less subjective.