Marshall 100w OT rewind

[andresound]

Hi guys, after researching some of our local forums for a recommendation, I sent a Marshal 100w OT in for a rewind. This company was 400 miles away from me. I had some conversations with the “tech” who was doing the job (because it was taking so long!), and I got the feeling they did not quite know what they were doing. Herewith is a copy of their job card. Before I connect, I was hoping some of you wise gentlemen would look at the way in which this was wired and offer advice as to the correct connections. My understanding, is ...
Brown = common
Black = 4 ohm
Yellow = 8 ohm
Green = 16

All of the above from the “top” winding?

[Stevem]

I understand where you live and all, but I need to ask, is a iffy rewind job enough cheaper to your wallet then a new one ?

Here in the states a direct replacement 100 watt stand up bell type ( non nylon bobbin lay down type) is not even 110 bucks.

What ever you do when you get one reinstalled in the amp be sure to wire in a 250 ohm 10 to 20 watt resistor across the 16 ohm tap so you remove one potential source for blowing up the new one!

[andresound]

Thank for the reply Steve. The rewind cost approx $50 including transport. If I purchased a new one overseas and imported it, it would be $70 just for shipping!! .

Regarding the 250 ohm resistor... herein comes the confusion... the wiring sketch in brackets at the bottom of the page shows different color scheme to the sketch at the top !

[martin manning]

The primary leads and center tap are easy to identify with DC resistance. Then you can easily measure the voltage ratios and sort out the secondary leads. With AC applied on the primary, the highest voltage on any two secondaries indicates the outer leads, and progressively lower voltages from either end to the other leads will identify the taps.

[Stevem]

You can also do the voltage test / secondary tap test out just by jumping the amps heater voltage into the secondary side and reading the voltage across the whole primary side.

[andresound]

The primary leads and center tap are easy to identify with DC resistance. Then you can easily measure the voltage ratios and sort out the secondary leads. With AC applied on the primary, the highest voltage on any two secondaries indicates the outer leads, and progressively lower voltages from either end to the other leads will identify the taps.

Thanks Martin, we can always count on you helping with advice. And Steve, great idea with the heater suggestion.!!! Great advise both of you. This is why I hang out here . TAG rocks!!!!

[andresound]

Ok by using the “top” sketch of the secondary and using brown as the common... these are my readings using a variac to set the heater tap to 5v AC applied to the primary.

5v AC applied to primary

Brown - black = .247v
Brown - yellow = .348v
Brown - green = ,486v

Does this make sense. Obviously no load attached to the secondaries. Also do these readings make mathematical sense (turns ratio for this OT)?

If I use the sketch on the bottom right of their drawing, there is an extra winding (marked as “start”), which has an added120 turns...which also a brown wire of different gauge...and I use this as the common, these are my readings

5v AC app to the primary

Brown - green = .477v
Brown - yellow = .612v
Brown - black = .711v
Brown - brown = .960v

[andresound]

My second set of measurements seems to suggest that the turns ration is roughly 5:1?
And the first set of measurements seems to suggest a turns ratio of 20:1?

So it seems the 1st set reflects a primary of 3.2k into 8 ohm? This must be correct??? I am NOT a maths wizz!! So please correct me if I am wrong

[martin manning]

Ok by using the “top” sketch of the secondary and using brown as the common... these are my readings using a variac to set the heater tap to 5v AC applied to the primary.

5v AC applied to primary

Brown - black = .247v
Brown - yellow = .348v
Brown - green = ,486v

Ok, 0.348/0.247 = 1.41, 0.486/0.247 = 1.97. Good for impedance ratios of 2 for each tap, so black is 4 ohm, yellow is 8 ohm, and green is 16 ohm. Then voltage ratio primary to 16 ohm secondary is 5.0/0.486 = 10.288, and 10.288 squared to get impedance ratio = 105.84. 105.84 times 16 = 1694. Looks good to me.

[sluckey]

You already have the number of turns given. They're on the printed table and also on the hand drawn pic. 1240 turns end to end on the primary and 120 turns end to end on the secondary (brown to green). So the turns ratio is 1240:120, or 10.33:1. Square that for the impedance ratio of 106.78:1. Put a 16Ω speaker across the brown and green wires to reflect 16 x 106.78 = 1708Ω back to the primary. Sounds about right for a quad of big tubes.

Or, brown to black is 61 turns. So the turns ratio is 1240:61, or 20.38. Square that for the impedance ratio of 415.34:1. Put a 4Ω speaker across the brown and green wires to reflect 4 x 415.34 = 1661Ω back to the primary. Still sounds OK for a quad of big tubes.

Or, brown to yellow is 86 turns. So the turns ratio is 1240:86, or 14.42. Square that for the impedance ratio of 207.9:1. Put a 8Ω speaker across the brown and yellow wires to reflect 8 x 207.9 = 1663Ω back to the primary. Still sounds OK for a quad of big tubes.

[andresound]

Thank you Sluckey and Martin

[martin manning]

This simple test should be done on all OT's before wiring into an amp. Wrong color or miss-identified leads occur in production transformers, so best to take a few minutes to make sure the part is what the documantation says it is. In this case the technician's sketch has the secondary lead impedances flipped top to bottom with respect to the colors.

[tubeswell]

Black (Common), Yellow (4), Green (8), Brown (16) is what he has written there - so do your Pri:Sec VAC measurements from those. (Or is that your writing?)

[andresound]

Black (Common), Yellow (4), Green (8), Brown (16) is what he has written there - so do your Pri:Sec VAC measurements from those. (Or is that your writing?)

Not my writing but, as it turns out ..., I have the OT back in the JCM800 and it sounds great. However the correct color scheme, ... was Brown (common), black (4), yellow (8) and green was 16.

Martin’s suggestion to always measure is spot on !!. Thanks to all who chimed in. I have now learned another chapter on OT’s and how to check and calculate.