Just found out that on my OT the red is NOT the centre tap on my tranny. I had read the reports of the blue and brown being reversed and my amp tech told me that red for centre tap is standard in Fender and Marshall OT trannies. I had completed the build and gave it to my amp tech to test and he found a problem which he thought was the OT being faulty. Nik suggested measureing the resistances red-blue-brown-red and sure enough red was NOT centre.

Easy if you know to check and kit builders should be made aware of it I think.

I just hope that the secondaries are correctly colour coded as per layout drawing.

Nik did admit that my transformer is wrongly colour coded and it does happed occasionally and that his transformer manufacturer will get a slap!

If yours is centre tap red then

red-blue = red-brown =  (blue-brown)/2