Transformers in Marshall® Amplifiers
There are a lot of myths and rumors around about the "old" transformers Marshall® used for their amplifiers in the beginning. As nearly everybody knows, even all parts were bought from Radiospares, the transformers too. All transformers were normal catalog products and could be bought by everyone. But which versions of the Radiospares transformers were used?Radiospares itsself changed their suppliers about the years and therefore the transformers changed too. For further informations, please take a look here.
The JTM45 was one of the first amplifiers Marshall® build and was fitted in the first years with the Radiospares Heavy Duty mains transformer. This type was only available for the European voltages in the range of 200-250 Volts. Radiospares itsself only offers this version. A 120 Volt tap never existed for this version. So be careful if you see cloned Radiospares Heavy Duty transformers with this 120 Volt tap, the original for this type never existed. The 120 Volt tap was first introduced by using the later Drake PT with the voltage selector on the top too.One other point is the Radiospares De-Luxe output transformer. I have seen five different versions of this "poor man´s" output
transformer of
the Partridge P5352, but which one was used in the JTM45?
You often see on other sites the version with the black panel, which make you believe, that this was the right one. But it wasn´t. The older version with the stucked label was the built in output transformer in the JTM45. 
if you take a look at the inside, you can see the differences with a naked eye. The older versions are sngle wound, one piece after the other, the newer ones were fitted on a multiple winder. But these are not the only differences, the newer one has an other winding arrangement.
The same things applies to the Marshall® 18 Watt Amplifier. The Radiospares Hygrade PT was availabe only for European voltages too and on the picture the right EL84 output transformer was used.As you can see, it is a hard work to collect all these things before to make a clone of the right transformer. I know, there a lot of companies arround, one or the other are writing articles, but are they giving you this deep inside? And the above story is only a very short one.