Metro is a good kit.
Also... www.tubesandmore.com has their own MOD kit, a marshall knock off.
The key - is the output iron. SPEND the money on good good iron.
Why are Mercury transformers expensive? Because people are willing to pay the price. Laminate iron varies. Wires pretty much the same, but coil cores and separators vary. The interleave techniques to matter, but not as much as you'd think. The REAL key to that "vintage" vibe is having a transformer that is ok electrically, and a bit understated magnetically. IOW, you want the coils to hold up, especially when its having problems gettting enough EM field going. That will make them run hot. The EM field is what makes it go. Less iron makes it harder to do. You want that - because as the transformer gets magnetically "saturated" sonic wonders begin to happen. The right amount of saturation comes from careful selection of the laminate metals, the way they interact with the coils, and the amount of laminates used. Some transformers might even use mixed lamination materials. Its a bit of an art but you want a certain percentage of saturation in the transformer, in the right frequency ranges (lows saturate first, but at what point and at what frequency), and you want to do that at a target operating point, and impedance. There's a bit more to it. There is also some things in single ended (and some push pull ) transformers that must be closely observed, and held to tight tolorance. One is the so called "air gap" in single ended (and some p/p trannies). Its sort of like a coil gap in speakers - not really the same but - must be held to a close tolorance in the control of saturation of the magnetics. And on and on.
Cheap transformers work. Select ones work like we want them to!
Also... www.tubesandmore.com has their own MOD kit, a marshall knock off.
The key - is the output iron. SPEND the money on good good iron.
Why are Mercury transformers expensive? Because people are willing to pay the price. Laminate iron varies. Wires pretty much the same, but coil cores and separators vary. The interleave techniques to matter, but not as much as you'd think. The REAL key to that "vintage" vibe is having a transformer that is ok electrically, and a bit understated magnetically. IOW, you want the coils to hold up, especially when its having problems gettting enough EM field going. That will make them run hot. The EM field is what makes it go. Less iron makes it harder to do. You want that - because as the transformer gets magnetically "saturated" sonic wonders begin to happen. The right amount of saturation comes from careful selection of the laminate metals, the way they interact with the coils, and the amount of laminates used. Some transformers might even use mixed lamination materials. Its a bit of an art but you want a certain percentage of saturation in the transformer, in the right frequency ranges (lows saturate first, but at what point and at what frequency), and you want to do that at a target operating point, and impedance. There's a bit more to it. There is also some things in single ended (and some push pull ) transformers that must be closely observed, and held to tight tolorance. One is the so called "air gap" in single ended (and some p/p trannies). Its sort of like a coil gap in speakers - not really the same but - must be held to a close tolorance in the control of saturation of the magnetics. And on and on.
Cheap transformers work. Select ones work like we want them to!
. I also bought a complete '68 era 100 watter that is flawless. George has just released a 50 watt kit with instructions that are beyond obsessive compulsive. Anyone can build that one. 
