Marshall JCM 800 power transformer

[jeff]

 Well my bro changed his mind and wants to go with a marshall JCM 800(thanks for the jubilee help). I was looking at the schematics and can't understand what PT to get. All the PTs I've seen for a  JCM 800 have been 345-0-345. The schematic shows a B+ of 367V for the 50W. Am I misunderstanding this. I thought with a SS CT full wave rect DC=1.4XAC so wouldn't that give a B+ of 483V? In order to get a B+ of 367V wouldn't I need a 260-0-260?

It doesn't seem to make sense to me for the 100W either because it uses a full wave bridge rect so wouldn't a 345-0-345 CT be 690V X 1.4 or 966V! Wouldn't the 100W need a  167-0-167V?

So for the 50W I'm thinking I need a 260-0-260V at what mA?

Thanks
 Jeff

[sluckey]

You don't use the same transformer for a 50 watter and 100 watter. Your calculations are correct for no load but you are overlooking the effects of loading. Here's the info you need...

For JCM-800 50 watt use Doug's MPT-50 transformer. (345-0-345VAC @ 150ma) (conventional rectifier)
For JCM-800 100 watt use Doug's MPT-100 transformer (350VAC @ 290ma) (full wave bridge rectifier)

[jeff]

So a 345-0-345 transformer with a solid state rectifier will produce 483V no load, but under load the voltage will drop 116V to 367V? I thought you only got significant voltage drop with tube rectifers.

[sluckey]

It all depends on the amount of load you put on the PT. The PT has internal/winding resistance. The more current the PT has to supply, the more voltage will be dropped internally. If you place a zero ohm load on the PT, the voltage will drop to zero.