50 watt JCM800 clone

[fixer2]

Hello it’s been a few years since I have been here I recently finished a build of a StewMac JCM800 I have owned for five years and never got around to building it till this year well I ran into a issue on first testing it I got to the second test where I put only the pre amp tubes in no power tubes yet and I get with the power on but stand by in off ( not in play mode) and I get at the filter capacitors 470 DC volts turn the stand by on ( to play mode ) and the voltage dropped down to 57 volts , what do you think it is I need to be pointed in the right direction and then I can find it I have the b+ wires going to the correct spots and I think I have the rectifier diode correct ? Very puzzling any help will be good