Request some assistance diagnosing a problem in the power amp of a
Marshall Artist 4203.
Amp runs 2xEL34's,12AX7 phase inverter and a solid state rectifier.

The problem is loss of power.
Usually within a minute or two of start up.
It can occur within a few seconds of start up, which makes me think it
is not necessarily heat related.

I have replaced tubes and filter cap.

The interesting clue is when I open the standby with signal present,
there is a brief surge of near full power.
I am guessing, but it's like the charge in the filter cap drives it
once the supply from the rectifier is removed.

Does this automatically suggest rectifier diodes as the problem?

I am new at this and have only a basic theoretical understanding - any
help greatly appreciated.
Schematic available he
http://www.drtube.com/schematics/marshall/3203pwr.gif


Thanks.