Hey thanks very much for the info!
The instructions I was getting were a little unclear as english is the second language and we had a significant time zone difference that delayed replies. But I did confirm the diode was shorted with the Fluke 117 meter and the amp did work with the replacement.
But I appreciate you letting me know the rectifier diode is not the correct replacement.
I do have the circuit diagram but not allowed to share online. The diode is indicated 120V and in parallel with a 10uF to ground on the B+ that feeds the 6SL7 plate.
There is a 5W resistor prior to that to reduce the 370V to 120V so the diode is there to regulate the 120V in case of high voltage if I understand the purpose from what you mentioned?
I was also given the prior version R300 schematic and there is no zener diode in that amp. Nor shown on the photo of their amp in house....so I am leaning towards just deleting it altogether as it seems it might have been an afterthought to help better regulate voltage. I would not want it shorting out again.
Will confirm with the builder.
Thanks again! You learn something new everyday on these forums.