Willsenton R300 Circuit Design Issue

So I was able to repair my Willsenton R300 after it died and made no sounds.

The dealer was good to work with me thru emails from China to troubleshoot the problem instead of shipping it to a repair center.

It turned out to be a diode in the preamp circuit that shorted out. The diode the builder used is a 1N5380B which is for use in 5V-200V circuits. The B+ voltage in the preamp is supposed to be 370V dropped to 120V by a power resistor adjacent to the diode. However I was getting 417V dropped to 140V. So within spec but would not take much of a current surge or voltage spike on startup etc to fry the little diode.

I replaced it with a 1N5408 rated to 1000V. (Less than $1 repair cost!)

So the reason I posted this is that I have read some reviews of users having crackling noise with the R300. It could be that diode.

Hopefully the builder updates the design with a sturdier component.