[mark norwine]

As a general rule, it's not a great idea to hit cold power tubes (or even preamp tubes for that matter) with high voltage.

With a glass rectifier, there's no high voltage until the rectifier tube heats up. By that time, the other tubes are warm....maybe not fully hot, but warmed up nicely.

A solid state rectifier delivers high DC voltage immediately. With no standby switch, you're hitting cold tubes with high voltage.

The copper cap is designed to mimic the sag of a glass rectifier tube, but is ultimately a solid state rectifier. It will present high voltage to your cold tubes immediately on startup. This is why your tech is recommending against its use. I concur.

In fairness, many old ampegs were built with ss (diode) rectifiers & no standby switch. My all-time fave amp (GS12-R) is that way. I've been smacking cold tubes for years....but it's still not a great idea.

If my amp had a glass rectifier & no standby switch, I'd stick with glass.

EDIT....That said, I wouldn't use a sovtek 5Y3 ever. Ever measured the forward Vdrop characteristics of one? It bears little resemblance to the specs of a 5Y3. And, after all, NOS American 5Y3 tubes are cheap & plentiful.