When does a 5V4G/GZ32 have to be replaced?

In my experience they rarely need replacing unless they fail.I would keep a spare on hand though because when they go bad they usually just stop working with very little drama (although the other tubes are not going to light up either).
If it makes you feel better you can remove it from the socket whenever you change the output tubes and shake it next to your ear to make sure that none of the internals have worked loose. I had one rectifier tube that was sketchy and had a piece loose inside- sometimes the amp would work but after moving or transporting it it wouldn't work at all...until I moved it again.

I would not check the voltage on the rectifier tube filament, if that is what your speaking of. The voltage there is several hundred volts, and *deadly*. Even if you could measure it, it wouldn't tell you the condition of the rectifier. The high voltage will be on all the pins. That tube changes the high voltage AC, to high voltage DC. Way to risky unless you no what your doing, and have a good high quality meter capable of handling the high voltage there. Plus you would need a schematic for, and understand it.

Pardon the grammar, I just got worried about losing a new member.

I'll to try to give you a little idea, why you wouldn't measure the 5 volts on the filament. There would be the full B+ plus high voltage on it *also*. That is why it is dangerous. Pin 8(rectifier tube) on this Dynaco amp (link below), has the output from pin 8 (high voltage DC ) coming out of it (rectifier tube) to feed the filter caps, and choke for the B+ high voltage. I don't know your amp, but on this one, that it basically where the high voltage for the B+ originates. With the schematic, it would tell the proper voltage that should be there. This amp shows 480 volts after the choke. Risky high voltage. That's why you need to know what is happening, and I don't recommend trying to measure it. Dynaco Mk3 link [http://www.triodeel.com/dynamk3.gif]