I've got a GZ30 rectifier I got a while ago with a high powered Tweed Deluxe to run it with 6L6 tubes. Most seem to use a 5AR4 so to be careful I took it out and used 6V6s with a 5Y3. I don't have that amp anymore and was wondering what I could use the GZ30 for?
I suppose a N.O.S GZ30 (and by N.O.S I mean Mullard, since these are the most well known examples of this tube) can be used in place of a 5Y3GT in most modern boutique amps that accept "multiple" rectifiers.
The Sovtek tube that is marketed as a 5Y3GT today must be built on a 5Z4GT/GZ30 platform from what I can tell by looking at the tube.
Empirically speaking, the GZ30 must be a slightly "stronger" tube than a 5Y3GT as the headroom in my amp is higher with the GZ30 than with 5Y3 type tubes.
The direct European equivalent of an indirectly heated 5Y3 type tube is the French made 5Y3GB.
Most tube manuals state that a 5Z4 has the same qualities as a 5Y3 (regardless of where it was made). Not stronger, doesn't take more current, doesn't handle more voltage.