Buy real old production Mullard/Philips GZ34’s (or two) from a reputable tube dealer before they become unobtainable @ sane prices.
Seen them recently in the $150-$200+ range.
Don’t fret about metal base et cetera as they all sound good and last if properly tested (even the early 70’s production).
Aside from Mullard/Philips I have a few 1983 Sovtek’s and much earlier Japanese 5ar4’s that sound good and are reliable, but that boat passed years ago as I purchased them NIB way back then.
Used Mullard GZ34’s 10-15 years ago in HiFi gear that were from guitar amps that I owned in the mid 1960’s on, as well as some from Dynaco gear that I purchased in the mid 1970’s.
PS:
I used them in Pilot 232/240 amps, which run the crap out of the rectifier/GZ34.
DeKay