What is the best sounding GZ34 / 5AR4 rectifier tube you've ever heard?

It depends on the system (as a whole) and your personal preference.

I have various Mullard/Blackburn), Amperex/Holland, later 2003-2004 (when they momentarily remedied the reliably issues) Sovtek as well as a few manufactured in Japan.

They all sounded good (to me) depending upon the gear/system I used them in and my favorite varied with each/different setup.

Reliability seems to be an issue with contemporary products, but I've mainly read about this in guitar audio forums.

Never owned a DH/SET amp that was tube rectified and have used them in vintage Pilot and Dynaco PP amps as well as Fender Princeton/Deluxe and Super Reverb guitar amps.

As an example, when I ran early production (fat sounding) Mullard EL84's in the the Pilot (232/240) amps I preferred the slightly lighter sound of the Sovtek and Japanese tubes.

If I used old production EI EL84's (lighter sounding than the Mullard's) I preferred the slightly richer sound of he Mullard/Amperex 5AR4's.

In my Dynaco (ST-70/MkIV) gear, which always ran Mullard XF1/XF2 EL34's, I preferred the slightly more detailed Amperex over the Mullard version in the ST-70's and the Mullard in the MkIV's.

I've forgotten the model/version codes on the old production Amperex/Mullard 5AR4's (recall 31/32 maybe), but can tell the earlier versions by looking at the plate construction (welded VS crimped tab construction).

This said both types sounded good/close.

Many say the metal base Amperex version is the holy grail, but I've only heard them in unfamiliar (to me) systems both HiFi and guitar/amp based - so no opinion on that.

Sorry if this does not answer your question.

DeKay

Bliss:

There are some major spec differences between 5ar4 and 5r4gy(s).

Aside from a considerably higher voltage drop the 5r4 also has a faster warm up time.

Not familiar with 5u4g, but once looked into the other and decided not to go there with my amps.

DeKay

Bliss:

This was for my Pilot 232, which in stock configuration already runs the outputs rather hard..

There were suggested mods @ the time (like adding a t/f choke to the PS), but I didn't want to bother as I had moved on to 300B and 2A3 amps.

I still have it, but @ this point it would have to be properly refurbished (especially the PS considering the higher AC wall voltages experienced now).

I reformed the original multi section cap twice, but can't imagine that that the 3rd time would be a charm.

Had to replace the large ceramic resistor off the rectifier on both of my Pilot amps (232/240) which is telling.

DeKay