5U4 versus GZ34 (5AR4)?

[mbratch]

I have an amp that will accommodate either one of these rectifiers. However, I did notice that the GZ34 runs a lot "hotter" in bias than the 5U4. In other words, if I bias my power tubes with the 5U4 in the amp, then replace it with the GZ34, then the bias will be way too hot (it nearly doubles) and it has to be adjusted down by increasing the bias resistance.

But here's my question: assuming I were to adjust bias in each case to the same bias current, which of these rectifiers is generally preferred?

I guess we can consider this a poll: do you like the 5U4 better, or the 5AR4? And why? What's the difference in behavior between these two given that they're adjusted to the same bias current on the same set of power tubes (all other things being equal)?

[Old Tele man]

the GZ34(european)/5AR4(usa) rectifier has a 17 Vdc drop.

the 5U4(usa) rectifier has anywhere from 44 to 54 Vdc drop depending upon which variant (5U4GA, GB, etc.) and manufacturer.

[mbratch]

Thanks OTM, that's informative. I'm wondering whether given that one adjusts the bias according with the given rectifier, how it (5AR4 v. 5U4) affects the sound of the amp, all other things being equal? And whether designers prefer one versus the other?

[Blue Strat]

A 5u4, or any rectifier that provides lower output than another, will give more sag, less headroom, looser bass, and less brightness.

Beyond this, it's all about personal preference and what volume you're playing at. You might prefer a 5U4 with the amp on 4 and a GZ34 with the amp on 8.