It's true that the GZ34 is a very good rectifier. However it took Philips/Mullard a while to achieve that. The datasheet
http://www.r-type.org/pdfs/gz34.pdf is very telling. The first two graphs show the forward voltage drop as a function of current through a single diode. "Hang on" you say "Why are there two graphs for one parameter ?". The answer is in the bottom left hand corner of the page - it's the date. At the start of 1954 if you drew 100mA through a GZ34 you'd expect a forward voltage of more than 15V. By the middle of 1958 that had dropped to just 8.5V. The change at 400mA is nearly as impressive - from more than 35V in 1954 to less than 22V in 1958.
Interestingly guitarists will pay a king's ransom for 'metal base' GZ34s - ones like this
http://www.jogis-roehrenbude.de/Roeh...by-Amperex.jpg. I believe these were only made in the very early days and perhaps it's the poorer performance (=increased sag) that they're looking for ?
Cheers,
GJ