I have a Supratek Sauvignon preamp.

It uses a 5ar4 as a rectifier. Can I substitute a 5z4gy with no problems. 

Quote from the link I posted:

American designation 5AR4, European GZ34 are one and the same, and there is no proper substitute for this unique, indirectly heated cathode type-- the Mullard version is perhaps the best rectifier tube ever. Definitely do not sub 5R4, 5U4 or any filamentary cathode type; they produce DC almost immediately which could cause cathode stripping on your precious 300Bs (and any other tubes) Micronetics in Switzerland (Tubes@mnfoc.com)is a great supplier of this and many other scarce tubes. I e-mailed them yesterday; they have nearly a thousand (!) Mullards, as well as hundreds of Valvo and metal-base Philips GZ34. The Mullards are $65 per-- considering the alternative of paying $14 for a Sovtek which will never come close to the Mullard in performance and will last perhaps 20% as long as a Mullard, $65 is not too bad. Ebay is another source, as is Brent Jesse Recording online. His prices are great and he is a decent guy. Andy Bouwman of Vintage Tube Services (616)454-3467 may also have these, but his specialties are VT231/6SN7, 6922/7308, and ECC European series tubes. Hope this helps, Chris

You could also call any of the referenced tube suppliers. I have purchased from Andy Bouwman before and he is very knowledgeable. Good luck.

I purchased plenty of tubes from Andy. He thoroughly test his tubes.
I have plenty of Mullard 5ar4 tubes, fat base, older double o getters. They are very robust, but are a little cool sounding compared to something like a Mullard gz32.
In my preamp I like the sound of the Brimar and Philips 5r4gy tubes. They are directly heated, which can be hard on my 6sn7's on start up.

Thats why I'm interested in the 5z4gy tubes which are indirect heated. 
Hi Joe,

You may already be aware of this, but according to my literature the 5Z4GY has considerably lower ratings for the parameters shown below than for the other tubes you mentioned. So the answer would depend on the specific conditions at which the preamp operates the tube. I have no knowledge of the specific design.

Max DC output current (ma):
5Z4GY: 125
5R4GY: 250
5AR4: 250
GZ32/5V4GA: 175

RMS supply voltage per plate (volts):
5Z4GY: 350
5R4GY: 750
5AR4: 450
GZ32/5V4GA: 375

Max peak current per plate (ma):
5Z4GY: 375
5R4GY: 650
5AR4: 750
GZ32/5V4GA: 525 

Best regards,
-- Al