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.
