Genalex GL GZ-34 Rectifier...Did I just get a bad one? Five months seems

Trelja. Thanks for the encouragement.  I still think the cryo treatment is the main problem. It may be fine for the other tubes. But the different materials expand & contract at different rates,. This may work in tubes which aren't pushed as hard as the rectifier tube in the ST-70. But it isn't a good thing for a rectifier tube IMO. Other articles point this out as well. Jim  McShane is also of your opinion that a good rectifier shouldn't have any problem pushing the ST-70.  And, I got 3-4 yrs on the original tube. So, even though the rectifier is somewhat of a bottleneck its not the be all end all without remedy. I agree with you that hundreds of thousands were built and ran great. Certainly many were pushed as hard as I push mine with less efficient speakers

I think my new Zyx cartridge is the culprit cause I wasn't playing quite as loud before. It sounds so doggone good though.

:) that is because there is less distortion, so you naturally turn it up louder.

consider folks who never previously built anything assembled all but a few of the 300,000 ST70s, the fact that most of them that came up on the used market sported their original (5AR4 and all) tubes, and the majority of that number still worked mostly fine even after all those many, many years.

Joe, this weakness in the ST-70 design is pretty well-known. The reason many of the originals worked is that they were Mullards, which were built over-rated in their day. Most modern 5AR4s meet spec, but they won't hold up to a ST-70 like a Mullard will.

I used to modified ST-70s for a living years ago. You can't get around this problem very well with solid state as the power supply voltages run too high, so you just have to deal with the fact that your ST-70 is going to eat modern rectifier tubes. There is a wonderful thread on audiokarma.org regarding all the various strengths and weaknesses of the design. I tried looking for it this morning but there are a lot of ST-70 threads over there so I apologize in not at this time being able to back up my comments with something you can get your teeth into. BTW that thread is overall pretty positive- in it you find out that the ST-70 challenged a lot of more expensive amps like the Marantz 8 by having excellent distortion figures and bandwidth. 

@atmasphere  

 The reason many of the originals worked is that they were Mullards, which were built over-rated in their day.

Does this mean that I need to get a NOS Mullard? If so, which one (if there are differences)?

BTW, I read an article by someone named Campbell who measured the ST-70 for accuracy and concluded that it was among the best measuring amp produced. However, his criticism was the single rectifier tube, noting it should have had 2.

If you want longer rectifier life, look for a Mullard made in the UK. Beware of fakes on ebay.

I would also look at the cost and compare that to the garden variety. If its more than 4 times the cost, I would just buy the cheaper tubes and get a bunch of them.

@atmasphere  Can you show me an example of a fake. I'm not sure how one would fake a tube. How is it done? Thanks