jpms, some of the posts in this thread deter me from contributing, but I can help you with your questions:
1) U54 is another designation for GZ37 and CV378. It's not a GZ34, but it's listed as a suitable sub in Mick's owner's manual.
2) All indirectly heated rectifiers have "slow start." Not all indirectly heated rectifiers can be safely sub'ed in your Supratek though. Nothing personal, but I find a lot of the recommended tube substitutions in this thread way too liberal.
1) U54 is another designation for GZ37 and CV378. It's not a GZ34, but it's listed as a suitable sub in Mick's owner's manual.
2) All indirectly heated rectifiers have "slow start." Not all indirectly heated rectifiers can be safely sub'ed in your Supratek though. Nothing personal, but I find a lot of the recommended tube substitutions in this thread way too liberal.