See the link:
https://www.decware.com/newsite/SE84CKCowner.pdfThis makes reference to a 274b and that some will arc on start up because they are not compatible with the first 47uF cap. There certainly are other variables that come to bear. I did have a conversation with Steve when I owned my SE84UFO2, some time ago, specifically about the use of the WE422a. He did not say that it would be a problem but that caution is warranted. In fact he stated that a variac would be a consideration.
Having said that I have had two 274B that were supposed to be rated for use with a 47uF cap that arced on start up. One in less than a year and one on the very first start up, that cost me $400.00 and was I POed!
I have used 5U4G variants that were rated for a max 40uF cap with no issues. I have also tried a Valvo AZ1 mesh plate that was rated for 10uF at best and it survived, but I held my breath every time I turned it on. It only cost me $80.00 plus the adapter, so I didn't care if it blew up like the 274b's. I wanted to hear what it sounded like.
A WE422a costs far more than $80.00 and far more than my GZ34 metal base. All I'm saying is that there is risk and that caution is warranted based on my experience. I'd hate to see anyone loose a WE422a regardless of cost.