Hello, and oh boy, we haven't updated this blog in a long time....!
We just added a page about Emission Tester and 12AX7.
The TL;DR of this page is that vintage Emission Testers have problems testing current production 12AX7 tubes. The page is our attempt at explaining this. You can find the same info on other sites as well.
This is a topic we have been wanting to cover for some time, and we recently had a few inquiries in a row. One was from a customer who purchased 12AX7 from us over a year ago...! This made us realize that this is a good information to have on our site, as it can result in a poor customer experience if this information is not known in advance. We hope the page can be of some help.