Mr. Masahiro Yonezu is the owner of the blog "Dr. AMP Lab." in Japan. Mr. Yonezu posted a series of articles on etracer. He also posted a lot of test results of various tubes with etracer. Get your Google translate ready because the following links are in Japanese:

「etracer」導入編

「etarcer」製作編①

etracer製作編②

etracer測定編「2A3」