There was a site with links covering this exact subject. For some reason the server is down or the web site has been removed.
The link provided below is only in "cache" with Google. If you find value in the comparisons I would print it out before they remove it. Hopefully the original will return with data intact.
http://216.239.39.100/search?q=cache:xu2DiX5j-SYC:www.hickoktubetesters.com/12au7.htm+hickok+tube+testers&hl=en&ie=UTF-8
The link provided below is only in "cache" with Google. If you find value in the comparisons I would print it out before they remove it. Hopefully the original will return with data intact.
http://216.239.39.100/search?q=cache:xu2DiX5j-SYC:www.hickoktubetesters.com/12au7.htm+hickok+tube+testers&hl=en&ie=UTF-8