This becomes an interesting read towards the bottom.
http://db.AudioAsylum.com/mhtml/m.html?forum=tubes&n=231311&highlight=6al5&r=&search_url=%2Fcgi%2Fsearch.mpl%3Fforum%3Dtubes%26searchtext%3D6al5
http://db.AudioAsylum.com/mhtml/m.html?forum=tubes&n=231311&highlight=6al5&r=&search_url=%2Fcgi%2Fsearch.mpl%3Fforum%3Dtubes%26searchtext%3D6al5