I would like to purchase a quartet of EL-84 GT's for backup. I liked the way GT's are numbered, however I can not find the correct bias number (7) for the tubes that are in my amp. On-line tube suppliers do not specify the GT numbers. So my question is, what good is this numbering system if you are unable to happen or by chance even find a matching tube? I'm not interested in changing the bias and I like the way my amp sounds.