Good linearity, and probably cheaper at the time the amp was designed than the 6FQ7.. Tubes with 6.3V filaments almost always cost more than equivalents with now nonstandard filament voltages, as no one in the commercial realm generally wants anything else, the QS 4 is obviously an exception.. FWIW I would look for replacement tubes while you can find them.
Yes.Thanks Kevinkr , can I use the 12cg7 ????
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