The 6FQ7 is a direct replacement and I've had very good sound from some NOS GE 6FQ7 tubes. Happy Holidays!
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I've been using the Mazda Brimar 6cg7 because it's a little shorter than the other brands and thus it fits inside the chassis without modifications (this is an issue with previous AVA models, but not the ones that came out after Frank suggested the change in tube). I like it, since it follows the prime directive, "don't screw things up". Comparison with the 6N1P that it replaced says that the change is subtle at best; also, the 6N1P has a little more gain, so the volume knob has to go up a click or two.