6CG7 interchangable with a 12au7?

[conundrum]

Can a 6CG7 be subbed with a 12au7, in a phase inverter application?:confused: They have the same pinout and similar ratings.

[Damon]

6CG7 doesn't have a centre-tapped heater. Pins 4 and 5 are for filament, one to each leg of the 6.3V secondary. In most 12AU7 circuits, pins 4 and 5 are connected together, and pin 9 (the centre tap) goes to the other leg. 6CG7 will not light up properly in most 12AU7 spots. Try 7316, 5814A, 6189, CV4003, M8136, 6067, B749, 7730, 6680, and possibly E80CC/6085 as subs for 12AU7/ECC82.

[conundrum]

I guess it's just as well, NOS 6CG7 aren't that expensive anyway.

[donnyjaguar]

I think that the 6CG7 draws twice the filament current of the 12A?7 tubes too. The 6CG7/6FQ7 are good tubes in my opinion. They are basically the 9 pin version of the 6SN7 that is/was popular with the hi-fi crowd, with mildly reduced max ratings. I made a Williamson Amplifiier years ago and used 6CG7 because they are cheap and available and it sounds perfectly fine to this day. IIRC, the 6FQ7 has a shield between the 2 sections which reduces cross-talk. This is probably more relevant in a stereo amplifier where you are amplifying L and R channels in one tube. When I was designing my latest amplifier I considered using 6CG7s, but wimped out and went with the standard 12A?7 ones and saved myself one amplifier section.

DJ