Without modification would it be possible for me to roll in some of my vintage 6CG7/6FQ7 tubes instead of 12AU7s?
The heater voltages are different, so no it won't work at all.
6FQ7 = 6V heater (pins 4-5)
12AU& = 12V heater (pins 4-5)
The 12AU7 can be configured to work on 6V, but the heater pinout would be different.
Short pins 4-5 for one terminal of the filament supply. Connect the other supply lead
to pin 9, which is the filament center tap. Now the two halves are in parallel, for 6V.
6FQ7 = 6V heater (pins 4-5)
12AU& = 12V heater (pins 4-5)
The 12AU7 can be configured to work on 6V, but the heater pinout would be different.
Short pins 4-5 for one terminal of the filament supply. Connect the other supply lead
to pin 9, which is the filament center tap. Now the two halves are in parallel, for 6V.
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Thanks for feedback ... would've loved to use my Mazda 6FQ7s but guess it'll be for another project
You can buy a pair of cheap socket converters, but the real issue is that the 6CG7 draws twice the heater current, which in not something the amp will tolerate well without modifications.