6CG7 instead of 12AU7

[whitelabrat]

Just for giggles I tried a 6CG7 in place of the 12AU7. Not bad. I haven't decided the best way to convert the board to use it though. I think the simplest method would be to cut the traces for pins 4, 5, and 9 and then run a twisted pair of wires from 8 and 9 on the J1 terminal strip to pins 4 and 5 on the board where the 12AU7 would be.

I just bring this up cause I've got a lot of 6CG7's.

I suppose it could be possible to make a switch to toggle between 12AU7 heaters and 6CG7 heaters.

Another option that may be less destructive would be to remove the socket from the board and wire in a chassis mount socket where pins 4 and 5 would go to pin 4 on the socket and then pin 9 would go to pin 5 on the socket.

[EWBrown]

To me, the 6CG7 / 6FQ7 is a nicer sounding audio tube than most 12AU7s, which were more intended for RF than audio use. 6CG7/6FQ7 is essentially a 6SN7 in a 9 pin miniature botle, with somewhat less maximum plate dissipation capability, because of the reduced plate size.

The 6CG7s also have an 8-volt version, the 8CG7 / 8FQ7, which would work well in conjunction with 8BQ5s, which leaves only the 12AX7 / 5751 which would need series resistors in order to operate their filaments from 8V. Think Plinkertron Yellow_Light_Colorz_PDT_06

12BH7s are another simple "plug 'n' play" replacement, for 12AU7s, and no circuitry changes are required in either case.

Another approach for PC mount tube sockets is to flip up the contacts for pins 4, 5 and 9, so that they do not make contact with the corresponding PCB through holes, and then hand-wire them to 6.3VAC as needed.

Or simply "flip up" pins 5 and 9 on the socket, let pin 4 make the usual connection with the board, and then wire in 5 and/or 9 as needed.

Similarly, the Russian 6N2Pi ,which is basically a 6V filament version of a 12AX7, with pins 4 and 5 filament connection, could also be similarly connected in place of the 12AX7 or 5751 voltage amplifier. This would also work for the rare and wonderful 6AX7, if you can actually find any of them Yellow_Light_Colorz_PDT_02 :o

/ed B in transition between states

[TerrySmith]

I managed to score three 6AX7 and two 6AU7 NOS Tung-Sol's from flea bay a couple months ago for about 2 bucks ea. And you can find NOS 7AU7's cheap and will work perfectly on 6.3v on pins 4 & 5.

I yanked the voltage doubler in one of my PAS3's and wired it as a FWB, then I stuck 6AX7's in the phono board and 6N2P-EV's in the linestage.

[EWBrown]

7AU7s and 7DJ8s are a good cheap priced alternative to their 6.3V filament brethren... as I say, think plink... Similarly, the 6V filaments will work on 6.3V, they just take longer to warm up. Probably not best for "critical" applications, but yes, they do work, and save $$$ at the same time.

/ed B in NH