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A 12at7 instead of a 12au7 in a brick preamp?

PostPosted: 12 Dec 2016, 09:54
by mildew
Ive got this bugle boy 12at7 thats my best sounding valve. Its low gain by at7 standards. My groovetube brick has a 12au7 in the last stage.

Am i likely to damage my brick by trying out the at7 ? - (obviously ill alter the gain structure)

(circuit diagram here...)

https://www.gearslutz.com/board/attachm ... -brick.pdf

Re: A 12at7 instead of a 12au7 in a brick preamp?

PostPosted: 13 Dec 2016, 00:48
by tictac
you may not damage the brick but you might damage your tube. The max heater to cathode voltage for a vintage 12AT7 is 90V; the 12AU7 rating is 180V.

Your schematic shows the cathode for V3 the tube in question is sitting at 190V....

There is a way to float the heaters on a DC ref. voltage to decrease the heater to cathode voltage but given the design of the heater supply is regulated DC this likely would not be a good idea or worth the effort.

If you really want the 12AT7 in your Brick I'd say replace the 12AX7 with it and keep V3 a 12AU7.

The gain will be slightly lower but I bet you'll barely if at all notice the difference. 12AT7's are considered by many audiopile types to be a superior sounding tube to the 12AX7....

TT

Re: A 12at7 instead of a 12au7 in a brick preamp?

PostPosted: 13 Dec 2016, 01:29
by mildew
thanks tictac - i cant seem to find the "say thanks" button on this forum anymore, but thanks for the info.