• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Can I use 12AT7?

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Hello!

Well, my "hi-fi" answer would be NO. You can't use a different tube: you'll have different operating points, gains, etc etc and the stage won't behave as it has been designed.

My "guitarist" answer is YES, and that's good to use a slightly lower gain tube in the RealMcTube (Rest In Peace Fred). You're not going to hurt anything, so try and see if you like it better. In those sorts of tube stompboxes, you REALLY want to get as many tubes as possible of different flavours (12AX7, 12AU7, 12AT7 but even others) just to hear how it sounds.

Though I don't like 12AU7 flavour, it sounded best in a guitar preamp that I prototyped (very similar to the RealMcTube). Much less gainy and more bluesy sound, opposed to a fuzzy cold 12AX7 sound. I won't expect the 12AT7 perform so different than 12AX7, gain is still too high.
 
Just keep the differences in character between the 'X7 and 'T7 in mind. The 'X7 is high Rp/low gm, while the 'T7 is high gm/low Rp.

The 5751 has a mu of 70 and it will replace the 'X7 without incident. The 12AY7 has a mu of about 40 and it may drop in too, as it also has the high Rp/low gm character.
 
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