12AT7 vs 12AU7 (interchangeable?)

[jn2630]

Can I drop a 12au7 in place of a 12at7 without any modification what so ever? Here is my schematic: click

Will it work in my application?

Sincerely,
Jake

[SY]

You'll get sound, but bad eq, low gain, and high distortion. 12AT7/ECC81 is pretty easy to get and cheap- why not use the right tube?

[Cycline3]

Jake,

That's an interesting question here. In many cases, you could drop in lower gain 12au7 and in those cases, it would probably work fine, if not under optimum conditions.

However, in what looks like a direct coupled phono stage with the equalisation wrapped around the tube in question, I would imagine that it's probably not a good idea. The operating point wont be exactly right for direct coupling and the different tube would likely change up the equalization some... how much, I don't know... it might not matter, it might change it quite a bit.

In this case, I would think it much smarter to spend $10 or so on a 12AT7 from somewhere and avoid the headache all together. Yeah?

[Miles Prower]

Originally posted by jn2630
Will it work in my application?

No. That's a feedback amp, and using a 12AU7 instead of a 12AT7 will change the open loop gain, and thus, the performance under NFB. You'd have to redesign the feedback network to compensate for the lower Avol.

[jn2630]

Thank you all for your input, I would agree that $10 is worth it for preventing a head ache. The schematic also calls for the 12ax7a and 12at7a. Does it matter if the tubes are 12ax7 and 12at7 without the "a"?

[SY]

This time, you're cool; the "a" isn't critical.

FWIW, I've tested a wide range of 12AT7 and found the currently available and relatively inexpensive JJ to be the best as regards distortion and noise.