"Matisse" preamp - 12AX7 12AT7

I have always been told and/or read, that you should never sub an AU for an AT, or an AX for an AU, or AX for an AT, or any of the above combinations. Some preamps, like the Lector ZOE, let you use whatever tube you want, but this is usually not the case.

The 5751 is a DIRECT replacement for the 12AX7, just less gain, again, depending on the circuit.

Maybe ask your question over here also:

http://www.audioasylum.com/cgi/etv.mpl?forum=tubes

Are you sure it's the pre and not your source? I've seen this when someone has a high gain piece plugged into a low gain input (ie mixing up mc & mm or cd into phono)?

hmm could be the source, but how do I test? I have only one source, one pre and one power amp...

An easy and inexpensive way is to put in a fixed value attenuator at the amp input. You can buy them or have a techie do it for you. Cheap commercial ones are made by Rothwell. I believe you can get different values. Cost about $60. They are fairly transparent. Worked for me when I needed to do that. BTW, you may also be able to use them between your source and pre-amp if you underlying problem is too much gain from your source.

There is absolutely NO danger in swopping the tubes you have mentioned. You will not damage anything, and 9 times out of 10 it will run OK, and give you the gain lowereing you desire.

But it may not sound as good as it would if the stage had been designed to take the tube you use - but it won't harm anything - and may work perfectly!

Of the tubes that you can sustitute, the 12AU7 has by far the lowest gain.

Just don't try fitting something that uses different pin connections - or uses far more heater current than the ones it was designed for.

Regards, Allen (Vacuum State)