Depending on how the gain stage is setup, you could replace the 12AX7 with a 5751. The 5751 usually has 30% less gain, depending on how the tube is used in the circuit.
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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:
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)
Okay well, to report back, I tried it and it did not really change much at all!!
I rolled in a few other sets of tubes to see if any differences could be noticed, and slightly less gain was apparent on the following combination;
Brimar CV4004, GE JAN 12AT7WC
This also sounds great (except for the first time I turned it on there was good deal of buzzing whilst turning the volume dial on low levels - although it went away after a while, any idea what could cause this?).
I will now head down the in-line attenuator path I think....