From bruce@ashbysolutions.com Thu Jul 25 06:35:54 2002
Subject:Re: 12AX7 vs 12AU7

Hi Galen,

>In various user reviews on tube preamps you can find claims such as:
>
>"...I recently changed the 12AX7 and put in a Chinese 12AU7. As a result, my preamp sounds a bit rounder and not as edgy or brittle..."
>
>My question is, if true, why wouldn't the manufacturer install the most appropriately voiced tube--especially if their product was developed for "warmth" rather than "distortion" (such as a Tube Works Blue Tube or similar products)?
>
>Are the tube swapping customers merely people with too much time on their hands or, are there other factors I'm missing here?

It really depends on the design of the preamp. If the preamp was designed to create a lot of overdrive -- i.e., for a lead guitarist's needs -- swapping a 12AX7 for a 12AU7 (or 12AT7, 12AV7) may provide a warmer, less overdriven response. I've played guitar preamps that had three channels with names like "clean," "overdrive," and "extreme," and the "clean" channel had too much grit for my liking! In this instance, a lower-gain tube would certainly drop the overall gain level, and may be just what the doctor ordered.

The operating word here is "may," however. Depending on how the tube or tubes are biased, the transformer (or transformerless) load seen by the tube(s), and other factors, the result may not be all that pleasing. In particular, swapping a 12AU7 (gain of 20) for a 12AX7 (gain of 100) can be too drastic for the design. To further cloud the situation, many newer tube circuits use a method called "starved plate" which biases the tubes with anywhere between 5-50V, rather than the 150-300V that the tubes were designed for, and the results of swapping tubes can be quite unpredictable in these designs. (No tube manufacturer I know of publishes performance curves for this kind of use.)

Additionally, it is even possible to design a 12AX7-based preamp with a lower level of grit. If a tube preamp is designed for a milder overdrive, and already uses 12AX7 tubes, moving to a lower-gain tube will probably leads to nothing but disappointment.

Regards,

-BW

--
Bruce Wahler
Design Consultant
Ashby Solutions™ http://consult.ashbysolutions.com
CloneWheel Support Group and HiNote moderator
978.386.7389 voice/fax
bruce@ashbysolutions.com