Tubes

[Earnest]

I have a Hammond M-143and it uses the 12AU7 tubes. Locally on Craigslist a poster just posted that he has a "pile of Sovtek 12ax7 tubes in a mixture of WA & WB. $5 a piece or make an offer on more than two". Are they interchangeable with the 12AU7 tubes? They are listed as in excellent condition.

[KOC62]

No, it is not a direct substitute.
The 12AX7 is a high-mu (higher gain) tube, while the 12AU7 is a medium-mu. The 12AU7 can handle twice the plate power dissipation. The biasing will also be different.

[Earnest]

Thanks for the reply.

[David Anderson]

The M100s use one 12AX7 and two 12AU7s, among others.