The fact that most popular amplifier manufacturers rebrand these as their own should tell you that they are considered the industry standard. If you're not sure what you're looking for from the sound of your amp when replacing old tubes, you can't go wrong with a Sovtek 12AX7.
The one drawback I have found with this brand is the quality. I have occasionally received a 12AX7 tube that has been microphonic and not suitable for preamp duties. However, they performed well in other tube positions and the price of these tubes are so competitive, I highly recommend them. I regularly use these in both my Fender and Vox amplifiers.
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