[Ender]

The tube on your Marshall is not used the same way as it is on a pure tube amp. It's a preamp tube for one, not a power tube and should last quite a while. And it's not even really used the same way a preamp tube on a pure tube amp is, it's a solid state amp so the entire power process doesn't go through tubes. A small power supply is added post-signal which runs through the 12ax7 to add a little bit of note bloom to simulate a tube amp. That's all, so the tube doesn't get a lot of power run through it. They still wear out though not like a tube in a tube amp will, nor as fast. You may not see the brown burn marks on the glass but you would hear some buzz that wasn't there before and the sustain on the end of your notes will be similar to when a battery wears out on a gain pedal

Tungsol's are good cheap 12ax7's that are as good as any out there.