No I said that if you put a tube that handles less current[12ax7] into a circuit meant for a 12at7 that is feeding it more current that you will cook the 12ax7 causing other components to fail.
You folks are all kidding, right?
1. There's exactly one rating that matters in preamp tubes (assuming the pinouts are the same), and that's the voltage.
2. The tube heater is a resistive load and will draw exactly as much current as it needs to draw. No more, no less. So there's no difference in tube life expectancy caused by substituting a device with lower or greater current draw. That's like asking whether a light bulb will burn out sooner if you hook it up with a heavier gauge lamp cord. It won't....
3. If your amp can't produce enough current for the tube heater, it will result in a voltage drop and the wires will heat up and the tube's sound won't be as good (or if the current limit is caused by a fuse, it will blow the fuse). Since we're just talking about a 12V current source, though, I can't imagine an amp design not being able to provide enough current. We're dealing with current in the hundreds of milliamps here. It's peanuts. The cheapest, smallest regulator or Zener you could buy should handle that little trickle....
4.
All of the 12A*7 tubes are designed for a 150 mA, 12.6V supply. While there is individual variation from manufacturer to manufacturer, if your amp doesn't provide at least 150 mA, it is broken by design, and if a tube draws more than that, it is defective. In other words, the supply for a 12AX7 has to provide exactly the same voltage and current as the supply for a 12AY7 or a 12AT7 or a 12AU7 or....
In other words, you won't have any problem swapping around preamp tubes. You probably shouldn't be swapping
power tubes without rebiasing, but you really can't go wrong swapping out preamp tubes as long as it's a 12A*7 or any other pin-compatible 12V tube (5965, 6072, etc.)....
One caveat: if the tubes happen to be arranged in a push-pull configuration, make sure you're using similar tubes for both sides or else you'll have a really weird output.
Over at the Carvin boards, some guys with way more knowledge than I have, stated that certain resistors in some amps may not be rated for the extra current draw. They also showed pictures of toasted resistors and circuit boards which resulted from using 12at7s. Another person stated that his amp fried when he substituted 12au7s in place of 12ax7s. Some amps may be fine with the substitutions but just make sure before you do it.
My bet would be a defective tube, but if not, Carvin should recall the amps. If they can't handle a 12AU7, you can pretty much guarantee that they can't handle a lot of 12AX7 models, either.
