Tube amps are notorious for 60 (North America) and 50 (most of the world) cycle hum. The primary source of this type of hum is AC filaments. Many amps avoid this problem by powering the tube filaments with DC. Anyone who has encountered a blackface or silverface amp with its original tubes noticed that the amp had 7025s installed, especially the silverfaces. The 7025 is a 12AX7 with a twist, pun intended. The 7025 was a Fender STR (special tube request) tube. The STR in this case was for a 12AX7 with reduced hum when the filament was powered by AC. The 7025 has a spiral wound filament. If you have amp that has 60 or 50 cycle buzz (e.g., the buzzy sound one gets when playing a non-humbucking guitar under fluorescent lights), it is worth looking for genuine 7025s. They do not need to be NOS. Pulls are fine as long as they are not microphonic. Alternative commonly available current production tubes with spiral wound filaments are the 12AX7EH and 12AX7LPS (only in heads). However, old production 7025s are better than current production tubes. As I mentioned earlier, pulls (used) are fine as long as they are not microphonic. Preamp tubes last forever.