Just as it says, a replacement rectifier. I tried it in a JTM45 and it sounded closer to a 2204 - less woofy low end, quicker and brighter attack, slightly less "body" to the notes. I understand it's best to rebias your power tubes after installation, but I didn't check it before and after. I mostly use it as an emergency, although it's hard to narrow a problem down in a couple minutes to the rectifier tube going out. Stiil, if you need it, here it is.