Is this the sound of tubes going bad or something worse?
Yesterday I fired up a Traynor YBA 2A. Great bass amp and I love putting p90 guitars and Gretsch guitars through it and turning it up to 11. Alas, doing so yesterday resulted in a weird vibrato to the sound. As soon as the amp started to break up, you could hear a warble that became more distinct as you turned up.
Tubes are a pair of old Mullards for pre and Groove Tubes for power. Bought the power tubes back in 2008 but the amp has very low hours since then.
No, seriously, it's hard to tell. A recording would go a long way to help identify the problem over the internet, but even that may not help unless someone has heard that exact same sound before. I've not heard a bad tube create a vibrato sound. Tremolo, maybe. In any event, the first thing I would try is to swap out the tubes for known good ones, and see if the problem goes away. If it does, then you'll know it was a tube problem. If not, take it to a tech.