Tube amp trouble

I've got a Crate Vintage Club 50 that's having some problems. The amp plays fine for about 30 minutes or so and then starts acting weird. First the tone starts varying from normal to real "thin" sounding (no bass). Sometimes the volume cuts to almost nothing. The other day the amp completely cut out. I turned it on today and it worked but after some time, it's got the same symptoms. I've bought all new tubes (preamp and power) but want to know what's wrong before I have to re-bias the amp. Thanks for any help.

I'm guessing that your problem is heat-related, since it takes a while for things to go wrong. The inside of a tube amp is a very dangerous place to poke around in, so I strongly recommend taking it to a tech for a look-see. I suspect a failing plate resistor, which should be an easy, inexpensive repair. Get the tech to install the new tubes and set it all up, and you should be fine.