My V50 Crate amp has developed a problem...the clean channel has stopped working and the OD channel is not sounding quite as it should. I have noticed that one of the 6L6GC valves has a small piece of metal broken off inside it, looks like it has come off a bent piece of metal at the very top of the valve. I'm going to replace the pair of them anyway, as I figure it's not good, but I'm wondering would this be the cause of the clean channel issue? I'll find out soon enough, but would be good to know ahead of time.
That's almost certainly the cause of the issues for both channels. All the sound from the amp goes through the 6L6GCs, as they are the power tubes that take the clean or OD sound and increase the power until it's enough to drive the speaker.
You should probably leave the amp turned off until you have a chance to replace both tubes with a matched pair - to make sure you don't cause any damage to the amp itself. You may also want to consider replacing the other tubes (probably 12AX7s?) just to make sure the amp sounds good as new again.