Hi, I bought a Trace Elliot Hexa Valve from 1991 through ebay. It sounds AWESOME! But there is a problem... After some 30 minutes of running, the input level starts lowering over a period of 15-30 seconds before completely going silent. The 12 band EQ can still be heard, you notice the hissing when maxxing out the top 3 frequencies so that part of the preamp, and the power section have no problem. The input level indication leds also go dark so the signal is lowered somehow. When I switch off the amp for about 30 minutes and then back on, all is working fine again. Is this some kind of overheating protection kicking in for the two 12ax7 preamp tubes? The amp sounds really really good and I want to start using it as my main amp, replacing the Mesa Bass Prodigy I also own (the Trace sounds a lot more hefty and less boomy). Can anyone give me some advice what to do? Thanks!