Happy Holidays! I have a brand new Epi Valve Jr Combo that works fine for a few minutes, then the volume fades down to nothing. The power switch remains lit, but no output. I removed the back panel and made sure the tubes were well seated, but other than that I'm at a loss for troubleshooting. It was an xmas gift, but I have the receipt. If your advice can't help me solve the problem, it's going back. Thanks in advance.