I have a little problem with my Mark IV's channel ledlights. I use my TC G-major's relay switches to change channels in amp. The problem is, that when I change to Lead channel, the led shines bright for a moment. After couple of seconds it turns darker. I hear some slight change in sound too. However, I doubt that the problem is tubes, 'cause they are quite new. Has anyone had this problem before? The amp is from the mid 90's.