I have a 1990 JCM 900 Model 2100. When i first turn the amp on sound is coming out, but after about 5-10 seconds the sound fades out until it’s completely inaudible. The tubes are glowing, and the head is on but there’s no sound. When i unplug the cable from my guitar and leave it in the amp, it doesn’t even make a sound. It sounds as if the cable isn’t plugged in at all. Any sort of help is appreciated!