I picked up a used Fender '57 Custom Deluxe. Initially it sounded great. After some playing time I found that at moderately loud volumes I'd get a bit of a farting/sputtering sound if I hit low notes. Not every time, but sometimes, and it took a while to warm up to the point where it would start happening. My first thought was that the vibration of the speaker was impacting one of the tubes, and the tube was getting ready to fail. It was an electronic sound, not a mechanical one. I had preamp tubes on hand so I changed those. No improvement. Still sounded great, still made the occasional noise. I ordered power tubes and put those in this evening. It sounded great at first but after maybe 10 minutes of playing it suddenly fizzled (an actual fizzle sound that lasted maybe a second or two) and now it's dead. Grasping at straws so I put in a different rectifier tube.... nothing. All the tubes are glowing. There are three fuses that I can see, and they are all intact. If I had more tubes I might put them in, but I don't. Wondering if there's any way for a newbie to do some preliminary investigating with a multimeter, see if maybe I can avoid a trip to the doctor. The doctor is some distance away and I'd rather play this thing than be without it until I can get it there.