I know, kind of a longshot, but who knows... A friend of mine brought his acoustic guitar preamp for me to troubleshoot. Apparently it spent some time in storage, and it didn't work afterwards. It uses a small tube (12au7a ecc82). I have zero experience working with tubes and do not have a tester. All I could do for now is check for shorts or opens between the tube's pins with a multimeter. All 3 heater pins are continuous, and all the other pins are isolated from each other as per the datasheet. Probing on the PCB, pin 8 (section 1 cathode) is also short with the heater pins, but not pin 3 (section 2 cathode). Without a schematic, I have no idea if this is how it's meant to be. The traces leading to these socket pins are in the middle layers, too. Any input welcome! Thanks!