Over the past several months I have been building a tube preamp using "kit modules" sourced from eBay. It's finally assembled - but it's doing some evil things that I think are related to grounding. The board in lower left is power supply board. This board has no connection to chassis. It has 12V regulated DC for the heaters and approx. 300VDC B+. The main board is obviously the preamp board. Again, no direct connection to chassis. The upper right board is the input selector. Chassis ground is through the upper left standoff - including the signal ground. It has it's own 12VDC regulated supply. RCA inputs and outputs are otherwise isolated from the chassis. As it is there is a hum (120hz?). The hum is loudest at min. and max. on the volume control and reduces greatly as the control gets closer to center. Connecting a ground wire between the heater negative and chassis eliminates the hum - BUT - the heaters on one side of the tubes glow brighter than the other - and the "Dark" side stops passing signal. Removing the ground wire causes the the dead channel to fade back in, but the hum returns. Puzzled....!