50 watt amp project

[TiCatFan]

Hi All; looks like i took about a year off from this project, but am back at it. Decided to remove all of the footswitch stuff till i am totally happy with the amp using panel switches. Also looked over the channel switching relays and tweeked them a bit, and redid the "Dumble" preamp overdrive switching, a couple of times. Currently I am happy with my progress, Zen preamp seems to work nice, channel switching seems to work well, and overdrive switching is working well. Everything is great, almost. There seems to still be an odd problem with the D-pre, well two problems. 1) - If I am playing the D-pre and crank the volume on the Zen preamp, it starts to howl. Well, don't do that, but it is still indicative of a problem. 2) - When I am playing the D-pre there is a background noise, on the guitar it is about the same as a low b-flat, and it hums about every 6 seconds for half a second. Strange. What is really strange is that if i switch the safety socket from bypass to active the hum stops.

Any ideas??

Thanks all...