PPIMV Squeal on 36 watt Lite build (Marshall style) - Curious.

[rockybottom16]

I have a 36 watt lite build (similar to an 18 watt lite, but with 4 - EL84's) with a PPI master volume. I notice that if I dime the volume control when no guitar is plugged in, the amp squeals quite loudly. Obviously, there's no need to do this, and everything works fine when a guitar is plugged in. I'm just curious as to what causes this. Anyone have any thoughts?

[Jazz P Bass]

My only thought is that the input jack tip is not grounded when no guitar is plugged in.

Did you use a switching jack at the input?

[rockybottom16]

Thanks for your idea, but yeah - I used switching jacks. The sound is more of a squeal than a buzz from an ungrounded tip.

[g1]

When your guitar is plugged in and guitar volumes at 0, amp does not squeal when amp volume full up?