Switching problem RG ? Anyone ?

[fikri]

Hi !

I have just finished my tube preamp and finaly got this annoying Squeel sound. My preamp consists of 2 channels completely separated and i uses DPDT footswitch to switch between channel (like the usual tru bypass on stompboxes). The problem is, when i switch to DIST channel, and then turn up the gain on the CLEAN channel, it squeels. And Vice versa. Why this could be happened ? eventhough it is completely separated ? And how can i get rid of this ?

[R.G.]

eventhough it is completely separated ?

Well, what can we conclude from the fact that it squeals when you turn the other gain up?

Yes, you're right - it isn't completely separated. It only seems to be.

Tube circuits are difficult to get right because they are so sensitive. The grid of a tube can pick up incredibly small signals by capacitance from quite a distance.

You get rid of it by three things:
1. Isolate the power supplies with R-C decoupling stages.
2. Isolate the ground lines between the two channels. Each channel should have a completely separate return path to ground.
3. Shield it. Any signal wires longer than about 1"/25.4mm which lead to a grid should be coaxial cable with the shield grounded at one end only .
A shield between the two channels might help as well. It would intercept the capacitive signals.

[fikri]

Thanks alot R.G. I will try your suggestions