2nd channel not silent - where is the problem?

[txstrat]

I own a double channel amp with a standard blackface channel and a british master volume channel. The british is almost the same like a 59 Bassman (plus gain control before the second grid).
Both channels share the first triode and I switch channels with a relay after that stage. The second pole of the DP relay switches the lead of both channels to the pi. As a matter of fact, I can't turn the british channel down. Even with the gain control and the master volume full down I can hear the sound (not even very faint but clearly audible). My biggest problem is, that it also effects the (clean) blackface channel. The amp had a second BF channel before and this didn't happen then. Could it be the cathode follower being the culprit? What can I do to silence the british channel? I added a block diagram for easier understanding.

Matt

[chipaudette]

The way that you've drawn your diagram suggests that the unused channel is floating. For example, when you're on your clean channel, it looks like the input to the dirty channel is not grounded...it's just floating. Similarly, it looks like the output of the dirty channel is just floating. It's possible that this is your problem.

I would suggest that you open up your amp and temporarily add a couple wires to ground the input and output of the dirty channel. Then, when you turn on the amp and attempt to play the clean channel, has the dirty channel now been silenced?

If so, consider the wiring of your relay and see if you can ground the unused connections. You might need to add additional relays.

If this didn't silence your dirt channel, I'm not sure what to tell you. You could have poor grounding in this part of your circuit (allowing the two channels to become coupled), you could have poor filtering of the B+ rail (allowing the multiple pre-amp stages to become coupled), or you could have capacitive coupling between wires that are physically too close together. Tough problem.

Chip

[txstrat]

Actually I switch either second grid to ground with the relay. The lead to the PI I switch just open (see pic).
Do you mean I need a separate ground wire to the star ground for both preamps?
Filtering didn't occur to me. I'll have a look.

Matt

[txstrat]

Just to let you know. There was a filter cap missing but I decided to bring the second BF channel back to life since I need the amp for a gig next saturday.

Matt