Sound cutting out Tremoverb

Hey everyone. I'm having an issue on my ToV head. I just bought it about a week ago and everything has been good up till now. I was using a Friedman BE-OD into the clean channel with no problems, now I have sold the pedal and was just trying to use the clean channel as a clean and classic rock channel.

The issue I'm having just started today and it is that the sound on the orange channel will cut out randomly. It will work fine and then suddenly quiet. It still makes noise that can be heard but it's very quiet, then it will come back to normal and cut out again. It's completely random and no knob wiggling or switch flipping seems to help. While this is going on the red channel is completely fine, it works as it should. I didn't have any problem with the clean channel when using the pedal in front of it but now that there is no pedal this has started!

First thought is that it may be a preamp tube, if that's the case which tube controls the orange channel? I have some spares around that I can try in it. It's not a consistent problem so I can't just "swap them till it works" because it's in and out.

Any help appreciated, thank you.

EDIT:

Just tried swapping all tubes. V3 and V4 did not make a difference. V1 seems to be the culprit. With the Mesa tube that was in it and a different Mesa tube it's still cutting out as before. I installed a Shuguang and found that it did not loose volume like with the Mesa tube but instead lost gain. So, it would fluctuate back and forth from gain to clean at the same volume. Installed a 3rd Mesa tube and now it's back to retaining the same tone but changing volume levels.