Mark IV's FX loops keeps going quiet

In the Fx loops of my Mesa Mark IV, I have a delay and an EQ boost.

When I got the FX Loop On button on the footswitch there is a great solo boost (thanks to the EQ pedal).

However, last gig, every now and then it would be quieter than the normal rhythm tone. Sometimes it would start normal, then dip in volume, only to be fine again the next time.

According to the Mesa's manual, the preamp valve in V2 controls the FX return stage.

Is it the valve that's going you think?

Try swapping V2 with V4, I doubt you use the reverb anyway ??

That's a very good point, I took the reverb out of it :lol:

This reminds me of an old fender trick. If youre not using the reverb could you just take that tube out and get a bit of headroom and run the PT cooler?

You MUST have a tube in V4 as one half is a gain stage for the lead channel !! :shock: V2B sounds like the loop problem and V4B is the reverb send stage.Just swap em see if it makes a difference.

Won't the lead channel be affected by the (possible) dodgy valve?

No coz it is only the B half that sounds dodgy. V2A is also a lead gain stage and that seems ok, just the V2B half is suspect, put it in V4 and see if the problem goes away. It's not a permanent fix by any means, but it will eliminate if the tube is faulty or not.