Mark IV Effects Loop problem.

[Mountain Fever]

New Mark IV--This morning I was taking my first stab at using the effects loop. After 'solving' the problem of no sound when the loop button was engaged by getting the return cable into the 'mono' input (duh) I was going along just fine.

Then, all of a sudden there was momentarily a change in the tone (20 seconds) and then the sound level went to barely audible. The sound was similar to when to happen to switch a tube amp off when there is still signal going on and it kind of fuzzes out.

Once I switched the loop out via the footswitch normal sounds (sans my digital delay) returned.

I guess the easy thing is to assume that the tube that handles the effects return is bad...which may well be the case. I have not yet bought backups beyond having a set of EL-34s for the outside sockets which will get tried at some later point. It is a little disappointing that an amp with about five hours of use is already experiencing problems, regardless of how minor they might turn out to be.

Questions:

Does this clearly seem like a problem with the effects loop tube?

Do you use the same brand/type of tube in all the sockets regardless of the funtion?

Thanks for your insight!

 

[Pinweaver]

If it's a brand new Mark 4 there should be a spare 12AX7 if you remove the front panel.
If you think a tube is the problem you try changing V2, that's the FX return stage. V2 is one next to the shielded V1.