Super Reverb RI trouble

[I Tell You What N.O.]

Hi,
My super reverb reissue has crapped out in the vibrato channel.

The first channel works fine. As the volume knob is turned up from 0-10 the volume peaks at 5 and then sinks to nothing as you go all the way to 10.

The big tube on the left (facing the back of amp) is making a "loose filament" type of noise as well. problem started after after traveling to a gig then got worse to this point I describe after the return.

What to do.

Also, can't I switch some preamp tubes and make the vib ch. work again?

[I Tell You What N.O.]

also

Just was checking it out again. No noise from the leftmost tube (looking at back of amp) but the second from the left lights up blue for a second when you filp the standby switch off.

[Enzo]

You can swap tubes around and see if the problem changes, yes.

All the little tubes light up inside? There are two little heaters inside the small tubes.

After that I would be checking for missing plate voltages on the involved preamp tubes, and possible an open or broken free coupling cap on the way to that volume control. And of course check the control itself for open element or short to frame.