help problem with quad forty valve amp

I have a Quad system comprising of Quad 988's and the Quad valve amps QC-24 Pre and 2 x Forty Valve amps. I came into my living room this morning, turned on the amps and suddenly realised no sound was coming from the right speaker.

I narrowed it down to a problem with the amp by switcihing cables etc, but all 3 valves are glowing, so fuse must be ok and atleast 3 of the 5 valves must be ok, but it has hardly been used, so anyone with any experience who could offer their opinions would be much appreciated.

Thanks

Rael