Postby ClubAndCountry » 07 Oct 2009, 04:51
I hope you mean 4-ohm *cab* :-).
If the valve fail lights come on you have a functioning HT supply, and output transformer on the primary side at least... that's good.
Is there *any* hiss through the cab when the amp is on - you may need to listen very closely, but you should hear some, and at least a pop or change when you flip from Standby to on (these amps normally hum *more* on standby than they do when running). If no, the problem must be in the speaker circuit somewhere - the output transformer secondary (not completely impossible, but extremely unlikely), the impedance switch, the speaker jacks, the cable or the cab... or a bad connection inside somewhere.
If yes, the problem is not in the power stage and running into the 4-ohm load was almost certainly just a coincidence. Try connecting the guitar directly to the FX return, and turn up the FX Mix and Master Volume controls - this bypasses the preamp and will allow you to check the phase inverter and power section. And do check that mute switch, if you haven't already!