I can't get much detail from the web on your amp.
However, if the pre-amp valves light up, but the power valves don't (double check this), then I would suspect that the filament supply to the power valves has expired.
This would normally be an unusual fault as the pre-amp and power amp in most amps use the the same supply, however it is increasing common for amps to run their preamp valves from a separate DC supply. If this is the case, then you could have LT failure to the power valves and not the pre-amp valves.
Can you check whether you get a signal from the pre-amp out? If you do then the HT supply is probably OK, and you are looking at a power amp LT problem.
The likely causes of this could be:
1) Broken connection/ cold solder joint in LT wiring.
2) LT winding on the transformer has failed.
Less likely:
3) Heaters to power valves wired in series, and one valve heater has gone open circuit. Can test this by using a multimeter to test continuity between pins 2 and 7.
VERY UNLIKELY
4) LT fuse has blown. (Most amps don't have an LT fuse, although this actually contravenes electrical safety regulations!!)
Incidentally are you using 6L6s or EL34s? EL34s have a greater LT current requirement and are more likely to cause faults 1 and 2 to manifest themselves.
Regardless, I suspect a trip to a competent tech is in order here.
