ok, so its a 100w head with four EL34s (or 6L6s). Take out the dead valve and also the mirror image valve in the same position...
i.e. V1 V2 V3 V4
If V1 is dead, also take out V4
If V2 is dead, also take out V3
Assuming you are running a 16ohm cab, just set the amp to 8ohms and rock away on half power. If your amp has blown the HT fuse and the fuse is accessible from the back panel, just replace the fuse after taking out the two valves as described above.
Instant fix on a budget and stretch extra life out of the valves you have left.