So, you show up to the gig, plug in, turn on your Marshall JCM800 series head/combo and... nothing... nada.... zip... the amp is DOA without signs of a struggle. What happened? Well, the heater fuse(s) just gave up on you, and it happens pretty often. The trouble is that the heater fuses are only accessible by opening the amp. Inside a JCM800 amp there are up to 4 heater fuses, 3 hi-T fuses, and an "IDIOT" fuse which is rated just slightly higher than the mains fuse and will blow if someone tries to "fix" a problem with tin-foil or a 20 amp replacement.
The heater fuses are there to protect the output power tubes and the transformers in case of problems. They are in series with the (6.3v) tube heater filaments. Tubes need to be heated by these filaments in order to get them hot enough to operate. When the amp is cold and you turn it on there is a sudden surge of current to these filaments, not enough to cause the fuses to blow but enough to cause them to soften up and melt slightly. As time goes by these fuses suffer from metal fatigue and finally one day.... today... the amp dies on the starting line.
Sudden Death Syndrome got to be so bad that here in Canada the national distributor would hard-wire bypass these fuses altogether in all new amps before they reached the stores. In retrospect, (the JCM800 series came out in 1982) I would suggest to leave these fuses in, as they serve a very useful purpose. I would recommend using 7 or 8 amp Slo-Blo values instead of the 5 amp that comes stock. The 50 watt model has 2 such fuses, the 100 watt version has 4. They are clearly identified as heater fuses on the small PC board inside the amp.
To make your amp more road-worthy always change ALL your fuses before a tour. Do NOT change the rating of fuses other than the heater fuses as this will put your amp in jeopardy, and you will suffer the disgrace of needing the IDIOT Fuse.