The 70's Model T didn't have a fuse, it had a push button circuit breaker. The circuit breakers eventually fail and are replaced with a fuse because the circuit breakers are somewhat difficult to find. For 120V North American mains, a 5 Amp slow blow will probably work. You need to find one that fits the fuse holder. Remember, the job of a fuse is not to protect the amp, it's job is to protect the house from burning down because the amp died.
