Hi Helmut!
Before you blow another fuse, it is my suggestion you build a lamp limiter.
Google it up and you'll see it's a very inexpensive circuit to build and very handy in your situation.
A schematic is not always necessary, a lot of amps of that era are not very complex.
The rectifier tube will likely have 4 wires coming off the transformer 2 yellow and 2 red ones, colors may vary and also fade with age.
the yellow ones will be at pins 2 and 8 the red at pins 4 and 6.
power tubes will have a wire from pin 4 to the output transformer.
You will likely have a field coil speaker,
A field coil speaker will have 4 wires running to it, 2 from the output transformer and one coming off the B+ from the recto tube and then returning back to the amp. before the days of affordable permanent magnets, the power supply would feed an electric magnet on the speaker and it would in turn act as a choke for the power supply.
NOW,
unless your dead set to restore this amp, you got a pretty good platform to gut and put a proper guitar amp into.
Post some pics if you can, nice juicy closeups of the guts if you can and a few to show some details of the cab. Maybe we can see whats going on.
First guess is a shorted electrolytic in the PS section. with any luck the PT is still good!
Ray