Blowing Fuses in a Tube Amp

bias can be the main problem. what tubes your amp is using?
i can tell you what tube pin you should check for grid voltage in order to have a correct bias before you place the new tubes there. also i can direct or perform an installation of the bias measurement circuit almost on any tube amp with potentiometers for a neccessary adjustement when needed.

Does the amp have a tube rectifier ? If it does, it is a good place to look.

I have a CJ MV-50. It was blowing fuses, I think it was the Svetlana EL34's. I replaced them with old Mullards that BARELY tested good and have not had a problem. So I think maybe you should try different tubes. The Mullards also sound better.

You also could have an output tube with an internal short. If you have a tube tester, you can find out right away, or you can simply replace the tube.

Or, you may have the wrong fuse type. If your amp requires a time delay (slow blow) fuse and you have an instantaneous peak type installed, that could do it also.

There are a lot of other unlikely possibilities like a bad capacitor, plate resistor or xfmr winding.

there are lots of variables on that issue to check but this value is finite.
1.the unstable bias supply can be due to the rectifying tube or ss diode or bridge; replace with higher quality rectifying diode(a-must) or ss-diode or the whole bridge.
2.malfunctioning coupling capacitor that needs to be replaced. try to find the better model than manufacturer-installed and do same for the functioning channel as well.
3.malfunctioning trim pot if such exists. simply that one needs to be replaced. plese note that potentiometers tend to wear-out and to prevent it vacume your amps with tubes unplugged approximately twice per week. meanwhile you can also clean the tube sockets(not so often but...) and tube pins.
4.malfunctioning self-bias supply that has a complicated circuit and if no or slight experience dealing with different models of power supply i'd recommend this amp to be serviced by tech.

Enzo,
In your cituation the biggest problem is not tubes but just poor design of a power supply and bias supply of most CJ amps believe it or not. Sound has to be = to the built quality.

Gs...,
Even if you test the tube going bad you still have to check the rest of routine I mentioned above since the blown tube is only a result of malfunctioned electronics. xfmr winding must be probably checked last but completely agree with plate resistor and coupling cap.