Several years ago we started using internal fuses on the rectifier tubes of many of our amps.  This was done to protect the power transformer and capacitors from arcing or shorting rectifier tubes in case the mains fuse doesn't blow.

These fuses are located on a small board connected directly to the rectifier tube socket on the inside of the amplifier.  They were never intended to be user replaceable.  However with today's rectifier tubes failures and generally short life spans are common.  When these tubes start to become problematic they arc on start-up which usually blows a fuse.  Sometimes they also simply short out.

We have three fuses on the board to deal with all the scenarios by which the tube could fail.  Two are for high voltage, one protecting each half of a center tapped transformer winding. These should be sized at 200mA for most applications.  On TORII JR we ran 500mA because the power transformer is much larger.

The remaining fuse is always a 5A which is for the heater in the tube.  Should the tube have a high voltage short to the heater (cathode) this fuse will blow.  When this fuse blows, the rectifier tube goes dark.  When the other fuses blow but this one doesn't, which is usually the case, the tube remains lit but is making no high voltage for the amplifier, so there is no sound.

Changing these fuses is dangerous because of high voltages potentially lingering in the capacitors.  Usually simply having the amplifier playing music and then turning it off will drain all the caps because the output tubes are hot and conducting current so once the amp is turned off the caps are drained within a few seconds.  However, if for example a rectifier tube began to work and then failed before the output tubes could conduct current (say in the first 10 seconds) and you shut the amp off right away it is possible the caps will not be drained.

On our new TORII MK5, the fuses are externally mounted and there are two for each channel.  On our new SE34I.6 and the TORII JRv2, the first capacitor has been reduced to 8uf instead of 47uf.  This will reduce the tendency for weakly built rectifier tubes of today, as well as yesterdays NOS rejects from arcing on start up.

On the new 300B amplifier there are no fuses because the first section has been designed with 4uf and a choke.

The best thing you could do if you're worried about rectifier tubes, is use a 5AR4 instead of the 5U4.  5AR4 is indirectly heated so it soft starts the amp.  So far the best one we have tested is the PSVANE 5AR4.

FYI/FWIW 5AR4 is the US designation of the GZ34. Other indirectly heated rectifiers include the 5V4G aka GZ32 as well as the GZ33 (a personal favorite) and the GZ37. And the ridiculously expensive Western Electric 422A which a lot people consider the ultimate 5U4 type rectifier.