Usually professional flash heads and monolights are equipped with user-replaceable flashtubes that may simply be plugged into, and unplugged from their sockets.
With proper handling, the flashtubes may last for tens of thousands of flashes under normal use.
Waiting for a flash tube to fail before you replace it is not always economical.
As with all equipment, the output of your flash tubes can change with use and your photos are effected.
They should be checked periodically to be certain you are getting the performance you expect from your flash heads.
A basic test is to simply take a meter reading with the old flash tube, and again with a new tube installed.
If you have access to a flash color meter, apply the same process.
With this information you can decide if the tube is too tired and should be replaced.
Tube Diameter: 0.4inch/ 1cm
Outter Diameter: 2.4inch/ 6.2cm
Inner Diameter: 1.7inch/ 4.2cm
Weight: 0.8ounce/ 22g
1x 400W Replacement Flash Tube