Wouldn't it be nice if you could gain a reasonable level of protection for your expensive output tubes by adding a simple fuse inline with the tube? I've done this with every tube from a 6LF6 to a 3CX1200A7 and it works. You have to install the fuse on the cathode circuit of the tube in the DC bias line. Since plate current and cathode current are nearly the same, you can prevent excessive plate dissipation by fusing the cathode side of the tube. This offers protection from over driving, incorrect tuning, or a shorted antenna.
For tubes like the 3-500Z you would install the fuse between the filament transformer center tap and the bias zener diode. This has only been tested on grounded grid, cathode driven amps. You must place a resistor across the fuse to prevent excessive cathode voltage when the fuse blows under load. Otherwise all of the plate voltage will attempt to reach the cathode when the fuse blows and this will short the tube. Think of the tube, fuse, and plate supply being in one series circuit. In a series circuit the majority of the voltage drops across the point of highest resistance. If the fuse opens under load without the resistor to limit the voltage drop, the open fuse is now the point of highest resistance.
The fuse value should be determined by the tube specification sheet for maximum plate current. Using a lower value fuse offers more protection. Not a bad idea since this fuse does not directly protect the grid of the tube. The value of the resistor across the fuse is very important so that it places the tube into cutoff but not with an extreme voltage spike. Get this wrong and you may have been better off without the fuse altogether. Get it right and the tube as well as the amp should survive most common mistakes at the expense of a fuse.
One value that works well with the range of tubes mentioned is 10,000 ohms wire wound at 10 watts. Use 25 watts if you have several tubes in one stage. Just increase the fuse size according to the number of matched tubes in the stage. If the current is over 2 amps, dropping the resistor down to 5 or 6 thousand ohms may be appropriate. I suspect this will work on larger tubes but have avoided trying it on the 3CX1500A7 because that tube is very delicate and should have electronic protection on both the grid and plate.
Electronic protection that uses opto couplers driven off the plate and grid current meter shunts is a superior method since it's reaction time is faster and it protects the grid. The fact is there are countless homebrew and amps similar to the SB-220 that can benefit from this simple fuse / resistor modification while the electronic protection may be might be more then the person is willing to install. It was nice to hear customers say they made the same mistake that blew up their amp the first time but this time it only burnt up a 25 cent fuse. Make sure you use fast acting fuses rated at least 250 volts.