There is some interesting background information on these amps. Johnny Frank, Dolomite, Cocaine Charlie, Tennessee Walker and about five other guys all copied the same flawed circuit starting in the late 1980's and now there are hundreds of them out there. They all had this bias issue if you drive them with too much power. Many also had plate transformers that were way to high in voltage and would arc the tubes.
It was almost funny to watch each builder attempt to deal with the bias zener diode shorting as they always overlooked the root of the problem. There is very little current being drawn by the bias feeding the control grid. It's a high impedance circuit. Even a small 5 watt zener can handle this current. So why do we see the biggest zener diode you can buy still failing sometimes?
It's not failing from DC current, it shorts out from RF getting across the zener diode. Way back in the beginning, someone forgot to install a proper choke in the DC bias line off the control grid so that the blocking cap has no place to drop the RF. Just about everyone after that copied the same mistake. Now if you feed a little too much RF into the amp, it shorts the bias zener and blows the fuse. Sometimes it can ruin the tubes too.
This lack of a proper choke in the DC bias line causes another problem. It's loading the RF drive feeding the control grid of the tetrodes. This reduces gain and wastes drive power as the RF heats the zener diode. I pointed this out to one of these builders many years ago and he told me "That's so they can't overdrive the amp and blow the tubes up". My response was "So if we get the tubes drawing lots of current by driving them hard, it's OK to let the bias fail under this condition and approximately double that current?" The look on his face was better than his response.