Apropos of nothing, I stumbled across some pics I had lost. They were copied to the wrong folder, and never tracked down.
Just the same, the issue of fixed bias for the 3-500Z tube gets troublesome as you pile more and more of them in parallel. Zener diodes are the routine solution, but zener diodes are famously easy to overload. They fail as a short circuit, most of the time. Piling up rectifier diodes in series will do the same job, but rectifiers are just tougher than zener diodes. This customer needed a robust solution for his eight-tuber. It's made with fifteen 25-Amp rectifier bridges in series. Connecting the "+" and "-" lug on each one gives us two diodes in series. And two of those series pairs in parallel. The surge rating of this style rectifier should handle the rowdiest worldwide operator, no matter how many tubes he pops. The heat sink is seen only from the component side. It's a foot and a half long, more or less. Apparently it's big enough, since this amplifier is still cooking along several years after installing this trick.
73
Just the same, the issue of fixed bias for the 3-500Z tube gets troublesome as you pile more and more of them in parallel. Zener diodes are the routine solution, but zener diodes are famously easy to overload. They fail as a short circuit, most of the time. Piling up rectifier diodes in series will do the same job, but rectifiers are just tougher than zener diodes. This customer needed a robust solution for his eight-tuber. It's made with fifteen 25-Amp rectifier bridges in series. Connecting the "+" and "-" lug on each one gives us two diodes in series. And two of those series pairs in parallel. The surge rating of this style rectifier should handle the rowdiest worldwide operator, no matter how many tubes he pops. The heat sink is seen only from the component side. It's a foot and a half long, more or less. Apparently it's big enough, since this amplifier is still cooking along several years after installing this trick.
73