Safe Amplifier Design

     We have not become a less litigious society as far as I can tell. I do not see poor lawyers standing at street corners, holding signs proclaiming that they will sue for food. Yet safety's dim glow, like a feeble pocket flashlight, in too many modern tube amplifier designs, troubles me as someone who wants to see greater acceptance of tube equipment, and should terrify the legal staff of the amplifier's maker.
    "We place sound first and all else, even safety, second," the high-end, high-priced tube amplifier manufacturer tells us. If only safety did come second and not ninth or tenth. I have looked in horror at amplifiers that required bias or hum nulling adjustments inside a rats nest of high voltage wiring and components. (I remember being told by a manufacturer of high quality tube amplifier ten years ago that he had to stop using thin ventilation slits on his amplifiers and use small circular punches instead. I asked why and he told me that children would feel compelled to drop coins in the thin slits and then their parents would use butter knives to remove them!)
     Admittedly, some protection devices compromise performance. But even race cars have roll bars and safety belts. And many safety touches cost nothing in performance.

Safety Issue One:
What happens with a cold tube?
    Following the maxim, "the fewer the components, the better the sound," some have tried direct-coupling the output tubes' grids to the previous stage, wishing to bypass the coupling capacitor altogether; often their experiment ends in catastrophe. The plan seemed reasonable: use a bi-polar power supply and cascade the first stage into the driver stage via coupling capacitors and then directly connect the driver stage to the grids of the output tubes. Less capacitors; therefore, better sound.  What went wrong? The problem is that until the cathode is hot enough to emit electrons, the tube does not complete the circuit.

     Requiring time, effort, and expense, safety comes at a cost. This is at odds with the ideal, as ideally, all design decisions would automatically place safety first and ignore the expense. In reality, "safe enough" is, usually, "good enough." (Remember, a perfectly safe airplane is one that never leaves the ground.)
     Having to deal with high voltages makes safety a more pressing concern for tube audio enthusiasts than it is for their solid-state following brethren. As little as 42 volts can kill and I understand that some electric chairs ran on only 240 volts; so when contending with the 1100 volts of a transmitting tube's power supply, a freshly shaved head is probably not necessary. Definitely, having respect for the high voltage's lethal ability is the first step in building a safer amplifier. So while it is impossible to design a perfectly safe tube amplifier, trying to do so is still worth the effort.

Safety's Focus
     More like a spotlight than a floodlight, shifting from place to place, safety's focus moves. For example, if you sell tube amplifiers (and if you heed your legal department's council), then you should point the safety spotlight on the customer. On the other hand, if you are building yourself an amplifier based on new-old-stock 300B output tubes, then you would probably shift the spotlight to the 300Bs. But if you are building yourself an amplifier based on Chinese 300B output tubes and you have Lother speakers, then you would undoubtedly move the spotlight to the speakers.
     Simple enough (so simple in fact, that I was pained to mention it} but how do we explain the safety brownout in which so many amplifiers are designed? Amazingly, as far as I can see, some tube amplifiers are designed in complete safety darkness. I have seen amplifiers with exposed high voltage terminals on the outside of the amplifier! 



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