high voltage safety
Originally posted by Thatch_Ear
Rob,
A .005/400 is a protection device and I use even at least a .1 to float a ground on a chassis, so no. Even though it is rated to 400V it barely counts as a capasitor. Anything that has some capasitance to it that you can use with chokes or electrolitics that will handle more voltabe than it takes to run a 300B or like triodes I consider to be high voltage. First of all it is not only rated to handle it but does, second of all not only is it rated to handle it but will kill you because it has high voltabe running through it.
I think you guys that fart around with transmitting tubes to drive speakers are nuts. I would be sitting in a plastic chair on a rubber matt and wearing rubber gloves. Anything hot enough to leap out and bite you like a snake is too much for my limited knowledge.
Thatch,
You seem to be referring to the energy storage capacity of a capacitor which is measured in Joules, rather than the voltage rating. The capacitance of your body on rubber soles is capable of storing 50,000 volts when you shuffle across a carpet in a dry room but the energy storage is so low that even though you make a 3/8" spark to a grounded metal object when you approach, you don't get electrocuted.
BTW, that insignificant 0.005uF cap if charged to 50kVDC would store 6.25 Joules. That is more than enough to blow you away permanently.
You are absolutely correct that working with the voltages and currents that apply to transmitting tube audio amplifiers require extra special care and attention. The energy levels stored in the power supply filters combined with the relatively high voltage that negates the safety factor of skin resistance makes such circuits beyond lethal and you seldom get a chance to tell about accidental contact. Such circuits should be built only by the most experienced in safe handling of high voltage circuits, and it doen't hurt to have a 'buddy' around to tend to your first aid should an accident occur. On the same theme any products sold to the consumer with such high voltage circuits and enclosures have to be very well designed so that all but the most determined nutcase cannot get himdelf zapped by accident. Open chassis with xmitting tubes using plate caps for example are a big no-no here. Even amps running common, almost household 6146's or 807's can be an accident waiting to happen in an open configuration with as much as 750 volts on the cap.
The drawback with tubes (yes I'll reluctantly admit there is actually ONE) is that they are inherently high impedance devices and to control any power, relatively high voltages are needed.
Even circuits for 45's or 300B's can give you a really unpleasant shock experience so it just makes sense to play safe at all times when building this gear or working on it with covers removed. Standing on a rubber mat, especially if your shop is on concrete floor is a good safety step. One hand in a back pocket while poking in a circuit with HV with a test lead is another good idea. The idea being that accidental contact won't cause the lethal current to take a path through your chest and possibly fibrillating or stopping your heart.
I have worked with high powers at up to a million volts in my lab and currents of 150,000 amperes at 15,000 volts and have never gotten a shock because I was super careful. I will treat my 805 amp circuit with the same respect.
The photo below is the last 2 feet of a 7 foot long, 1/2 million volt streamer dancing on a welding cable ground wire the size of your thumb on a concrete floor in my HV lab. The current was short duration pulses but the pulses were tens of amperes, representing many megawatts of peak power. Around here we call this high voltage.
Rob
