Safety and the Bottlehead
As more and more audiophiles come to the long forgotten conclusion that building your own tube gear is the best way to sonic heaven, the issue of safety around high voltages becomes terribly important.
With the near demise of commercial tube audio gear in the late 60’s, and the similar decline of kitbuilding in the late 70’s, safe test and construction techniques have been well nigh forgotten by most audiophiles who were there ‘way back when’ and information on the topic of dealing safely with the kind of high voltages present in the current crop of kit and DIY tube designs seems to be covered lightly if at all.
Tube audio gear tends to operate a much higher voltages than the current day solid state audio equipment. While a high power solid state amp might draw amps like an arc welder, it usually runs at 75V DC or less. Not to say that these kinds of voltages and currents can’t hurt you, but a typical tube circuit may operate at anywhere from 120VDC to as high as an ‘electrifying’ 1500VDC or even higher!
While these voltages are intimidating ( in fact many experienced tube DIYers limit their construction to circuits running at 500V or less), that high voltage is only half of the safety issue.
Is high voltage dangerous in and of itself?
Spend some time around a small town TV repairman and eventually you will see him check the HV supply of a picture tube by touching the high voltage lead from the voltage tripler with his bare finger. A small snap will ensue,his hand will fly back, and he’ll say, “Yup, it’s working”.
Is there a destructive streak in TV repairmen? That supply is probably at a potential of 5kV to 10 kV!
Nope, the repairman knows that there is virtually no current available from the supply, so while the high voltage may give him a slight ‘zap’, the supply can’t push enough electrons through our intrepid TV guy to really do any short term damage.
NOTE: we are not condoning this practice, in fact there are studies that show repeated microshocks may indeed be detrimental to the nervous system – we’re just trying to explain by example!
But- let’s consider a single ended 300B amp. Now we have a power supply that can supply far less potential than that TV high voltage supply, maybe only 450 volts, but it may be able to push out 160 mA of current (if it’s a stereo amp). Before you grab a bare terminal in that supply while in your bare feet on the basement floor, you might want to make sure your life insurance policy covers acts of incredible stupidity.
It is interesting to note that in the days of tube powered test gear Tektronix corporation use to put a warning inside its oscilloscopes reminding the technician that the lowly 400 volt supply, which supplied as many as thirty or forty current hogging tubes, was far more dangerous than the intimidating looking tube rectified voltage tripler circuit.
So yes, those 900V+ 211 and 845 and circuits demand a lot of respect from the builder, but watch those 250V 2A3 amps too!
OK, so how to stay safe?
First some basics. Always wear shoes when working with electronic gear, preferably rubber soled, and particularly when standing on concrete floors. This is because the high potential source will want to find it’s way to ground potential. If your body supplies a path to ground, that’s where the juice will go, homes. Right through you! The rubber soles will insulate you from ground.
Another classic path to electrocution is from one hand to the other. If you grab the chassis of an amp or preamp with one hand, and touch a live terminal with the other, guess where the current will flow. Right through you!
The oldtimers figured out a good way to avoid absent mindedly performing this shocking display – train yourself to always keep one hand in your pocket when reaching into live gear.
The reason these paths are two of the most critical is because they cross through one of the more electrically sensitive organs in your body – the heart. Because the heart is slighly to the left side of the chest cavity, it is actually slightly safer to use your right hand than your left hand when reaching into or touching a probe to live circuits, as the path to ground through your feet does not pass quite so directly through your heart as current passing through the left hand would.
I’m safe, you’re safe, now what?
Make sure you stay away from mains wiring! Remember that the power cord is live even if the equipment is switched off. And don’t forget those mains usually supply 15-20A before the circuit breaker will trip. If you don’t need a piece of gear plugged into the wall to test it (say you need to check a resistance), don’t leave it plugged in! If it must be plugged in, consider use of an isolation transformer between the wall socket and the equipment to be tested, which will create a current limit on what the AC mains can supply.
A ground fault interruptor is a must have in garage and basement shops with concrete floors, and anywhere else where the floor could get wet. Install them in your shop if they are not already installed.
A few tips
I use test leads on my meters that have optional alligator clips that can be slipped over the pointed probe tip. If I can avoid holding a probe in my hand when checking a live circuit I do, and the clips make this possible. The alligator clip is particularly useful for the grounded probe. Even if you must measure several points, attaching the negative probe to the ground plane or buss lets you concentrate on the hot test points, which can save not only your precious hide, but help you to keep from shorting the test point to ground from an errant slip, which is the number one way to blow underchassis components.
Never, I repeat, NEVER lean over live equipment or put probes in where you can’t see them!