Just read through this thread and there's a lot of good stuff here - and also some nonsense - like the bloke who would have us believe that 25V can be dangerous! Yes OK a 20000 uF capacitor charged at 25V and discharged through one's metal watchstrap would be very nasty - but a shock?
Rather worrying, the number of people hereabouts who are building/repairing amplifiers and yet do not know how to discharge a capacitor; a case of cart before the horse?
Anyway one point that has not appeared: since DC measurement is so important when working on or developing amplifiers, I always have the bench meter connected to the amp on which I am working always to the HT supply to the output transformer(s). Thus after switching off, I pull out the mains lead BEFORE picking up the soldering iron and then assure myself that the bench meter is measuring (and continuing to measure) millivolts before I do anything. I have always found that valve amplifiers very quickly discharge all their capacitors whilst cooling down, though this does not mean that bleeder resistors are not a good idea.
Interesting that a -160V shock is every bit as unpleasant as a +160V shock isn't it?
Paul
Rather worrying, the number of people hereabouts who are building/repairing amplifiers and yet do not know how to discharge a capacitor; a case of cart before the horse?
Anyway one point that has not appeared: since DC measurement is so important when working on or developing amplifiers, I always have the bench meter connected to the amp on which I am working always to the HT supply to the output transformer(s). Thus after switching off, I pull out the mains lead BEFORE picking up the soldering iron and then assure myself that the bench meter is measuring (and continuing to measure) millivolts before I do anything. I have always found that valve amplifiers very quickly discharge all their capacitors whilst cooling down, though this does not mean that bleeder resistors are not a good idea.
Interesting that a -160V shock is every bit as unpleasant as a +160V shock isn't it?
Paul
Dangerous voltages?
I read through the "danger" of 24 volts. Did the writer not mean 240 volts? Ask ANY auto electrician or mechanic working on large diesel trucks (Those that are with a gearbox and not diesel electrics) and they will tell you that APART from induction developed back EMF (starter motors /solenoids etc.) 24volts is handled without any problems. Ask a telephone exchange technician that worked on those old electromechanical (Strowger)units and we handled 50 Volts no problems. The current from those batteries can burn your metal watch strap making it red hot in milliseconds but shock? I must still get a shock from it. and I worked for many years on lift trucks with 72 volt batteries and I cannot remember a shock from them
EXCEPT if an inductance like a motor or contactor coil was in the picture 
Hi Guys,
I read through the "danger" of 24 volts. Did the writer not mean 240 volts? Ask ANY auto electrician or mechanic working on large diesel trucks (Those that are with a gearbox and not diesel electrics) and they will tell you that APART from induction developed back EMF (starter motors /solenoids etc.) 24volts is handled without any problems. Ask a telephone exchange technician that worked on those old electromechanical (Strowger)units and we handled 50 Volts no problems. The current from those batteries can burn your metal watch strap making it red hot in milliseconds but shock? I must still get a shock from it. and I worked for many years on lift trucks with 72 volt batteries and I cannot remember a shock from them
EXCEPT if an inductance like a motor or contactor coil was in the picture
Hi Guys,
I read through the "danger" of 24 volts. Did the writer not mean 240 volts? Ask ANY auto electrician or mechanic working on large diesel trucks (Those that are with a gearbox and not diesel electrics) and they will tell you that APART from induction developed back EMF (starter motors /solenoids etc.) 24volts is handled without any problems. Ask a telephone exchange technician that worked on those old electromechanical (Strowger)units and we handled 50 Volts no problems. The current from those batteries can burn your metal watch strap making it red hot in milliseconds but shock? I must still get a shock from it. and I worked for many years on lift trucks with 72 volt batteries and I cannot remember a shock from them
Yes that's right; one can get a nasty little tingle from the back emf developed in a relay coil; the reason for those 0.1uF 500V caps fitted to protect the diodes in alternators. The lighting column switches on Lancia Fulvias often gave me a little wake-up call - probably about 400V!
7N7
Here's a good story how I about how almost died I picked a Crate stealth 50 about couple years ago well about 3 months ago I score some 1955 CBS 6v6 black-plates for so I was stoked and popped em in biased up etc well had amp out of the chassis sitting on top of 4x12 on few blocks of wood I was metering/checking voltages/resistors like usual I noticed I left my Delay pedal on hooked through effects loop on right side of the amp well with left arm I reached across amp to turn it off guess my arm got too close to edge of the PCB and I Mean HOLY **** defiantly cap Discharge into my arm 450v 47uf ran up my arm into shoulder into neck head area at this point I was convulsing twitching very violently held me there for about 10-12 seconds it finally let go I slumped to floor about blacked out man and still to this day I have quarter sized burn mark on my arm so this **** can kill you I'm just extremely lucky, basically don't get about half drunk and play around inside and amp you might get careless like I did.
Some basic points to add. Designers and builders of high voltage valve amplifiers should write a maintenance and repair booklet, containing all test points, voltages, and operating current. Layout extremely important, since arcing distances from exposed connecting points vary with room temperature and humidity. Use different colored hookup wire for different voltages. Build the power supply before starting with the amp - and test it thoroughly. Take frequent breaks from the project - 60Vdc @ 20mA can kill a human being!
you sould work with a RCD inline with your ac supply and a islation transformer with MEN config done on the ac side you have to this yoir self most transformer are not wired right i found this in my country and check and or do the MEN config use a RCD abd a seris globe config witch inables you to see it the device is drawin lagre amounts of current etcI want to live.
In the interest of safety, I would like to collect whatever safety tips and practices this forum kindly and experienced members will share, so that all may benefit.
First, some warnings and disclaimers:
And please note that I have *no* amplifier hardware on hand, and will not for a while. This is a discussion only, and I will only be talking about doing stupid things, not actually doing them.
- Any voltage can be deadly.
- Amplifiers are dangerous--each person (me included) is responsible for deciding if they are willing to risk their life, and for ensuring that whatever information they receive is correct
- This forum and its members accept no responsibility for any death, injury or property damage that result from any of these suggestions--your safety is your own responsibility.
Okay, with that out of the way, I would like any tube equipment electrical safety advice you care to share, both in general, and for what I will classify (for the purposes of this discussion) as UltraHighVoltage--1000V to 1500V.
I'll start with what I think I know, and you can add more or correct me if I am wrong.
I can think of two basic rules of thumb--
From those rules of thumb, I can think of these specific practices:
- Make sure any charge-storing devices remain discharged while working on the equipment
- Avoid creating a circuit which would pass current through the heart.
- Put a drain load across all significant charge-storage devices (e.g. B+), and leave them there for the duration of your work--some devices can redevelop voltage if left unloaded.
- Verify that all charge-storing devices are discharged before starting work.
- Always work with only one hand--keep the other hand in your pocket.
- Use hook probe for secure connections and to avoid shorts.
- Wear rubber gloves and rubber soled shoes--no bare feet or socks.
- Make sure your design has the chassis safety grounded--connected to the ground wire of the power plug.
Okay, what do you think about these practices, and what can you add?
One member spoke of a "bench isolating transformer". What is that, what does it do, and how is it installed/used?
Also, what extra design and usage practices apply to UltraHighVoltage (1000V to 1500V) electronics? I'm thinking about things like component choices, test procedures (standard DMMs are only rated to 600V), design practices (danger of arcing), etc.
Thanks for your attention and advice. Although I hope for courtesy, I'd rather be flamed figuratively than literally.
Best,
George Ferguson
No, you don't "just accept it" - that is why you look things over several times to make sure it makes sense before you turn on the power. Mistakes and malfunctions do happen, but following safe practices will isolate you from the most serious consequences. Some of us work with designs operating at 1kV and above where there is no room for carelessness.
hi guys the old saying is with high voltage if your are not confident or do not know what you're doing you should not touch it i have been in the repair business fixing electronics since i was 13 i am now 37 and i am still amazed what people try to do or what they read on the internet these voltages will kill make no mistake there will be no second chance
Paul, in this world of risk assessments and hazard identification it is better not to be too blase about what is 'dangerous'. Sometimes the act of getting a tingle from 25V, or it causing a part to smoke or get hot, could lead to a secondary more 'dangerous' reflex reaction if the situation is unexpected. An unexpected tingle, even if it doesn't exceed the theshold of whichever standard you're complying with, is still a hazard. Ciao, Tim
In this thread I saw a reader asking about a "Isolation transformer" This is a transformer with two ISOLATED windings The secondary is the SAME as your supply voltage. This is used with mainly service guys doing television sets with "live" chassis. This is to prevent electric shocks handling the chassis. If you should touch ONE lead on the secondary then you should not get an electric shock. I used them working on those old AC/DC sets with their live chassis This is for safety I also used it working on appliances
The other use of this transformer is how do you connect your MAINS driven instruments. Those days most of the instruments were tube based and needed a mains supply to work.This made such a transformer essential if earth connections were made. Fully solid state oscilloscopes are not that old and spectrum analysers etcetera and these were not all "floating" as the solid state equipment that run for ages on a smallish battery. Those 90 volt batteries were dear and most of the electronics those days worked with an H.T. of 300 plus volts and the display tube ran with 30000 volts plus. AND THEY WERE BULKY AND HEAVY you needed a forklift to handle them. A 50 Kg box was not fun to move around "Those were the days!!!"
And never make fun such a way like on the picture! It can kill.
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