How dangerous is it to build an SET? - diyAudio
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Old 18th April 2010, 06:24 PM   #1
robertd is offline robertd  United States
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Default How dangerous is it to build an SET?

I'm writing to get a sense of how dangerous it is (shock) to build an SET power amp? I'm very interested in undertaking this project but will forgo it if shock/electrocute risk is prohibitive.

I have some soldering experience - resistors in my phone pre, driver in a speaker, switch in a pre, but my question is, can I take some reasonable steps so building an SET is a safe project, or will there always be a risk that I do something accidental or stupid and get zapped?


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Old 18th April 2010, 07:02 PM   #2
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Hi, I don't know if you have seen this "Sticky Thread" at the start of the Tubes / Valves Forum, but there is good info on this there.

Safety Practices, General and Ultra-High Voltage

Building tube amps isn't dangerous, per se, it's the builder's absent mindedness that gets him into trouble and zapped. The best rule to use (it's akward though) is the "one handed rule". When you have the amp powered up and are working on it, you have one hand deep in your pants pocket and only work on the amp with the other hand. This prevents the HV from going from one hand to the other (and through your chest / heart).

Last edited by danFrank; 18th April 2010 at 07:07 PM. Reason: missing words
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Old 18th April 2010, 07:24 PM   #3
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Another good basic practice is to install bleeder resistors across the power supply caps, so when you power the amp down, their charge bleeds down over the course of a minute or so. Otherwise, your PS caps can hold a HV charge for quite a while.

I also use clip leads when testing a live amp, and power down while monitoring B+ voltage before moving any leads. With clip leads I don't need any hands in a live amp. Testing/troubleshooting is a little slower, but perfectly acceptable to me.

Also always use grounded plugs, and make sure the chassis is bonded well to the ground wire. It's the most important connection in the amp.

Staying below 450V-500V power supply voltage is also a good idea safety-wise.
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Old 18th April 2010, 07:34 PM   #4
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One hand rule.. Carpet, nonmetallic chair, shoes, long shirt, all help that work the best. It's all a matter of keeping an open circuit through you. You can take yourself right up to few hundred volts, thousands if you're a little loonie, so long as you don't start any arcs or make any contacts. Although getting rapped through a section of the arm doesn't feel that great, the most damage you suffer might be caused by how you jerk away. I've run myself across some sheet metal edges that way. Somehow the probe tips figure out how to get in the way of where my hands want to go pretty often, which is usually not too fast unless I think I'm getting shocked or I need to "change a connection" in a hurry.
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Old 18th April 2010, 07:40 PM   #5
tomchr is offline tomchr  Canada
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Any project that connects to the mains is inherently dangerous. That said, there are steps that you can take to minimize the risk of electrocution. Monitoring B+ and other high voltages used is one. I use two voltmeters for that reason. One is permanently hooked to B+, the other measures whatever I want. I don't mess with the circuit until B+ has discharged to below 5~10 V.

Don't work on the circuit if you're drunk, sick, tired, or in a hurry as you are much more likely to make mistakes that could turn out lethal.

Make sure all exposed metal parts of the chassis are grounded to the green wire safety ground of your house.

Circuits that operate up to 250~300 V don't generally require any special components. Most hook-up wire is rated at 300 V. For higher voltages, keep in mind that most 1/4W resistors are rated at 250 V so if they have a higher voltage drop across them, you'll need multiple in series. 600 V hook-up wire is pretty easy to find. Once you get above 500~600 V sourcing parts becomes a challenge. Not impossible if you know where to look, but you won't find much at Digikey, RatShack, and the like places.

Stay safe. Work slowly.

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Old 18th April 2010, 07:54 PM   #6
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Oh man, the tired thing is great advice. Maybe 80% of everything I blow up happens after midnight. The rest of the time it's usually when I'm sure of something without checking.
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Old 18th April 2010, 08:20 PM   #7
Loren42 is offline Loren42  United States
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One hand rule: Make it your right hand. It is furthest from your heart.
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Old 18th April 2010, 08:28 PM   #8
defect9 is offline defect9  Ireland
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Originally Posted by Loren42 View Post
One hand rule: Make it your right hand. It is furthest from your heart.
Sucks for the lefties.
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Old 18th April 2010, 08:31 PM   #9
Loren42 is offline Loren42  United States
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Unless your heart is in the wrong place.
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Old 18th April 2010, 09:37 PM   #10
Glowbug is offline Glowbug  United States
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What's life without a little adventure...
The machine does not isolate us from the great problems of nature but plunges us more deeply into them. - Antoine de Saint-Exupéry
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