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John Warren

High Voltages and Tube Amps

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There's a very real possibility of lethal shocks working with or tweaking a tube amplifier.  In HiFi, they've been around for over 70 years and, combined with musical instrument amps, 10s of millions (if not 100s of millions) manufactured and distributed throughout the world.  And even today they're still popular and in wide use albeit amongst a niche group of audio and musician types. 

 

And then there's kits.

 

You'd think that death by electrocution would be a running plague associated with tube amps but I don't see it and find that a bit curious.  

 

Thoughts, comments?   

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A worse situation was the ubiquitous hot chassis table radios of the 30s through 60s.  In spite of that hazard I never heard of anyone even getting a shock from one in spite of many having their backs removed for ventilation, knobs removed for whatever reasons, etc.  I am sure things happened but it was seemingly a rare occurrence.

 

Maynard

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Car spark plug wires have 50,000 volts on average these days. Amperage is low but still a very serious shock. I have never heard of a mechanic suffering damage from ignition spark and they get shocked all the time. One rule to remember all audio technicians know about when working on tube gear is one hand in pocket. You do not want the voltage to go through your body. If you get shocked with one hand in pocket you are more likely to get hurt moving too fast when it happens. 

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I was so paranoid about seeing the electric blue jesus that I studied high voltage safety for years before working on a tube amp. I don't regret it and treat everything with respect.

 

 

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4 minutes ago, seti said:

I was so paranoid about seeing the electric blue jesus that I studied high voltage safety for years before working on a tube amp. I don't regret it and treat everything with respect.

 

 


Oh, you mean the Blue Fire! I used to work around machinery running 460V 3-phase = instant death.

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Until i read this, I had the ambition to one day put a 'kit' together but now I think it is wiser not to walk that path... 

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4 hours ago, ILI_MeloManiac said:

Until i read this, I had the ambition to one day put a 'kit' together but now I think it is wiser not to walk that path... 

You will have no problem as long as remember the rule with a live amp "one hand in pocket." Do not let scare tactics prevent you from diy. 

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Getting back to my initial question.

 

I suspect that the risk of lethal shock, though quite real at 500VDC, is in fact lower than what theory would suggest.   In other words, most shock events at 500 VDC deviate sufficiently far enough from the conditions required to cause a fatality.   

 

Now 1kV and higher is a different kettle of fish.

 

 

 

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I was taught in the Air Force Academy on electronics it's not the voltage but amps that kill you - even a tenth of an amp (0.10 amps!).

 

 

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23 minutes ago, Dr Morbius said:

I was taught in the Air Force Academy on electronics it's not the voltage but amps that kill you - even a tenth of an amp (0.10 amps!).

 

 

The adage I heard was "volts jolt, mil(iwatt)s kill" 

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2 hours ago, Dr Morbius said:

I was taught in the Air Force Academy on electronics it's not the voltage but amps that kill you - even a tenth of an amp (0.10 amps!).

 

 

That's actually 100mA, so plenty to stop your heart.

how-electricity-affects-the-body.thumb.jpg.4048c2b7e6506273798072f928b5d4c1.jpg

 

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I don't like to poke around a live amp. I just clip in enough multi meters that I don't have to move probes around. Then I turn the amp on take measurements turn it off. Then drain all caps before working on it again.

 

 

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1. Know that the thing can kill you if you complete the circuit with your body.

2. Probe with one hand only, keep the other behind your back. Long-sleeve shirts can also help, cover that skin.

3. Have respect for the pixies, they're angry and looking to get out.

4. Engage braincase, move smoothly, don't ***k it up.

 

Easy, no?

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I do a fair amount of restoration and modification work on old broadcast tube gear, many of which require experimentation and making of response measurements while live.  Things with outputs up to 10W.  That's never bitten me.  Go slow and don't touch anything until you've thoroughly assessed all the Murphy's waiting to strike.  

 

Tube gear should have bleeder resistors built into the power supply, they should be a balance between minimal current draw impact in of themselves when powered on, and reasonably rapid bleed down of high voltage once turned off.  Once bled down, they also prevent any memory voltage from rising, as is possible in an amp lacking a bleeder.   Almost all the old pro broadcast gear I work on is this way, and voltage measurement confirms a lack of stored voltage within a pretty short period of time.  Safe to work on!  

 

I also own an Ampeg V4B bass amplifier (four 6L6 type amp, 100W) I've had to work on several times.  Higher voltage (700ish?) and much higher current.  That's bit me before, even following the one hand rule.  Extremely painful.  High power amps are an entirely different matter.   But, if you're only working on the preamp section of that amp, voltages and currents are much lower, might startle you but that's it.  Think about WHERE in a circuit the highest current draw potential is, keep it foremost in your mind

 

I have a Hewlett-Packard 300V power supply that actually frightens me: 1.5A current capacity.  Have never seen a need to turn that (arc-welder) on.  Not sure what possessed me to acquire it!   

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Speaking of HiGH VoLTAGE any idea what this thang is and I ain't turning it on?

79503780_2633648036753982_4154033987200745472_o.thumb.jpg.7ed0a5d6a8fa80b1b1b42cacc8ec2c39.jpg

79378246_2633645473420905_7094699039941197824_o.jpg.737b2d3c9551b4a06a91b3355e3bd270.jpg

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You might want to consult with a certain Doctor.................I believe his name is Frankenstein - either that or a Staff Sgt that's in charge of launching ICBM's.

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On 12/14/2020 at 3:16 AM, John Warren said:

Thoughts, comments?   

Thank God.

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On 12/16/2020 at 2:18 PM, Dr Morbius said:

I was taught in the Air Force Academy on electronics it's not the voltage but amps that kill you - even a tenth of an amp (0.10 amps!).

 

 

But it takes voltage to make that current flow.

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7 hours ago, seti said:

Speaking of HiGH VoLTAGE any idea what this thang is and I ain't turning it on?

79503780_2633648036753982_4154033987200745472_o.thumb.jpg.7ed0a5d6a8fa80b1b1b42cacc8ec2c39.jpg

79378246_2633645473420905_7094699039941197824_o.jpg.737b2d3c9551b4a06a91b3355e3bd270.jpg

The detonator for the first A-bomb?:)

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