# Danger in tube amps



## GuyB (May 2, 2008)

I hear and read, here and there, that there could be great danger to fiddle inside a tube amp, electric shocks, etc. I'm not at all a tech guy and I would like to know exactly what that is, where is the danger ? Is it always dangerous ? Is there a way that we can eliminate that risk of shocks ? Is there something in particular that we should never touch ? Is changing the tube possibly dangerous ?


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## Rugburn (Jan 14, 2009)

Inside the chassis or underneath the chassis in a box you'll find filter capacitors. These are principally where high voltages are stored even long after the amps been turned off. These can be safely drained, but you should learn to do this with the help of a qualified individual. Changing tubes is generally a safe and easy way to troubleshoot noise and performance issues. The one caveat being that you know how your amp is biased. Some amps allow you to change power tubes without re-biasing the voltage, while others will require re-biasing to operate safely and and tonefully.

Shawn


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## StevieMac (Mar 4, 2006)

Yep, there's a potentially lethal shock hazard waiting in your tube amp. As stated, it's certainly safe to switch preamp tubes but power tube swaps generally require a rebias so my advice is to leave inside-the-chassis work for the pros.


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## GuyB (May 2, 2008)

Thanks to both of you. And with a photo so I can see what could happen, I'm well served !

But, my Traynor YCV 50 Blue does not need the power tubes to be biased (well, that's what they say) so, can I change them also without risk ?


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## keeperofthegood (Apr 30, 2008)

Well, if you are the kind of guy that likes poking his finger into the socket when changing a light bulb then for you I would say there is risk.... otherwise, treat the tube the same as a light bulb with the knowledge that the power source for that tube is not necessarily without power even when the amp is unplugged. So, pull on the base, and don't touch the pins while they are still in the socket.


Here, read this

http://en.wikipedia.org/wiki/Capacitor#Hazards_and_safety

In tube amps, you have a power transformer putting out between 225 and 500 volts AC being rectified and filtered and supplied to the plates of the tubes. Yes, when powered up, that will give you a heck of a shock. When powered down, the filtering capacitors will STILL HAVE that power supply voltage stored in them, anywhere from 225 to 500 volts easily. There is more to it than this yes, but that gets into maths. So, it is something to not be terrified of, but very respectful of.


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## GuitarsCanada (Dec 30, 2005)

If all you want to do is change the tubes, there is no danger there. As long as you dont open up the chassis, which is where all the nasty little hidden dangers are, the exterior was basically built with the untrained in mind.


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## GuitarG. (Apr 1, 2010)

Discharging caps is not difficult. There are a few ways of doing so, but I would advise on having someone show you.


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## GuyB (May 2, 2008)

Thanks a lot to everybody. You have giving me the answers I was looking for.


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## zontar (Oct 25, 2007)

And when I think of opening up the back of my old tube amp to poke around and see what it looked like--good thing the only things I ever did were put a loose tube back in, and hide stuff in the back (which I removed if I turned the amp on, and it wasn't anything illegal--just stuff I wanted to keep away from my brother.

Most of my amps are transistor, and I have no reason to open them up.
So I should be safe.


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## parkhead (Aug 14, 2009)

an amps power supply uses the same mechanism as heart shock machine, electric cattle prod or photoflash to reserve power for instant delivery on demand 

if you poke the wrong spot with your finger you can get the full cap discharge at once... 

in most cases it just makes you swear(evil words you never though you'd hear yourself say at full volume) ... possibly drop the amp, knock it off the bench 

fun stuff, ususally you get zinged taking a heavy amp out of its cabinet "jockeying for position" before you even get it on the bench to discharge the caps 

P


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## washburned (Oct 13, 2006)

So, a question.....has no one heard of draining a capacitor charge by disconnecting the amp from the power supply while it is still on, i.e. pull the plug? Or is this an unacceptable practice for some reason?


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## keeperofthegood (Apr 30, 2008)

washburned said:


> So, a question.....has no one heard of draining a capacitor charge by disconnecting the amp from the power supply while it is still on, i.e. pull the plug? Or is this an unacceptable practice for some reason?



Yes, that is valid suggestion. As long as the emission lasts long enough, time constant wise, to allow for draining the capacitors.


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## Fader (Mar 10, 2009)

"The time required to charge a capacitor to 63 percent (actually 63.2 percent) of full charge or to discharge it to 37 percent (actually 36.8 percent) of its initial voltage is known as the TIME CONSTANT (TC) of the circuit."

So even when you short a cap to drain it, it will never completely discharge. Right?
5 RC should give you 99% discharge.


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