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Discussion Starter · #1 ·
TL;DR - Bring your amp to temp, shut down without using the standby switch to safely discharge your caps.


We all know tube amps kill millions of people every year and mere mortals should avoid them like the plague. For those of us willing to live on the edge and experience the glory of tube greatness I found a new take on discharging the filter caps prior to maintaining or modifying an amp.

My previous go to video was this one prepared by Premier Guitar;


While researching mods to my Blues Junior I stumbled across the late BillM's (cancer not electrocution) explanation and procedure for discharging caps.


Any amp gurus care to comment?
 

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I wouldn't just rely on bringing the amp up to temp and turning off without flipping the standby. While that may work for some amps it wont work for all. Not all amps have bleeders on the caps.

The safest way to deal with caps is to make a discharging tool. Take a 10 watt resistor thats between 5k-15k and an alligator clip test lead. cut the test lead in half and solder each end to the ends of the resistor you have. wrap it up with electrical tape/insulated heat shrink tubing. Now to discharge your caps clip one end of the resistor/wire tool and clip it to the metal chassis. then clip the other end to the caps you wish to discharge. the resistor will slow the rate of discharge so you wont get a big spark and youll caps will be discharged within 30 seconds or so.

As well, its always best to check with a meter before you start touching anything.
 

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Good info. RobRob has a good page on this as well:

Safety

I once managed to somehow blast myself by touching a cap in a little digital camera that ran off a couple AAA batteries. Couldn't believe how powerful it was. That put some major context on the idea of goofing with a tube amp without taking proper precautions.
 

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I'm sure many of us have had close calls while working on tube amps. ironically we're the lucky the ones. the unlucky ones aren't around to post about it anymore.
 

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One of the gnarlier close calls I had was from wearing a button down shirt with metal buttons. I should see if i still have it. the button on my right sleeve came into contact with a standby switch. Fortunately it the button took the zap and not me
 

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I don't remember the time I touched a cap in a tube amp, but I remember the look on my friend's face just after!
The amp I was working on got thrown across the room from the force of my knees hitting the workbench, then a hail of tools hitting the floor. Luckily I wasn't grounded!

I now use a few methods of discharge the caps, and every relevant cap is checked via multimeter.
 

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I don't remember the time I touched a cap in a tube amp, but I remember the look on my friend's face just after!
The amp I was working on got thrown across the room from the force of my knees hitting the workbench, then a hail of tools hitting the floor. Luckily I wasn't grounded!

I now use a few methods of discharge the caps, and every relevant cap is checked via multimeter.
now thats not a bad idea. having a spotter! ;)
 

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Discussion Starter · #9 ·
Ok Question. If the amp is unplugged and you're sending the residual voltage to ground where does it go? The amp is unplugged so the 350 volts have to go somewhere. Is it just absorbed by the circuitry?
 

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Ok Question. If the amp is unplugged and you're sending the residual voltage to ground where does it go? The amp is unplugged so the 350 volts have to go somewhere. Is it just absorbed by the circuitry?
I was wondering that as well & thinking it might be a good idea to wire up an extension cord/outlet box with only the ground connected, then plug in to that to discharge. I guess if that's not necessary then that would be pointless, but you'd figure the voltage would have to go somewhere.

???
 

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In 1840 James Joule discovered what is universally known today as Joule's Law: E = I2R where E is the rate at which electric energy is dissipated when an electric current passes through a resistance. He was working on a Fender Super reverb at the time.
 

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I wouldn't just rely on bringing the amp up to temp and turning off without flipping the standby. While that may work for some amps it wont work for all. Not all amps have bleeders on the caps.

The safest way to deal with caps is to make a discharging tool. Take a 10 watt resistor thats between 5k-15k and an alligator clip test lead. cut the test lead in half and solder each end to the ends of the resistor you have. wrap it up with electrical tape/insulated heat shrink tubing. Now to discharge your caps clip one end of the resistor/wire tool and clip it to the metal chassis. then clip the other end to the caps you wish to discharge. the resistor will slow the rate of discharge so you wont get a big spark and youll caps will be discharged within 30 seconds or so.

As well, its always best to check with a meter before you start touching anything.
THIS! Read it twce, then read it twice again tomorrow.

It is so easy to make a bleed tool, there are no excuses. Here's mine:

Wire Technology Electronic device Cable Electronics accessory
 

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Absolutely don't trust the amp to bleed itself. I routinely find amps, both musical instrument and hifi that are still live days after being unplugged. To be hyper critical, I wouldn't want this guy touching my stuff.
Ok Question. If the amp is unplugged and you're sending the residual voltage to ground where does it go? The amp is unplugged so the 350 volts have to go somewhere. Is it just absorbed by the circuitry?
It is dissipated as heat. As long as there is a complete circuit current will flow. Ground does not necessarily mean the third prong on your plug. It can be any reference point in a circuit. Current flows from high voltage to low, so if a cap is say floating above ground but still has a voltage across it, it can be live, and your fingers (or whatever body part you choose) can complete the circuit and current will flow. This is why you always check across the capacitors unless you know they are connected to common.
 

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In 1840 James Joule discovered what is universally known today as Joule's Law: E = I2R where E is the rate at which electric energy is dissipated when an electric current passes through a resistance. He was working on a Fender Super reverb at the time.
Nobody got this??!!...What a shame !
 

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In 1840 James Joule discovered what is universally known today as Joule's Law: E = I2R where E is the rate at which electric energy is dissipated when an electric current passes through a resistance. He was working on a Fender Super reverb at the time.
Nobody got this??!!...What a shame !
Well duh, it was obviously a Deluxe Reverb.
 

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I agree with the others, don't count on the standby switch method, double check with your meter. And caps can acquire a 'residual voltage' where they partially charge back up again on their own. Probably not to a lethal level, but (after verifying caps are drained) I always keep the standby switch in 'play' position to keep the caps at zero volts. Or you can keep your discharge tool attached, just make sure you don't forget it when you fire the amp back up again!
 

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In 1840 James Joule discovered what is universally known today as Joule's Law: E = I2R where E is the rate at which electric energy is dissipated when an electric current passes through a resistance. He was working on a Fender Super reverb at the time.
I appreciate the humour but will nitpick your response nonetheless. :D
P(power)=IsquaredR, not E. E is used interchangeably with V to signify voltage. (E for electromotive force)
In fact, E=I x R (edit: error corrected)
 
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