Just tried dischargind the filter caps

[antispatula]

On my Fender Twin Reverb. This was the first time I have tried discharging any caps that actaully could potentially have high voltages in them.

So I turn off the amp and unplug it. I wait 2 days, just to be safer. Then I put on thick rubber gloves and take off the pan on the bottom of the chasis to expose the filter caps. There are 2 caps that have a rating of 220 mf. They were pretty darn big. I very carefully touch the leads (I'm holding the leads with insulated plyers too.....it was my first time, I was scared ok?!) to the 2 terminals of one of the big caps and I get a reading....of .3 volts? Wow, THAT was dissapointing. So then I plug in the amp and play it for 20 seconds, then turn it all off and carefuly measure one of the big caps again, and the reading is only 4 volts....This was measured less than a minute after the power was turned off......So what's up with that? I did notice some fat resistors near the big caps, are these used to somehow bleed the caps so they drain instantly? Any insight would be good.....

Oh, and the reason I wanted to drain the caps was so I could open the chassis and clean the pots, but I couldn't even find a way to get the chassis off the amps body, is there some trick to these twin reverbs?!

 

[soundchaser59]

If you find a way to make a cap that can hold 150+ volts for more than a few minutes after you turn it off, you could get rich quick!

 

[antispatula]

If you find a way to make a cap that can hold 150+ volts for more than a few minutes after you turn it off, you could get rich quick!

So why are people always like IF YOU TOUCH A FILTER CAP, you DIE no matter WHAT!!!!!! EVEN IF IT'S BEEN TURNED OFF FOR A TRILLION YEARS!

So fat caps can't really hold much of a charge after a couple minutes? That makes me feel a little more safe......

 

[mshilarious]

I did notice some fat resistors near the big caps, are these used to somehow bleed the caps so they drain instantly? Any insight would be good.....

Very possibly. Any modern design should have such resistors, and if it's really clever, a LED to indicate that the voltage has dropped (OK, I just really like LEDs ). Even so, the voltage drop is not instant, it depends on the resistor values (also the loads from the rest of the circuit when off), the lower the value, the faster the voltage drops, but the power dissipated must remain within the rating of the resistors. I think your caps are probably 220uF rather than mF, they should take maybe just about a minute to discharge to 1%.

The biggest danger from working with tube amps is that extremely high voltages are present when the amp is on. The caps are a secondary danger. Better safe than sorry.

 

[antispatula]

Very possibly. Any modern design should have such resistors, and if it's really clever, a LED to indicate that the voltage has dropped (OK, I just really like LEDs ). Even so, the voltage drop is not instant, it depends on the resistor values (also the loads from the rest of the circuit when off), the lower the value, the faster the voltage drops, but the power dissipated must remain within the rating of the resistors. I think your caps are probably 220uF rather than mF, they should take maybe just about a minute to discharge to 1%.

The biggest danger from working with tube amps is that extremely high voltages are present when the amp is on. The caps are a secondary danger. Better safe than sorry.

hey thanks a lot for the insight man, I appreciate the info. Yeah, by mf I meant micro farads, I should have just said uf. What does mf mean then?

Yeah, I've got an original fender twin reverb. And luckily, I think the bias pot is conviniently out of the chasis right next to the "on" switch, making the chance of me killing myself go way down!

 

[mshilarious]

hey thanks a lot for the insight man, I appreciate the info. Yeah, by mf I meant micro farads, I should have just said uf. What does mf mean then?

mF is millifarads, or 1000 uF. "u" is used as a substitute for the greek letter "mu", which is a pain to type . . . it looks like this: μF

 

[boingoman]

So fat caps can't really hold much of a charge after a couple minutes? That makes me feel a little more safe......

That's so wrong, don't be an idiot, check them every time. Even if you go to lunch and come back a few hours later. Capacitors store energy. That is their basic function. CAPACITORS have CAPACITANCE which is their CAPACITY to store energy. Not only can high-voltage caps hold a dangerous charge for a time ranging from days to years, certain types will recharge themselves to a sizable fraction of their original charge over time. I have seen filter caps in amps with no bleeders or out of the amp recharge to almost 100 volts overnight, with the amp unplugged. People usually store discharged high-voltage caps with the leads shorted to prevent this. Caps don't discharge themselves very quickly unless they are leaky.

Check the caps. Every time. What if one of your bleeders opened up and isn't working anymore, for instance?

Amps can kill you. Plugged in or not. After reading this thread, it seems to me reading things on the net can kill you too.

Sorry, the misinformation given in this thread so far fucking pisses me off, as it could end up with you dead or injured from false confidence.

 

[punkin]

While your rant was a bit stressed, I completely agree. A smart design might include a bleed off circuit but that would add to the over-all cost of the amp and well, simply doesn't make the amp any better or more valuable. In today's world I wouldn't bet on a "smart" design but rather a cost efficient one. Gambling on it could burn you...literally. Always do the safe thing when probing around in amps....discharge the caps. If you don't have a strong background in electronics...just stay out.

BTW...it's not the volts that will stop the organic pumper...it's the current.

 

[boingoman]

While your rant was a bit stressed.

Just a bit.

 

[Thurgood]

Bump to Boingoman. Stress or not, always re-bleed and check. Funny story: worked as teenager with an old dude/old school E-Tech (read all old tube amps in shop.) Well, being the inattentive teenager I was working on an Altec tube amp that i did not drain. That in it self was stupid, but I was wearing a silver cross attached via a silver chain around my neck. I leaned over and guess what.??????? A huge arc resulted, melting the cross, burning my neck, dropping me to the floor unconscious..... but alive. Lesson learned. Being alarmist about the potentially high voltage and the attendant dangers of working on tube amps is a lesson that must be emphsized to all the growing number if DIY'ers and tube amp enthusiastes (does that rhyme with testes?) Be safe, be aware and check. Out.

 

[soundchaser59]

Capacitors store energy. That is their basic function. CAPACITORS have CAPACITANCE which is their CAPACITY to store energy. Not only can high-voltage caps hold a dangerous charge for a time ranging from days to years, certain types will recharge themselves to a sizable fraction of their original charge over time. I have seen filter caps in amps with no bleeders or out of the amp recharge to almost 100 volts overnight, with the amp unplugged.

....Sorry, the misinformation given in this thread so far fucking pisses me off, as it could end up with you dead or injured from false confidence.

You actually believe a capacitor that is not connected to a circuit ("out of the amp" as you put it) can recharge itself??

If that were possible, somebody would be a heckuva lot richer than Warren Buffet or Bill Gates by now!

 

[boingoman]

You actually believe a capacitor that is not connected to a circuit ("out of the amp" as you put it) can recharge itself??

To a percentage of the original charge, yes. From Wikipedia:

"High-voltage capacitors should be stored with the terminals shorted, since temporarily discharged capacitors can develop potentially dangerous voltages when the terminals are left open-circuited."

You should read this before giving anyone more of your "insights". Maybe the whole page.

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

 

[punkin]

Tell ya this...I'm gettin' a charge out of this thread, that's for sure. Maybe we should just cool down a bit.

Got a lot of smart folks here and some with electronic backgrounds and some without. Going back into my "wayback machine", I was taught that very large (VERY large) caps, can pick up a light charge from being improperly stored (where one of the plates has opportunity to come in contact with other materials presenting a difference in potential can in fact lightly charge a large cap), since we're still talking 'bout big ass ol' caps here, just a slight charge might be enough to kick your ass or give ya a light burn. Old school (that's me again) reminds me of recieving and unpacking these afore mentioned "big ass 'ol" caps and finding a shorting strap connected between the leads. So, I suppose, there's atleast a possibility that there may be something to this but, I have no imperical evidence of this ever happening. It might make for some fun and educational research.

If we remain as kids playing nicely with others and just focus on the safety aspect of tweakin' our equipment, those that don't know for certain what they're doing probably shouldn't be doing "it".

Peace my bruthas

 

[ggunn]

To a percentage of the original charge, yes. From Wikipedia:

"High-voltage capacitors should be stored with the terminals shorted, since temporarily discharged capacitors can develop potentially dangerous voltages when the terminals are left open-circuited."

You should read this before giving anyone more of your "insights". Maybe the whole page.

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

A capacitor in an amplifier is not open-circuited. What wikipedia is referring to is the fact that a cap can accumulate charge over time from ambient electric fields, but in order to do that, it cannot be connected to any possible leakage paths. This is not true of a capacitor installed in an amplifier.

 

[jpw23]

you should play with the caps in an a/c system....last one I changed was a 440v 80uf ....packs quite a punch!!!! BTW....bleed resistor or not, I always short out the terminals before touching!!!

 

[soundchaser59]

I could remove every capacitor from every amp I have, then let them sit for ten years, and when I come back none of them will have "recharged themselves". I've built a lot of kits, and none has ever come with any caps strapped.

we were talking about caps taken from guitar amps..... not from public power stations....

paranoia over-ruling common sense......

 

[jimistone]

That's so wrong, don't be an idiot, check them every time. Even if you go to lunch and come back a few hours later. Capacitors store energy. That is their basic function. CAPACITORS have CAPACITANCE which is their CAPACITY to store energy. Not only can high-voltage caps hold a dangerous charge for a time ranging from days to years, certain types will recharge themselves to a sizable fraction of their original charge over time. I have seen filter caps in amps with no bleeders or out of the amp recharge to almost 100 volts overnight, with the amp unplugged. People usually store discharged high-voltage caps with the leads shorted to prevent this. Caps don't discharge themselves very quickly unless they are leaky.

Check the caps. Every time. What if one of your bleeders opened up and isn't working anymore, for instance?

Amps can kill you. Plugged in or not. After reading this thread, it seems to me reading things on the net can kill you too.

Sorry, the misinformation given in this thread so far fucking pisses me off, as it could end up with you dead or injured from false confidence.

I heard you can tell if the caps have a charge by sticking your tongue to both leads....kind of like checking a 9 volt battery.

Has anyone tried this?

 

[boingoman]

I heard you can tell if the caps have a charge by sticking your tongue to both leads....kind of like checking a 9 volt battery.

Has anyone tried this?

Yeth. I have tried thith. It hurth.

 

[boingoman]

I could remove every capacitor from every amp I have, then let them sit for ten years, and when I come back none of them will have "recharged themselves". I've built a lot of kits, and none has ever come with any caps strapped.

Those are new. They've never been charged.

 

[boingoman]

A capacitor in an amplifier is not open-circuited. What wikipedia is referring to is the fact that a cap can accumulate charge over time from ambient electric fields, but in order to do that, it cannot be connected to any possible leakage paths. This is not true of a capacitor installed in an amplifier.

Are you sure your amp is in working order? What if a cap lead is broken, or you are replacing them, or any of a number of other scenarios?

Ambient electric fields? Like if you place it near a generator or a lightning strike?

No. Charge remains in the dielectric after the cap is discharged. The phenomena is called dielectric absorption. Over time, the charge in the dielectric migrates to the poles of the cap, creating a voltage potential across the terminals again.

In an amp with the tubes pulled, there is no connection between the terminals of the filter caps, ie, no leakage path. And cold tubes have incredibly high impedance.