What does a capacitor do?

[Bigsnake00]

What purpose do they serve in guitar amps? I have an old amp that I am trying to get working, and I am trying to do some troubleshooting before I take it to my tech. Is it possible for an amp to power up if the caps are bad?

 

[Tadpui]

A capacitor temporarily stores an electrical charge and then releases it over a period of time. These are usually used in conjuction with diodes in order to convert an alternating current into a direct current.

As far as their use in a guitar amp, you've got me. I do know that if the capacitors are bad, that an amp will malfunction and/or fail to work.

 

[bennychico11]

i'd say no. generally if anything is bad in the amp, then it won't work properly. the whole circuit board is important to the amp. one part not working can ruin the whole thing. Get a DMM and test the cap (go to radioshack). press the + and - ends of the DMM against the leads of the cap and if it charges up (or charges down...depending on what side you put the leads on)...then it will work fine. If the cap doesn't charge (up/down)...or looks like it's been blown...then it's not working

 

[mshilarious]

Capacitors have a lot of different functions in a circuit. Some are used as filters for EQ circuits, or to block DC from audio circuitry, or as mentioned above in power circuits. Whether a bad cap takes out your amp or makes it sound bad depends on what the cap is supposed to do in the circuit.

 

[Le Basseur]

Is it possible for an amp to power up if the caps are bad?

Generally speaking,yes,but depends on WHAT kind of cap you're talking about and it's function in the circuit.Also,it depends on what do you mean by "bad".

I have an old amp that I am trying to get working, and I am trying to do some troubleshooting before I take it to my tech.

Please do that NOW and don't try anything if you don't have at least some basic knowledge about tube stuff.

A capacitor temporarily stores an electrical charge and then releases it over a period of time.

...what about filtering,if you talk about HT electrolitics,or blocking DC,if you're talking non-polarised?

These are usually used in conjuction with diodes

Please define "in conjunction" !
...Oh boy,my English is weak,but you got me on that!

Get a DMM and test the cap (go to radioshack). press the + and - ends of the DMM against the leads of the cap and if it charges up (or charges down...depending on what side you put the leads on)...then it will work fine. If the cap doesn't charge (up/down)...or looks like it's been blown...then it's not working

Sorry mate,but this "test method" you're suggesting here is a less-than-basic one and is irrelevant in most cases.This method doesn't show if a electrolytic is leaky (...CURRENT leaky!) or not,doesn't reveal if it's low-ESR or not,etc...it shows almost nothing important.
With such a kind of misinformations and superficial approach,the noobie's risks are very high,from severe hi-voltage shock-hazard to a f%¤&d-up amp.
I don't intend to offend anyone,but these three posts floored me!
Please do some serious reading on these subjects (documentation is all over the Net) and use your common sense!This way,you'll know at least what are you talking about...
Good luck,
Regards,

 

[onlyfingers]

Please do some serious reading on these subjects (documentation is all over the Net) ,

Gee, Le Brasseur, Bennychico and Tadpui are on the net. I guess you meant to do some serious reading elsewhere. If documentation is all over the net, why don't you tell us where to go? Your advice would be especially appreciated because of your obvious command of the subject and quick grasp on how little we know on the subject. Please, no PhD-level texts.

 

[Larry Gude]

Bigsnake...

That's a good definition:

A capacitor temporarily stores an electrical charge and then releases it over a period of time

As you asked what they are, I only point out for safeties sake that you understand what that description means. A capacitor can very well still be charged even when your amp has been off for some time. They WILL light you up and possibly hurt you if you don't understand how they work and how to discharge them before working on them and you could get shocked down stream of the cap.

I'm no expert, so, find out what you need to know or get a tech. Capacitors can kill you.

Caution.

 

[Le Basseur]

Gee, Le Brasseur, (...). Please, no PhD-level texts.

1.Typo,it's "Le Basseur"
2.I really wouldn't dare!

Here are some useful links.You'll note that there are alot of basic info on these sites, not only "the cap stuff" but almost everything concerning tube-related guitar amps basic infos.After reading those,I'm sure you'll understand why I reacted the way I did!

http://www.geofex.com/ampdbug/ampdebug.htm

http://www.geofex.com/
(go to Tube Amp FAQ)

http://www.aikenamps.com/
(go to Tech Info)

http://www.drifteramps.com/safety.html

http://www.harmony-central.com/Guitar/guitar-amp-evolution.html

http://sommatone.com/articles.html

http://www.pritchardamps.com/pritchardamps/tech_talk/audio_myths.htm

Happy reading!
Regards,

 

[Le Basseur]

A capacitor can very well still be charged even when your amp has been off for some time.

As I said earlier,in most amps there are some bleeder resistors in the HV rail wich discharge those caps to a safe level.However,it's perfecly true what you said because the caps aren't never fully discharged (they usually retain between 18-60 VDC) but,assuming that you waited long enough,that should be harmless.
If you want complete safety,you can discharge them completely by using a lightbulb.
Cheers,

 

[steve350]

I have an E.E. degree. I could spend all day discussing the different types and ways caps are used. In an old amp, they can and will go bad. They dry up. Unless you know how to read a schematic and work a multimeter and/or scope, bring it to a good repair shop.

Here is a link that will make your head spin about caps. Have a farad day!

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

 

[Le Basseur]

Thanks Steve for helping me out!

I could spend all day discussing the different types and ways caps are used.

Doh...please,don't.... ...at least,not now!

Unless you know how to read a schematic and work a multimeter and/or scope, bring it to a good repair shop.

That was exactly my point....however,onlyfingers reacted as I'd want to humiliate someone here,wich ceartainly isn't the case!

 

[onlyfingers]

Thanks for the links. (And if you correct people's spelling on bulletin boards, you must be a busy guy.)

 

[Fendophile]

Capacitors are basically devices that consist of two plates. Between the two plates is a material called dielectric insulator. It prevents the two plates from shorting out when power is applied. If a DC voltage is applied to a capacitor, it will charge up to a certain capacity (hence the term capacitor), then current flow will cease, essentially blocking DC current from flowing past that point.

AC current will flow through a capacitor continuously. DC current will only charge up the plates to a certain capacity and then will cease.

Capacitors are used to isolate certain parts of a circuit from another in the DC current sense, while permitting AC current to pass.

Capacitors are also used as filters in the power supply, to smooth out the current from the diodes in the power supply. When a cap goes bad, quite often you will hear the effects of raw AC coming through the speaker - briefly.

Take it to a tech. It is not as simple as plug and play.

 

[steve350]

In college we use to take an electrolytic cap, put each lead into the 2 holes of an A.C. wall socket and then throw it to a classmate across the room. Talk about a hot potato!