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How to Spot a Bad Filter Cap


badbluesplayer

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Here's a picture of a Fender Blues Deville showing a bad filter cap that's just failed. It smells bad when it starts going. You can see the electrolyte blowing out the end. It will tend to cause some 120 Hz. noise in the amp. This one had some bad 120Hz. overtones/distortion down near the low B/Bb area. You can tell the 120Hz. hum from 60 Hz. hum. Bad filter caps are associated with 120 Hz. hum.

 

Most of the filter caps in the newer Fenders are not that great anyway and this amp will sound way better once all the electrolytic caps are replaced.

 

C220A929-0397-42BF-8AE6-9A87AAEF6B7D_zpsa1dfnprz.jpg

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Nice shot of that part of the board. You can see the op-amps there too, eh?

 

Question for you? how can you tell 120hz from 60hz hum? I can't say I have ever noticed which is which, or if I could tell myself.

 

I have read that the filter caps in a lot of newer Fender amps are bad from the gate. By that I mean, without going "bad", they supposedly don't work all that well. In particular according to Dr. Weber.

 

I like to think I can hear bad filter caps, but honestly, I don't know that I could put a finger on it as to a decription. I CAN say for sure the average guy (including me) can hear the difference especially getting an amp back from the shop. "Bad" being a kinda out of tune, but very sick yucky sound that really has nothing to do with distortion or break-up.

 

Unbiased tubes or mis-matched power tubes as well. Even before saturation. Methinks.

 

I know you have, fix, and experience a lot of amps there is why I ask.

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Guest Farnsbarns

 

Question for you? how can you tell 120hz from 60hz hum? I can't say I have ever noticed which is which, or if I could tell myself.

 

It's double the frequency/an octave higher pitched.

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Question for you? how can you tell 120hz from 60hz hum? I can't say I have ever noticed which is which, or if I could tell myself.

 

I have read that the filter caps in a lot of newer Fender amps are bad from the gate. By that I mean, without going "bad", they supposedly don't work all that well. In particular according to Dr. Weber.

 

 

60 - This frequency is pretty much etched into people's brains.

http://www.youtube.com/watch?v=SVZ2P0KsLic

 

120 -

http://www.youtube.com/watch?v=pC4Uzt0qm2E

 

Gerald Weber says that all the caps in modern Fenders are NG because of insufficient capacity.

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What brand of caps are you going to use?

 

 

I'll use F&T caps, for the big filter caps anyway. They're pretty good and are about half the price of Sprague Atoms. Sprague Atoms are kind of the gold standard. I'm replacing all of the electrolytic caps, and the small ones will be american or F&T.

 

The IC's like in this amp, are Taiwanese and have a bad reputation.

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Hmmmm, still can't see the bad filter cap... Wait, which one of those components is the cap and which one is the bad one... Just want you to point it out so on my next amp build I wont install any bad caps... :blink:

 

It's the second from the right, the one with a little whitish junk coming out the end around the wire.

 

You European guys - the noise you will hear will be 100 Hz instead of 120 Hz, because it's twice the 50Hz. that your normal power runs at.

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It's the second from the right, the one with a little whitish junk coming out the end around the wire.

 

You European guys - the noise you will hear will be 100 Hz instead of 120 Hz, because it's twice the 50Hz. that your normal power runs at.

 

Yes. And it's logical: 12 Hz - 60 Hz range is the most dangerous for the human heart, in case of electrical shock. :rolleyes:

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Somebody replaced the speakers in this amp and changed them from 8 ohm speakers to 16 ohm speakers. That changed the speaker load but they didn't rewire the speaker jacks like they should have. The amp wasn't getting full power through the speakers. I'll try to rewire it and see if I can't make it nice and loud. [scared]

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Amperage kills not frequency. AC has frequency, but DC does not. In the USA we use 60 hz and a lot of the rest of the word uses 50 hz.. 1/10 of an amp across the heart for about 1 second can be fatal. That is less amperage than in a 60 watt bulb.

 

That`s true.

 

My statement was about the venticular fibrillation. It is most likely to happen when being shocked by current in that frequency range.

 

Bence

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I installed new filter caps and power supply resistors. I rewired the speaker jacks, 'cause the speaker had been replaced with difference impedance models and they didn't rewire the output tranny accordingly. [rolleyes]

 

7D3B62A0-57FB-47B6-98D3-B2AF99762783_zpsh7tgujhu.jpg6C107E1F-46B0-4FEF-AA51-4CA6E6A98E26_zpsyu9jkiuk.jpg

 

Sounds like a million bucks now. I still need to replace all the other electrolytic caps. But at least the power supply and speaker wiring problems are solved! [thumbup]

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60 - This frequency is pretty much etched into people's brains.

http://www.youtube.com/watch?v=SVZ2P0KsLic

 

120 -

http://www.youtube.com/watch?v=pC4Uzt0qm2E

 

Gerald Weber says that all the caps in modern Fenders are NG because of insufficient capacity.

Thanks for these. Very helpful.

 

Tell you why: From my experience, it isn't really a "hum" I hear that makes an amp that needs caps sound bad, it's the whole deal. That noise gets into what's being amplified, and makes the whole deal sound more "sick", I guess. Or kinda out of tune.

 

It's always been mostly a guessing game, a judgment call, as to when an amp gets into the shop and gets new caps. I am thinking if I can listen for this, it might take the guesswork out.

 

Amps with good caps sound a whole lot better than one that needs caps. I think it's on the level of what tubes are in it.

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Guest Farnsbarns

Amperage kills not frequency. AC has frequency, but DC does not. In the USA we use 60 hz and a lot of the rest of the word uses 50 hz.. 1/10 of an amp across the heart for about 1 second can be fatal. That is less amperage than in a 60 watt bulb.

 

However, Btoth76 is quite right, because our bodies have capacitance we "filter" out DC. With any capacitors, there is a nominal frequency which allows the most current to flow, that's how your tone controls work. The human capacitor likes something in the range Btoth76 suggested, making it the most dangerous.

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