Bad Caps..What are the symptoms?

Darrell Hendrix · Post by **Darrell Hendrix** » 20 Apr 2004 3:35 pm

Heres one for all you electronics folks out there. How can you tell if the capacitors that filter the power supply on your amp have gone bad or need to be replaced? I'm talking about the big ones that are wired to the power transformer. Thanks!!

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I would almost give anything to play just a little like John Hughey !

Ken Fox · Post by **Ken Fox** » 20 Apr 2004 4:50 pm

The caps serve several purposes. Typically in a Fender amp the two large ones are for primary filtering and will typically allow a lot of hum when they go bad. But not always! They can also become a resistive path or direct short for DC, load the power supply down and even blow fuses (I had one in a Twin recently do that). They can also cause distorion, had that in a Session 400. In a Fender tube amp there are typical 2 to 4 more caps under the can. They are for decoupling the audio stages and providing a bit more filtering. They typically are on the supply lines to the plate load resistors in the various preamp stages. If one opens up, you will get "motorboating" sounds or feedback. I just had a Fender Super 60 with that problem in the shop. As the old caps become resistive they provide a path for DC, so looks for burned resistors.

The other caps in the amp(not electrolytics)are for interstage coupling of the audio or tone circuits and should never pass DC! If they leak DC the next stage will be biased positive and cause audible, mushy distortion (non-symetrical clipping). In thj tone section you will get scratchy pots that you can't clean[This message was edited by Ken Fox on 20 April 2004 at 05:54 PM.]

C Dixon · Post by **C Dixon** » 20 Apr 2004 5:18 pm

Ken is correct.

The large caps are "eletrolytic capacitors". Unlike regular paper and mylar, or creamic, caps, they rarely short. In most cases they either open entirely or loose capacity (partially open). When they open, like Ken says, they can cause hum or they can cause coupling between stages that creates distortion, feed back or other similar problems.

carl

Ken Fox · Post by **Ken Fox** » 20 Apr 2004 6:16 pm

You are right Carl, they rarely short out. That Twin was a first for me! It had a 20uf cap shorted (one the of the B+ filters for a preamp stage). Usually they just get resistive, load down the power supply and caused other audible problems.
I must confess I am still learning every day about amp repair. I started into in about 5 years ago as an active hobby. I have been involved in electronics for years as a limited energy electrician and technician (commercial sound, fire alarm, nurse call, video, intercom, etc) and now working as an estimator and NICET certified Fire Alarm Designer. All that has helped me, but hands and the Fender and Steel Forums have helped me the most.
I learn a lot from all you guys on the Forum. If I every lead someone wrong here, please let me know! There are a lot of guys with a lot more knowledge on this Forum tham I have for sure! However, I do enjoy trying to help where I can, as this Forums gives so much back to me.[This message was edited by Ken Fox on 20 April 2004 at 07:18 PM.]

Greg Derksen · Post by **Greg Derksen** » 21 Apr 2004 5:39 am

Ken, what kind of Caps or Brands would you suggest for older Tweed amps? I've Heard
some Recapped Tweed's that lost some of the Mojo after being re capped? Mustard Caps?
Greg

Ken Fox · Post by **Ken Fox** » 21 Apr 2004 6:08 am

I really do not have a good answer for you on that! I use Sprague Orange Drops and Atom caps. A lot of amps change tone after a cap job, especially the cathode bypass caps. The olds ones have usually drifted very high in value, allowing more bass to be amplified. After a full electrolytic cap job a change in low end is usually noticed. A lot of thing drift in vaslue, resistors especially. All this adds to the unique sound of an older amp. Of course, it did not sound that way when new! [This message was edited by Ken Fox on 21 April 2004 at 07:08 AM.]

Ray Minich · Post by **Ray Minich** » 21 Apr 2004 6:31 am

<BLOCKQUOTE>quote:<HR> I'm talking about the big ones that are wired to the power transformer. Thanks!!
<HR></BLOCKQUOTE>

Hopefully you are talking about the big ones that are wired to the load side of the rectifier (but may be physically near the transformer on the chassis).

There is a case where a capacitor may be wired to the transformer, it's rare but it happens. I have a 28 volt center tapped 18 amp transformer on my shelf here that has a separate winding that goes only across a big electrolytic cap. Why?, to hold the core in saturation. It's another way of performining output voltage regulation. This xfmr came out of a golf cart battery charger, I use it and a 50 amp bridge to reform caps and nicad batteries.

Jay Fagerlie · Post by **Jay Fagerlie** » 21 Apr 2004 6:47 am

Darrell,
quote"How can you tell if the capacitors that filter the power supply on your amp have gone bad or need to be replaced? "

To answer this question directly, a sure-fire way to tell if your filter caps are in need of attention, play the lowest Ab note you can (on a standard 6-string, that would be the fattest string at the 4th fret), and slowly bend it up to an A note while listening carefully. If there is any unfiltered hum, it will show up as a 'beat' frequency, like when you're tuning. It just so happens that a harmonic of the power supply frequency is between these two notes. I've used this method for years to check filtering....quick, easy, and no test equipment involved!
I hope this helps,
Jay

Bill Ford · Post by **Bill Ford** » 21 Apr 2004 7:13 am

I had a Session500 that sounded like there was a distortion unit on at certain cords/positions, turned out to be bad caps in the preamp.

BF

Lee Baucum · Post by **Lee Baucum** » 21 Apr 2004 8:43 am

Jay - I do the same thing, only I use the low G#.

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Lee, from South Texas
Down On The Rio Grande

Jay Fagerlie · Post by **Jay Fagerlie** » 22 Apr 2004 5:43 am

Lee, you must be using ET......
I use JI, so it has to be an Ab.

J