General tech questions

Capacitor failure modes


I need a refresher...

If a capacitor were to eventually fail, in any of many ways, what would be the observations?

Both for an Amp Filter box and Tone Controls, and for a Guitar Tone an old amp or old guitar will succomb.

Dry Out?

Higher or lower Capacitance and/or Resistance/Impedance?

Faster loss of Highs or All Highs no Bass?

Open Circuit? Same question...

Short Circuif? Etc...

How likely is it there would be intermittent, sometimes good sometimes questionable conditions?

Time for me to get ahead of the curve on some of my gear...


When a big electrolytic blows it sounds like a rifle going off. I've had them pop in xenon projector lamp power supplies. Smaller caps fizzle out.


In less dramatic cases, old dried out/leaky filter caps will cause a steady 120 cycle hum regardless of volume level.

Leaky coupling caps can cause lower volume/higher unpleasant distortion.


I understand the Amp test to be the Bb at the 15th fret on the G string...anything but a pure, clear, solid tone is a telltale.

Now, for guitar Tone Pots it gets squirrelly...

Depending on the 2 pickup, 2 Volume and Tone Pot configuration, the wiring like this Kay I bought, which is '59, can add nuances. I did easily, without anything but a screwdriver, switch Pickup positions such that my stronger pickup is at the bridge. Who knows what or might have been previously reconfigured in a 57yo guitar, but I believe everything to be all there and original. Pot movements, at some point on the dial, suddenly become rapid either on the Volume or Tone. Clearly different than what I am used to for sensitivity. And the pickup location switch doesn't matter. Just sensitive.

But now she seems to be perfect...Bridge pickup holding its own and louder for Volume whereas before the switch the Neck was dominant. Using the controls only darkens the Tone. I have to figure Audio Taper pots with 50's wiring, but now Pickup position is much better.

I am trying to, without a teardown, gauge the effects of age on the caps and resistors. I think it to be good that darker Tones occur from movement. Wide open everything is very bright P-35 single coil Tone.

Just trying to equate age, knowing I have this and other guitars creeping forward into mid-life crises!


I have a 1997 Fender B bender Telecaster with the delta-tone system. Delta-tone bypasses the capacitor and the guitar sounds outstanding without the capacitor in the circuit.


Old tone and coupling caps often leak DC this makes things go haywire in amps,It's not a positive thing, and they're capacitance values drift making them essentially different valued caps. If you want the amp/guitar you are playing to sound like it was designed to sound and it has ancient past-it caps in it, recap it. There is no advantage to worn out parts. On a guitar tone circuit it's a lot less crucial but it should ideally be within spec unless you really like the sound as is. Volume controls usually 'darken' your sound . There's a way you can use a bypass resitor that will allow some treble to bleed through as you turn down the volume keeping the tone the same.


There are a few schools of thought on re-capping amps. Some shops will look at a 50 year old amp and recommend a global cap job. Replace everything. Other guys, like Skip Simmons will go thru an amp and replace only the leaky caps. If it ain't broke, leave it alone and enjoy it's glory. Of course there is no advantage to worn out parts, but some capacitors have great longevity. Melted wax impregnated caps, no brainer. Toss 'em. Molded caps like Astrons, Ajax and the brown and blue blobs in BF Fenders have pretty good staying power. And I've yet to come across failed ceramic disc caps like are found in 60's Valcos. Sleuthing out the bad cap is much more time consuming.

Everyone seems intent on replacing old electrolytics. "If they're 50 years old, they must be bad". I have a bunch of amps from the 60's that have their original Mallory cap can filters and they're dead quiet. Just bring them up on a Variac when waking them from years of silence. If it hums, replace them. I find a lot more resistors out of spec than I do caps.


What I have found out thus far makes me feel reasonably good about the Kay for now. When the day comes to get inside, it will be refreshed. The tech who had the guitar apart for review before I bought it felt everything was fine, better to leave it original. A second tech and I reviewed it as well after, without teardown. At that point, having the stronger neck position was most noticeable.

I am reluctant to keep popping off these plastic pot knobs for fear of cracking them...needs to be good cause.

I have to figure paper-in-oil caps and carbon comp resistors.


Pick a catagory:
Filter caps & other electrolytics, tone control circuits, coupling caps, Tremolo caps etc. I'm already assuming you're concerned with caps in guitar amps or guitars.



I'm just trying to manage my herd as it ages. Having real life experiences from others as things wear out is the goal. If I lived where I could just drop something off for a checkup by someone who works through these issues every day it would be a no brainer. Like in NYC...

The Kay '59 K1700 is probably my first concern, although at the moment after swapping pickup positions it seems perfect. Can these older caps and resistors drift so much to cause second-guessing?

Thus far, I think things are still good, but...time, temperature, and oxygen continues to age everything and anything.


Guitar caps? Just play the guitar. If it keeps you up at night, replace them all. It won't make any difference, but it's only a few dollars worth of parts. Since there's no voltage in a guitar, resistors will drift if anything is going to change. Crumbling wire can be more of a problem. Inside a guitar, caps, resistors, or pots make absolutely no difference in tone.


Unless you have the old BumbleBee caps in which case you should just throw them out.


Bahaha Billy Zoom to the rescue! People love those old bumblebee caps in their guitar tone cicuits, yet the sound they are chasing was produced when the caps were relatively new in relatively new guitars. They're notoriously leaky and they drift in value.

People recommend replacing the electrolytics caps in amps because when old electrolytics die they can take down more expensive harder to replace parts. Replacing them is insurance against a burnt transformer down the road. New ones are cheap and work fine.


No filter caps inside a guitar. In electronics school back in the 60's, they taught us to just throw away any BumbleBee caps whenever we found them. That was when they were still pretty new.


Yes no electrolytics in guitars, usually just one cap in the tone circuit. I think the fellow was referring to amps as well initially. That's hilarious about turfing out the bumblebee caps when they were relatively new, especially considering the current mania surrounding them.


Replace electrolytics and power tubes every twenty years, whether they need it or not.


I remember being in a guitar store when the guy behind the counter was inspecting a lovely old '61 Strat. He had the strings off and was about to check under the scratchplate for pot codes when some guy comes up and tells him not to blow the dust out when he opens the guitar. What?? He insisted that the dust inside the guitar was original from the factory and a valuable part of the guitar.

I really think some folks take this stuff too far.


Now I'm worrying -- still got the original caps in my 1969 Traynor Bassmaster head. At least the power tubes are only eight years old so I'm good to go for years with those -- in fact I'll probably conk out before they do.

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