How do you know when it's time to change electrolytic capacitors?
- Thread starter baretta
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Spinning45s
Active Member
If a capacitor or capacitors go bad, you'll likely hear a hum, a buzz, or some kind of distortion.
Otherwise, if the system sounds good, I'd say leave well enough alone.
Otherwise, if the system sounds good, I'd say leave well enough alone.
If talking about filter caps; when they fail hum will be increasing. Until then they are ok.
Decoupling caps as in cathode decoupling , when they fail (with reduced capacity) amplification will
be reduced, possibly masked by NFB. If they fail by short, dist will increase.
@baretta , it really depends on many variables, the age of the caps, the conditions they were stored in, the brand and type of cap the OEM manufacturer used, etc. If the original caps are in pretty good condition, you may not really hear any difference, but you are re-setting the aging clock to zero with fresh new caps, so reliability will be greatly improved.
dr*audio
Fish fingers and custard!
1. If I'm working on a 40+ year old piece of equipment and I have to remove a pc board to replace a bad component, I'll replace every electrolytic cap on that board because they have exceeded their expected life.
2. You can tell if caps are bad if you see electrolyte leaking from them, if the top or sides is bulging, if there is hum, or if a channel is low or has no bass.
3. If I find a bad cap in a 40+ year old piece of equipment, I'll usually replace all of them.
2. You can tell if caps are bad if you see electrolyte leaking from them, if the top or sides is bulging, if there is hum, or if a channel is low or has no bass.
3. If I find a bad cap in a 40+ year old piece of equipment, I'll usually replace all of them.
One way to tell is that the temperature the can operates at can be a very good indicator of the cap's health (or lack thereof)
tonyg88
Super Member
Usually they were using less paper and wax caps when solidstate came out so that made the caps work a little longer
My first solid state amp, a Dynaco SCA 80, erupted with a truly spectacular flame, like a miniature volcano, shooting out of one of the huge electrolytic filter caps after a couple of decades of use. The slings and arrows of outrageous fortune missed their mark that day and the entire amplifier did not erupt into a conflagration. Instead, it simply belched a cloud of toxic smoke into the room.
Electrolytic filter caps in the power supply should be replace after a couple of decades. Again, the failure mode is not guaranteed to be benign to the amplifier, the power transformer, or the rectifier tube.
Coupling caps should never be electrolytic, but whatever construction they are, they will not last for more than a few decades. A failed coupling cap can take with it an output tube and, for those subject to the helpless prey of destiny, the output transformer too.
There is a tide in the affairs of men which, taken at the flood, leads on to fortune, or if ignored, leads on to failure and painful expense.
Electrolytic filter caps in the power supply should be replace after a couple of decades. Again, the failure mode is not guaranteed to be benign to the amplifier, the power transformer, or the rectifier tube.
Coupling caps should never be electrolytic, but whatever construction they are, they will not last for more than a few decades. A failed coupling cap can take with it an output tube and, for those subject to the helpless prey of destiny, the output transformer too.
There is a tide in the affairs of men which, taken at the flood, leads on to fortune, or if ignored, leads on to failure and painful expense.
Semanon
Super Member
Caps CAN change the sound or a device. But it will happen so slowly that a listener would never notice it. And a recap might change the sound, too. Or not. I’ve done recaps where it sounded different from when I started, and others where there was no discernible change.
I change them because, an Dr*Audio notes, they have passed their use by date, and I’m in there anyway.
IMO, if you want to do a decent test of whether caps are causing any issues in your equipment without disassembling it, you need to assess the following at minimum:
1. Is there any hum at the output? This will indicate filter caps failing by developing high ESR or by losing capacitance. You can use a dedicated ESR tester for this, but they cost some money. Measuring the power supply rail directly is probably better than listening for hum at the output, but you'd have to know what "normal" ripple levels for your power supply should look like. Filter caps failing in this way won't normally cause damage but it will reduce the performance of the amplifier.
2. Is there excessive current draw through the B+ (or any other power supply) winding of the power transformer? This could indicate a filter cap with high leakage or a short fault. Caps with such a fault will probably also get hot so you can feel them as a quick and dirty test. This is a dangerous type of failure because it can draw excessive current and kill your B+ winding and/or rectifier. If your power supply has multiple rails with decoupling and dropping resistors (it probably does) then I would also check current across those and make sure it's reasonable.
3. Are all your tubes biased correctly? This can be measured either by checking the grid-cathode voltage of the tube or the plate current draw. You can also look for red plating and distortion, which can be symptoms of this issue, but you can have dangerous incorrect bias without either. Incorrectly biased tubes usually indicate a failing coupling capacitor but could also be an issue with a cathode bypass capacitor leaking/going shorted. In the case of fixed bias, it could be an issue with a bias supply capacitor. Unfortunately, there's not really any way to know what "correct" bias is unless you have a schematic with marked voltages/current draws or you have an understanding of how the tube is usually operated. Definitely one thing to check for is whether your tubes are biased well over the maximum plate dissipation spec supplied by the tube manufacturer.
4. Is gain unexpectedly low or is there distortion? Could be cathode bypass capacitors developing high ESR or losing capacitance, which doesn't cause any critical problems but will reduce gain.
This list isn't comprehensive because amps can use caps in other ways that cause failure, so as ramjet says it has to be assessed on a case by case basis. But these are the main serious capacitor failure modes I can think of in a simple cathode biased amplifier. Personally, if I checked all these things and found everything in order I'd feel comfortable running an amplifier with capacitors that are a few decades old. A simple visual inspection is not enough and some of these failures will not have an obvious impact on sound quality but can still damage your amplifier.
For anything 1970s or earlier I think it's replace on sight for tube amp capacitors. The failure rate is simply too high and the consequences too serious to leave really old caps in place. Also, recapping vintage tube gear is simple compared to recapping SS so might as well.
1. Is there any hum at the output? This will indicate filter caps failing by developing high ESR or by losing capacitance. You can use a dedicated ESR tester for this, but they cost some money. Measuring the power supply rail directly is probably better than listening for hum at the output, but you'd have to know what "normal" ripple levels for your power supply should look like. Filter caps failing in this way won't normally cause damage but it will reduce the performance of the amplifier.
2. Is there excessive current draw through the B+ (or any other power supply) winding of the power transformer? This could indicate a filter cap with high leakage or a short fault. Caps with such a fault will probably also get hot so you can feel them as a quick and dirty test. This is a dangerous type of failure because it can draw excessive current and kill your B+ winding and/or rectifier. If your power supply has multiple rails with decoupling and dropping resistors (it probably does) then I would also check current across those and make sure it's reasonable.
3. Are all your tubes biased correctly? This can be measured either by checking the grid-cathode voltage of the tube or the plate current draw. You can also look for red plating and distortion, which can be symptoms of this issue, but you can have dangerous incorrect bias without either. Incorrectly biased tubes usually indicate a failing coupling capacitor but could also be an issue with a cathode bypass capacitor leaking/going shorted. In the case of fixed bias, it could be an issue with a bias supply capacitor. Unfortunately, there's not really any way to know what "correct" bias is unless you have a schematic with marked voltages/current draws or you have an understanding of how the tube is usually operated. Definitely one thing to check for is whether your tubes are biased well over the maximum plate dissipation spec supplied by the tube manufacturer.
4. Is gain unexpectedly low or is there distortion? Could be cathode bypass capacitors developing high ESR or losing capacitance, which doesn't cause any critical problems but will reduce gain.
This list isn't comprehensive because amps can use caps in other ways that cause failure, so as ramjet says it has to be assessed on a case by case basis. But these are the main serious capacitor failure modes I can think of in a simple cathode biased amplifier. Personally, if I checked all these things and found everything in order I'd feel comfortable running an amplifier with capacitors that are a few decades old. A simple visual inspection is not enough and some of these failures will not have an obvious impact on sound quality but can still damage your amplifier.
For anything 1970s or earlier I think it's replace on sight for tube amp capacitors. The failure rate is simply too high and the consequences too serious to leave really old caps in place. Also, recapping vintage tube gear is simple compared to recapping SS so might as well.
Bill Ferris
Lunatic Member
FWIW, All things being considered, since the mid seventies, when the amplifier is capable of operating without any serious issues (no visual cap leaking, heating, excessive mains current draw while idling) on the bench during check out, I start sinewave driving whatever the type amplifier is, up to the onset of clipping, while looking at the scopes trace for any ripple as the sinewave`s waveform starts flattening, and if I see any, I deem the main filter caps are either indicating their tired, or in some cases their OEM originally selected Mfd. values were a bit too low for best, or at least, improved performance of the amplifier, in my 45+ yrs. experienced opinion.
I also follow up and measure the AC component across all other lower level stages PS decoupling caps of the amplifier with my bench Fluke meter, or the scope looking for their filtering performance quality while driving the amp with pink noise after the main caps have been replaced. to see if any significate AC signal component is visual across their low level stage`s decoupling caps, then their not doing their job properly, and will affect the SQ performance, via increased TIMD. (Transient Inter Modulation Distortion)
So, that`s how I have dealt with the cap replacement question since, though sometimes the suspect amplifier is so old, that the wholesale PS`s electrolytic caps replacement is mandatory for max reliability, and SQ performance, if that is important to the customer, in which it always is, for musician`s instrument amplifiers, even though doing so elevates the bill.
("Give me Tone, or give me Death" has been their usual mindset over the yrs. !)
I also follow up and measure the AC component across all other lower level stages PS decoupling caps of the amplifier with my bench Fluke meter, or the scope looking for their filtering performance quality while driving the amp with pink noise after the main caps have been replaced. to see if any significate AC signal component is visual across their low level stage`s decoupling caps, then their not doing their job properly, and will affect the SQ performance, via increased TIMD. (Transient Inter Modulation Distortion)
So, that`s how I have dealt with the cap replacement question since, though sometimes the suspect amplifier is so old, that the wholesale PS`s electrolytic caps replacement is mandatory for max reliability, and SQ performance, if that is important to the customer, in which it always is, for musician`s instrument amplifiers, even though doing so elevates the bill.
("Give me Tone, or give me Death" has been their usual mindset over the yrs. !)
