Why use electrolytic in this Fender reverb circuit?

[nuke]

I'm puzzled by why Fender uses a polarized electrolytic capacitor in the reverb circuit of the Blues Junior and other "blues" series amplifiers with solid state reverb.

They use a 0.47 µf 100v electrolytic. It doesn't need 100v obviously, but that makes it the same size as the 22 µf at 25v and the 47 µf at 16v electrolytic caps used elsewhere.

There are any number of film caps, which even in lots of 5000 are less expensive than the electrolytic in the same order qty. So, it probably isn't cost, as all the film caps also come from the same vendor they buy their electrolytic caps from. ( ICC ).

Other than, "that's how its always been done" is there any electrical purpose for choosing an electrolytic that anyone can fathom?

fender-reverb-circuit.jpg

[martin manning]

I'm betting it's cost. They are probably getting a good price from IC.

[nuke]

I'm betting it's cost. They are probably getting a good price from IC.

Maybe. But it isn't reflected in wholesale quantities (5000 units). There are 0.47 ICC films that are less expensive than the same value ICC electrolytic and Fender do seem to buy both from ICC.

I can't fathom a reason, maybe cost, since we don't know what kind of deal they get. I'd guess inertia more than any other reason, but just wondering if I'm missing something.

[Stevem]

And while it may be cost effective for them, it’s not good to run a electrolytic that far under its rated voltage because they will not stay formed and there life will be shortened by some 25%.

[martin manning]

I'd guess inertia more than any other reason, but just wondering if I'm missing something.

Good point.

[nuke]

And while it may be cost effective for them, it’s not good to run a electrolytic that far under its rated voltage because they will not stay formed and there life will be shortened by some 25%.

Yeah, exactly, which is why I'm wondering about it.

I'd think anything would work, except for perhaps certain types of ceramic that exhibit microphonic effects. It is installed in a loud guitar amp.

[TUBEDUDE]

Cost is king. In the battle of degraded lifespan vs. expense the bean counters usually win.

[Astronomicum]

it’s not good to run a electrolytic that far under its rated voltage because they will not stay formed and there life will be shortened by some 25%.

If you could direct me to some reference material that describes this effect, I would like to learn more about it. I have read a few papers on derating describing how it increases capacitor life, but have been unable to find that there is a point where derating goes too far and actually decreases "life".

[R.G.]

I would have to dig for an on-line resource, but it's down to the electrochemistry.

Electro caps are made by using electricity and chemical environment to force-grow a layer of aluminum oxide to completely cover the underlying aluminum. The conditions are such that current flow in thinner spots builds that spot faster, evening out the insulation thickness. When the insulation gets thick enough, determined by the voltage and leakage currents, the cap voltage is big enough, and the cap is wound and sold.

The oxide layer is not stable. It slowly un-forms over time. Using it at a voltage near its target voltage goes a long way towards leakage currents re-forming leaky spots. Sitting on a shelf with no bias voltage, it un-forms faster than when it's in use. The re-forming in use is not as good as when the cap is made because the electrolyte in use is not the special forming chemical bath when it was made; the chemistry is a bit different. Eventually, an un-formed spot develops that goes into local thermal runaway that's more destructive than re-formative, and the cap heats up, encouraging more hot spots, and it eventually can't hold off the working voltage. Time for a new cap.

Using a cap way below its voltage probably limits its life to near its shelf life, which may be five to ten years, but the actual time is only guessable.

I personally object to that electro's use there because it's not fed a polarizing voltage at all. Electros NEED a DC voltage across them to keep the re-forming stuff happening. A tiny AC voltage across an aluminum electro is maybe OK, but aluminum/aluminum oxide also makes a crude oxide diode with a forward conduction voltage of 1-2 volts. Any reverse voltage at all increases the un-forming effect, but more than the diode voltage will make it go hugely faster, depending on the current. They can die in seconds. The received knowledge is to never, ever run a polarized aluminum electro without a DC bias.

Er, unless you want it to die after a while. A possible, evil motive for running a electro with no DC bias is to have it fail one shelf life later. That's probably well past the warranty. Just saying. But then as Napoleon said, never ascribe to malice what can be explained by incompetence.

[Stevem]

RG. So what’s the deal with non polarized electrolytics ?

[nuke]

Ditto what R.G. wrote.

Heck, C18, the 22u @ 25v on the left side of the schematic on the drive side of the op-amp is in the same sort of application, but they need the bulk capacitance there, so kind of understand why.

But an 0.47uf is easily and preferentially replaced with a film. Or a ceramic.

Electrolytic caps will breakdown just sitting in the drawer. In fact some capacitor manufacturers offer re-processing services to large customers who want to use inventory that's been aging on the shelf. (you'd have be talking many thousands of valuable capacitors for such services).

The presence of DC across the electrolytic capacitors near their rated voltage, below their operating temperature, with little AC current flow, maximizes the life the capacitor.

The no-bias, with AC is the worst application for a polarized electrolytic.

I can't imagine why Fender still (or ever) use this part in the amps, they're actually more expensive than same value film caps, even in the thousands qty.

I wondered if they were relying on the leakage current or parasitics of the electrolytic or some other property I'm not aware of.

I have new/old-stock Sprague Atoms, made in USA, from over 20 years ago in my parts bin. I have found that they're usually quite OK. The (original) Atom line was designed for repair shops, with very long shelf life. I have pulled them out and set them up to reform on a power supply at full rated voltage. I've yet to find one that doesn't meet the limit of a new part, even after 20 years of controlled storage. But letting them "re-form" for a day will usually reduce the leakage current a little bit.

I would have to dig for an on-line resource, but it's down to the electrochemistry.

Electro caps are made by using electricity and chemical environment to force-grow a layer of aluminum oxide to completely cover the underlying aluminum. The conditions are such that current flow in thinner spots builds that spot faster, evening out the insulation thickness. When the insulation gets thick enough, determined by the voltage and leakage currents, the cap voltage is big enough, and the cap is wound and sold.

The oxide layer is not stable. It slowly un-forms over time. Using it at a voltage near its target voltage goes a long way towards leakage currents re-forming leaky spots. Sitting on a shelf with no bias voltage, it un-forms faster than when it's in use. The re-forming in use is not as good as when the cap is made because the electrolyte in use is not the special forming chemical bath when it was made; the chemistry is a bit different. Eventually, an un-formed spot develops that goes into local thermal runaway that's more destructive than re-formative, and the cap heats up, encouraging more hot spots, and it eventually can't hold off the working voltage. Time for a new cap.

Using a cap way below its voltage probably limits its life to near its shelf life, which may be five to ten years, but the actual time is only guessable.

I personally object to that electro's use there because it's not fed a polarizing voltage at all. Electros NEED a DC voltage across them to keep the re-forming stuff happening. A tiny AC voltage across an aluminum electro is maybe OK, but aluminum/aluminum oxide also makes a crude oxide diode with a forward conduction voltage of 1-2 volts. Any reverse voltage at all increases the un-forming effect, but more than the diode voltage will make it go hugely faster, depending on the current. They can die in seconds. The received knowledge is to never, ever run a polarized aluminum electro without a DC bias.

Er, unless you want it to die after a while. A possible, evil motive for running a electro with no DC bias is to have it fail one shelf life later. That's probably well past the warranty. Just saying. But then as Napoleon said, never ascribe to malice what can be explained by incompetence.

[R.G.]

RG. So what’s the deal with non polarized electrolytics ?

Polar electros oxidize one of two foils, the other being left bare. The bare aluminum foil serves only to make good contact to all of the electrolyte, which is the actual conductor on one side of the oxide. The oxidized foil is the other conductor, and the oxide layer is the insulation.

For non-polar, they oxidize both foils. That way no matter which way the voltage is, one foil is protected by a properly-biased oxide layer, and the other is running right at the edge of conduction. With an alternating signal, neither oxide layer gets abused very much.

Ideally, non polar should not be used with a DC bias, as the smallish current through the "incorrect" oxide layer lets it degrade, and eventually you have a (mostly) polarized cap. But they will last a long time anyway.

We can fake the both-sides-polarized by hooking two polarized ones back to back. You get simuiar electrochemical action, but use two caps to do it. The oxide insulated layer is the nominally positive one in a polarized cap, so hooking the two negative leads together is a near approximation of an NP.

[LOUDthud]

Not mentioned so far... If an engineer wants to use a part that isn't in Fender's part catalog, there is a mountain of paperwork to fill out.

[R.G.]

Good one, L.T.!!!
Difficulties for the designer when the project is due tomorrow is a powerful incentive to do >>something<< now.

[nuke]

Not mentioned so far... If an engineer wants to use a part that isn't in Fender's part catalog, there is a mountain of paperwork to fill out.

lol, yeah inertia.

Probably explains all the axial electrolytic caps they are still using, where radials are available from many vendors, much cheaper and would easily fit the space available.