Do new electrolytics need to be formed?

I'm going to replace the power supply cpas in mt 18 Watter clone. The old ones, at 8 years old, seem too young to die, but it certainly sounds like bad caps (see older thread linked below).

Anyway, I just ordered to new caps from AES: a JJ 40/20/20/20 500V multisection, and a Sprague Atom 40 uF 500V. Do I need to form these new caps before using them?


http://www.tdpri.com/forum/shock-brothers-diy-amps/315431-how-old-old-modern-filter-caps.html

No, new caps do not need to be formed (not talking about unused caps laying around for years). Concerning your 8 year old electrolytics, they can dry up with heat. Even a new electrolytic cap can go bad.

It is never a bad idea to bring the voltage up slowly on a electro cap that has not been used in a few years, but very few will actually need it. Your new cap, from a reputable supplier that actually moves stock will should be fine without it.

Once read that Leo Fender never worried about forming the caps in his amps.

Neither did he worried about power tube pairing...

Just heat up your iron and go to work.

OK, as soon as the caps get here.

O.K. I don't particularly want to be the over-conservative tech and dissenting voice here - but have any of you actually "formed" a cap on a T0-6 Sprague? I have. I did about ten of them last night...including four "brand new" 100uF/450 Nichicons and four Sprague 47uF / 450's

On a T0-6 you can actually monitor the DC leakage current during the initial forming process. You can actually SEE the electrolyte's behavior as it polarizes and the DC leakage gradually decreases. In some capacitors, you can also see micro-bursts of current spikes, as tiny short circuits between the foil occur and then "self heal" during the forming process.

EVERY single cap I formed up demonstrated significant need for forming, based on the initial vs. final DC current leakage measured by the TO-6 during the process. Some of them had been sitting for six months, some for several years. There was an obvious and direct correlation between the severity of initial leakage and the time sitting.

Maybe I'm too cautious, but based on what I observed, I would always form mine before installing them. Maybe in bigger markets where you know the caps are "fresh", this might not be as important? I just know what my test gear is showing me in my one case.

Just my 2C

CBG

Formed? Never heard of that before.

OK, really stupid question here. Won't caps, brand new and never used or used but sitting idle for long periods, form or reform on there own, simply from plugging in and running current through them? Isn't that precisely what Fender and Marshall and Vox and all the other makers of classic amps did? While forming a cap slowly may well be a good thing, what harm or difference is there to the cap or amp when you you simply plug it in and fire it up? No disrespect intended to anyone, I'm just trying to learn.

Yeah, no offense taken. It's a great question.

Yes, in a perfect world, a perfect polarized electrolytic capacitor when hit with a large inrush of polarized current (in the form of the DC B+ in your power supply) will either ALREADY be, or will more or less instantly fully conform its dielectric polarization to that of the presenting polarity of the current to which it is connected (The B+ of your power supply to the "+" terminal of the capacitor)

Trouble is, that would be a "perfect capacitor". There is no such thing. Electrolytic capacitors are not perfect. Even though they are "pre-polarized", the fact is that they contain minute imperfections where the chemical dielectric does not polarize, or loses it's polarization, or where the dielectric is of insufficient physical thickness to prevent a miniature short circuit. Any of these conditions, creates a "shorting point" in one or more very tiny locations in the dielectric surface, allowing the B+ current to flow directly to ground at that (those) point(s). Inside the capacitor this tiny short circuit point quickly heats, arcs and burns out, USUALLY creating an "open" circuit (again) which allows the capacitor to return to it's normal state (blocking DC current) This is the so-called "self-healing" operation in a capacitor.

When the above scenario happens on a tiny scale, the capacitor "self heals". When it happens on a very large scale, the capacitor smokes, burns out and is destroyed. When it happens on a small scale, you never know it. The capacitor merely heats a bit, and the only way to discover it ever happened is to disassemble the capacitor and examine it, destroying the capacitor. So, "out of sight, out of mind" here.

The scenario which causes the above condition to be at its worst is the INSTANTANOUS application of large current/voltage (as in the scenario you described) "full inrush of B+, after months or years of storage"

The principle of "forming" a capacitor is the gradual increase in voltage, which gives the dielectric time to fully polarize without shorting out.


Practically speaking, I can confirm from direct observation under test conditions that this operative principle is not an old wives tale. I've personally seen it, measured it, and demonstrated the effect on real world capacitors under real-world conditions.

Is (re)forming absolutely necessary? Depends. Depends on the age of the capacitor, how long it sat in the box and the distributor, how well it was constructed, and other factors.

In the golden era of tube technology, when the manufacture of very high voltage capacitors was a daily undertaking and capacitor inventory turned over very quickly, it was probably not a big deal. That's just not the current state of the industry in 2012.

Personally, I would;

1. Variac up old amps which have been sitting for extended periods of time
2. Use a T0-6 or other controlled forming device to measure current leakage. If necessary, I would form any off-the-shelf new capacitors which show DC leakage.


Cheers,

CBG

Great posts!

This is great, I've actually been thinking about this lately. Is there a way to form a cap without buying a Variac???

O.K. I don't particularly want to be the over-conservative tech and dissenting voice here - but have any of you actually "formed" a cap on a T0-6 Sprague? I have. I did about ten of them last night...including four "brand new" 100uF/450 Nichicons and four Sprague 47uF / 450's

On a T0-6 you can actually monitor the DC leakage current during the initial forming process. You can actually SEE the electrolyte's behavior as it polarizes and the DC leakage gradually decreases. In some capacitors, you can also see micro-bursts of current spikes, as tiny short circuits between the foil occur and then "self heal" during the forming process.

EVERY single cap I formed up demonstrated significant need for forming, based on the initial vs. final DC current leakage measured by the TO-6 during the process. Some of them had been sitting for six months, some for several years. There was an obvious and direct correlation between the severity of initial leakage and the time sitting.

Maybe I'm too cautious, but based on what I observed, I would always form mine before installing them. Maybe in bigger markets where you know the caps are "fresh", this might not be as important? I just know what my test gear is showing me in my one case.

Just my 2C

CBG

No, never on an OT-6, but it is a pretty easy jig to set up, just an adjustable DC supply, a large resistor, 1Meg is convenient, and volt meters to monitor voltage across the cap and across the resistor. I have done quite a lot of it really. Ever reform one backwards? Oops, maybe just best to throw that one away. When I started building with tubes in the early mid 70's Mouser, Digi-Key and Newark only sold to business accounts. Apex Jr did not exist, nor did a host of other wonderful small suppliers. Surplus was a major source of parts, and it was best to check every cap. These days, it is rare to see 2ma of leakage current once the cap voltage starts to stabilise , and that is what 10-15seconds tops. I made the decision that my resources were better spent else where once I started working with new caps regularly. Sure if you form every cap, you might cull out a bad one every now and then, or save a few blown fuses, but you are not going to save yourself a lot, and if you are like me, not enough space to have everything available at all times, it is something that can be safely left undone, if you are working with new caps. Now from what you describe, you have a dedicated tool set up and ready to go, so your investment is a different calculus.

I don't know how well it forms caps. (if at all) But I use a light bulb limiter after working on an amp or when I get one I know nothing about. Handy tool that costs almost nothing to make.

Hi Celeste,

Yes, I did something like what you describe until I got my TO-6. Now I use it for my cap q/c work.

Like you, I've been doing this since the dark ages of the 70's myself, and I too remember those bad old days when parts were scarce, Vendors were not so responsive, and things were slow to arrive and so forth.

I'm glad you're having such great success with your high voltage cap suppliers. I thought I was too.. and I admit I was shocked (no pun intended) when I checked that batch of new caps last night. two different brands, two different vendors. Heck, I did it almost as an afterthought. These were, after all, "new" Nichicons, right? The results were quite surprising to me. Finding four out of ten "new" caps still with 10ma (or more) leakage current after a minute or two at 200-300vdc was more than a little surprising...at least to me. Not the tester either. Some caps would test perfect - others would form up quick, others would form up slow, a couple never would and were obviously headed for the dustbin.

This whole thing is a little disturbing to me, at a practical level. I'm not looking to waste bench time if I can lock down "good" parts right out of the box from a supplier...but I'm starting to become a bigger believer in the importance of forming caps and the life/death cycle of 'lytics. Looks perhaps far more real that I was giving it credit for? Jury's still out some - maybe I had a coincidentally bad batch?

Anyway - I'm building a Twin Reverb filter board with them, so we'll see how they perform.


Cheers,

CBG

Well, I built myself a cap-forming power supply over the weekend. Couple of cheap 1A 12-volt transformers, wired as an autotransformer, into a bridge, through a 39K 6.5 watt resistor. Seem to be working. I reformed the two caps I had stored away since 2004, and now I'm doing the new ones. I'm a cap-reforming fool.

O.K. You guys know how I love data and tests and actual measurements as opposed to speculation. So here's my report on the "Capacitor Forming Debate"

This is NOT exhaustive - but it is accurate within the limited scope of the data.

1. I examined my rather extensive collection of NOS vintage electrolytic capacitors. I found eight (8) NOS capacitors, four from Sprague, four from Beaver. These are 1960's examples. They are absolute mint NOS sealed package examples, each set of four being identical in age, having been stored indentically since new in sealed packages. There were no evidences of any flaws, bubbling, failures, or leakage in any of these eight examples. They were pristine.

Set A were Sprague 7/450
Set B were 20/450 Beaver

To test the concept of the need/benefit of capacitor forming I performed the following tests.

Test One: "Old cap power up after years at idle" this test simulates what happens when an old capacitor is suddenly given full current without the benefit of forming First, the TO-6 Voltage was preset to 375VDC, using a known good capacitor as the bogie load. Next, one capacitor from each batch was selected at random. It was connected to my Sprague T0-6 tester preset to 375VDC. The capacitor was "pre-tested" to verify that it indeed had measurable capacitance. (Both did, reading very near their rated capacitances) After it was established that the units were not shorted or open, the 375VDC load was instantanously applied. RESULT: In both cases the leakage current instantly shot up to dangerously high levels. The capacitors both failed within a very short time thereafter, (one more spectacularly than the other.)

Test Two: Low Voltage Forming - Another two examples from the same source batch were selected at random. They were pretested as before - then connected to the T0-6, which was slowly brought up from zero to 30vdc and then left there. The capacitors were allowed to "form" for 30 minutes each. Then the capacitors were rapidly subjected to a voltage rise culminating in 375VDC. RESULT: Before either capacitor could reach the full 375VDC, it became obvious that forming them at low voltages did not do the trick. As soon as their voltage shot up above 100VDC, their leakage also began to rise quite spectacularly...though not to quite the same levels as the first test. Both capacitors were held at 375 until it became obvious by measurement that they were experiencing micro-failures of the dielectric. Both capacitors were heating and it was obvious they were headed for a failure...so the test was aborted. These capacitors were tested at their maximum safe voltage, about 150VDC with 15ma leakage. They did hold a charge, albeit rather weak. I would call them "damaged goods" at this point.

Test Three: current regulated forming. Two more capacitors were selected form the remaining four examples.

Each was in turn connected to the T0-6, and the voltage was raised slowly until the leakage reached 5ma. No attention was paid to the voltage achieved. Everything was about leakage current. When the 5ma level was reached, the capacitor was left there. After a few minutes the leakage would fall to below 1ma. At that point the voltage would again be raised up until the 5ma limit was achieved. This process continued until full rated voltages were achieved. It took about six discreet "raise and wait" steps to achieve this condition.

Once the capacitors in test 3 had been taken through this exercise, they were tested for capacitance. Both were robust, normal in behavior, and fully able to do their job. Further, they could now take instantaneous "on current inrushes" without failure. (instantaneous inrushes of full voltage is similar to what happens when a SS rectifier amplifier is fired up).

After establishing the above, I repeated Test three on one of the remaining NOS examples, with identical results. I repeated test one on the other example, again experiencing total unit failure.

After that, I repeated test three on numerous "good used" examples of vintage electrolytic capacitors. Most of them came back to life well. A few were obviously bad as soon as the test commenced. Only one gave ambiguous results.

CONCLUSION:

It was abundantly obvious to me from my research that Test Style Three constituted the best practice for NOS vintage capacitor forming.

My limited experimentation (described above) was enough to convince me that

1.) careful and proper forming of vintage capacitors is a real and beneficial practice.
2.) It cannot be done "willy-nilly". Rather, it is subject to a specific set of behaviors which, if present, restore an undamaged and undegraded capacitor to full "as new" performance.
3.) Further, NOT practicing current regulated voltage forming of capacitors is almost certainly detrimental to their life and performance.



Cheers,

CBG

I accidentally formed my caps by plugging into an outlet controlled by a dimmer that is usually turned down to about 50%. I was very concerned when my first DC voltage measurements came in at 70ish volts... :)

Nice... :-)
Bet that got you that brown sound everyone's craving.

CBG

O.K. I don't particularly want to be the over-conservative tech and dissenting voice here - but ..........


Interesting post: I haven't heard the term forming a cap in years!

Question: have you measured leakage on a new cap then put it directly into service for a few weeks and then measured again? It would be interesting to see how new manufacture techniques stand up to the old need to form.



.

So, for the simple review of my post above.

1. Using NOS vintage caps, I've demonstrated that instant blasts of B+ can and will ruin otherwise perfectly usable vintage capacitors.

2. I've demonstrated, again using identical examples from the same lot of vintage NOS caps, that properly bringing them up to full rated voltage while keeping the leakage current below 5ma does in fact restore them to full and proper functionality - apparently without regard to their age - provided they are not physically leaky.

3. Finally, I've demonstrated that there is a difference between low voltage forming and current limited high voltage forming. The former does not appear to work, the latter does.

That's the summary - gained from actual bench tests, not conjecture.

CBG

Hey CBG, pretty much what I expected with the old caps, but the post about new caps has me reforming my original answer. Thanks for your post there. I never thought new caps would be an issue, but perhaps forming time is being cut to speed production.

In the Gerald Weber books he says you should form new caps.
But I learned on the internet that he knows nothing of such things.
Now I don't know what to believe. :lol:

because GW says it does not make it true, but it also does not make it untrue. My experience was that new caps with relatively current production dates would not need it, but with CBG's recent tests of new caps it seems that either the initial forming is not being done as well as it use to be, or my older samples were an anomaly. I think that suggests that if you can do current limited forming of your caps then you should, just to be sure.

I think it is time for me to build a forming rig to use with my bench supply so forming is easy enough not to put off.

Thanks, all. Now that I have my forming supply built, I figure it's a "better safe than sorry" proposition, at least for new caps. I really hoping to get around to opening up the amp and actually installing these new caps this weekend.

After the first set of data noted above, I've conducted some additional tests with capacitors who's age and condition can be determined with a reasonable degree of certainty.

In my admittedly limited testing, there seems to be a clear and inverse relationship between age and need for forming. This is just as Celeste suggested in an earlier post. "fresh new caps" (less than a year old) don't seem to need it. The capacitors I checked, which varied in age from more than 2 years to almost 40 years old (all NOS) seem to get progressively worse with age in terms of the need for forming...with a definite and distinct drop off in "freshness" at about five - seven years.

It does seem that;

1. Old caps - even 40 year old NOS examples - CAN be rejuvinated, if not leaking - by using a particular approach to reforming. This feature might particularly be of use to amplifier restorers, who for vintage appearance reasons on museum piece amps want to preserve original appearance.

2. There is such a thing as a "fresh" cap. Again, I admit I doubted this, but it appears that Celeste and others were right. I'm not sure when they go stale, but as Celeste and others noted, reliable suppliers probably turn their inventory often enough to make it safe to go straight to circuit with "fresh" caps.

Very interesting to see these subtleties. Earlier on the forum, I had been rather blindly posting about "old caps are fine", yet many other posters were not having that result. That's what motivated me to start the testing. I now believe my original conclusion about popping old caps in and blasting them with straight B+ was a mistake.

Back to the test bench!


CBG

CBand7

"measure again after putting into service"

Great idea - Yes, In a manner of speaking, I did do a limited "retest" of caps which had been formed. Without getting into subtleties which my equipment might not be able to detect, it appeared that once an electrolytic capacitor was properly formed, they afterwards functioned like "fresh" caps, withstanding instant "full B+" voltage levels, whether they were two years old or two decades old. This was ONCE they had been reformed.

However, for a fuller expression of your test suggestion, I'll wait a few weeks and retry some of these oldest caps which were reformed, then sat for a month, and see if they truly still had the goods...or if they slip back into stale torpor.




CBG

I would be really surprised if they degraded in the next few month. As I understand it, the combination of the acidic electrolyte and polarized potential across them causes an oxide coating to form on one of the plates. The in the absents of the potential, the coating degrades at a stable rate regardless of age. So once formed, the coating is "like new". In an "unformed" there is no coating or it is inconsequential, so the cap can be formed backwards.

Have you checked ESR of your old reformed caps? I would be interested to know if old unused caps have elevated ESR like happens to old used caps. I have no knowledge of the mechanism through which ESR rises.

Celeste,

Yep... I think you're right on.

re: ESR in unused vintage caps (that have been reformed)

Looking at a test rig to measure ESR... (this)

http://emcesd.com/tt020100.htm

but not sure my scope and pulse generator are up to meaningful and accurate measurements...but I'll make a stab at it and let you know.

Results will be interesting. Low ESR on properly (re)formed NOS vintage caps...that REALLY would clinch reforming as a legitimate exercise, wouldn't it.

Cheers,

CBG