Forming Caps with Variac

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boots
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Forming Caps with Variac

Post by boots »

I just stumbled across a variac in an "antique" shop.

I've never bothered with "forming" caps when building amps - I just slap 'em in and they do fine, as far as I can tell.

Now that I have a variac, I will probably form my caps from now on. How do you go about doing this? Do you just install the caps in the amp and power it up, bringing up the voltage a little at a time? Do you pull the tubes out of the sockets while forming?

I remember hearing long ago that it is a bad practice to run tube filaments much outside their specified voltage range. This was in respect to large radio transmitting tubes. I always understood that low filament voltage was bad as well as high voltage.

If that is true, it seems like a bad idea to bring the AC supply voltage up slowly with the tubes in the sockets, as that would certainly be running the tube filaments outside their specified voltage range. I guess pulling the tubes before forming the new caps would be OK.

You would still need to leave the rectifier tube in place though, wouldn't you? What happens when you run the recto filament at 2 or 3 volts instead of 5?

Can the caps be formed with unrectified AC?

Maybe it would be better to build a cap forming jig and form the caps before installing them in an amp?

Am I making this too difficult? It seems like it ought to be a pretty simple matter to form caps, but my mind is running away with all the details.

Any opinions?
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Structo
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Re: Forming Caps with Variac

Post by Structo »

I'm pretty sure that you don't need to form new caps that you may order unless they have been on the shelf for years.

NOS caps are a different story depending on their age.

Frankly, I don't see the attraction to using old caps because new ones are built so much better.
Lower ESR and more compacts sizes for a couple reasons.

You can always harvest the paper tube of an old cap and insert the new cap into it to keep the look.

Larry of Larry Amps wrote a great article on another forum.
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Cliff Schecht
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Re: Forming Caps with Variac

Post by Cliff Schecht »

The two methods I use for reforming old caps:

1) Get a capacitor tester that tests caps at their rated voltage. We have an old one in the lab up at school that I bring my old HV e-caps in and test them on. It goes up to 600V which means I can reform caps up to this voltage easily. It also measures ESR and capacitance, both useful merits for figuring out whether a cap is good or not before I drop it in a circuit.

2) This is how I used to reform caps before having a dedicated cap leakage tester at my disposal:
Here’s how to reform old caps (assuming they’re salvageable):



Take the unformed or old cap and measure its capacitance value with a Fluke DVM. Be CERTAIN that you haven’t charged the cap prior to doing this. It may read fairly close to its rated capacitance value but there’s a good chance that a lot of the dielectric has deformed and the cap will leak a lot of DC current when put into service.



Reforming the cap rebuilds the dielectric layer that the manufacturer established on the cap when it was new. Once reformed, the cap will not only have its rated capacitance but it will also leak very little.



To start: Put a resistor in series with the cap you’re going to reform. Choose the value of resistance to limit the short-circuit current to no more than a couple of milliamps (watch the resistor power rating as well, in case the cap is shorted and stays that way).



Next, hook the cap up to a variable power supply (minding polarity, of course) and slowly bring the voltage up to a value that’s a bit under the rated WVDC of the cap. IF you don’t have a variable high-voltage supply, you can hook the RC setup to a non-variable HV supply and just switch the thing on, although it’s preferable to apply the forming voltage slowly. Leave the cap hooked up to the HV supply for a few minutes and then check the leakage current flowing into the cap (measuring voltage across the limiting resistor and computing the current works fine). The cap’s leakage current shouldn’t be more than a couple of milliamps…some of the older ones may leak up to perhaps 10mA.



After the cap has sat under the charging supply for a while, disconnect the HV supply and then discharge the cap through a low enough resistance to run it down fairly quickly without making a lot of sparks while doing so. Don’t dead-short it to discharge it; it makes a lot of neat sparks and noise but the high surge current is really hard on the internal structure of those older caps. Then hook the cap up to the HV supply again (through the limiting resistor), leave it for a few minutes and then check the leakage current before discharging as above. Repeating this process a few times usually reforms just about any old cap that’s still in usable shape.



After the last forming run, discharge the cap one last time through a resistance and this time, place a shorting wire across the output for several minutes. A lot of electrolytics exhibit enough dielectric absorption to allow them to recharge themselves to significantly high voltages if left unshorted after being charged and that charge can fry your capacitance meter when you go to check the capacitance one last time.



So, after the cap has sat with the short across it for several minutes, remove the short and check the capacitance with your DVM. It should read within its tolerance limits. Since it’s been reformed, it won’t leak much either.



The term “dry” electrolytic is a bit of a misnomer. Modern, polymer dielectric electrolytic are truly dry in that sense but the older aluminum-foil and paper electrolytic all had some liquid in them. They were called “dry” since they didn’t have liquid sloshing about in them. Instead, the electrolyte was typically held in a paste form. If properly stored, some pretty old caps are still usable today. It’s easy enough to check the seals to see if they’ve broken or become embrittled and a capacitance/DC leakage test will also tell you whether they’ve survived or not.



If the electrolytic caps you have were used, then it’s probably best not to try to salvage them. The heat they endured for the years they were in service in tube equipment was usually enough to compromise their integrity or even kill them. NOS parts are worth trying to reform unless they’re really old (WWII or earlier).
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Reeltarded
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Re: Forming Caps with Variac

Post by Reeltarded »

It may be just a ritual I decided on but I place the caps in direct sunlight for a day or two before I hit 'em with voltage to solve the crystals I imagine are in a 'dry' one.
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rooster
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Re: Forming Caps with Variac

Post by rooster »

That article by Larry was amazing, pretty much what you might expect from a cat who builds such great looking amps. And his real focus was TONE, BTW. He expressed that the caps used in early Marshall's were capable of almost life giving juju, sonic life anyway, and again, I respect his opinion.

If I remember correctly he was using NOS versions but also used ones in old amps, too. It would be great if he would catch this thread. If I remember correctly he had a 72 hour variac burn in, running very low voltages for many hours. He said he was very successful at resuscitating these old caps.

Not to dis his method, but it was suggested to me at one time by a man named John Fouche to put the caps in question in a shoebox with a lid and than into an oven set to 150 degrees for hour or so before installing them in an amp.??? Yeah, sounds kinda crazy but I did this with a dozen or so old Mallorys and they are still working. That was 15 years ago.

I think the real question is whether or not you think these old parts have any virtue or mojo. I do and I also think Larry does, too. Sure, I use new caps too, but I'm glad I didn't toss those Mallorys out. YMMV.
Last edited by rooster on Sun Aug 19, 2012 6:08 pm, edited 1 time in total.
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roberto
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Re: Forming Caps with Variac

Post by roberto »

Structo wrote:Frankly, I don't see the attraction to using old caps because new ones are built so much better. Lower ESR and more compacts sizes for a couple reasons.
This is not so much better. Well, not in guitar amps.
rooster wrote:He expressed that the caps used in early Marshall's were capable of almost life giving juju, sonic life anyway, and again, I respect his opinion.
I agree with him. I've done some AB tests and by ear, old caps up to LCR have a better response, more midrange complexity. Something you can't obtain adding small caps in parallel or so on.
rooster wrote:Not to dis his method, but It was suggested to me at one time by a man named John Fouches to put the caps in question in a shoebox with a lid and than into an oven set to 150 degrees for hour or so before installing them in an amp.??? Yeah, sounds kind crazy but I di this with a dozen or so old Mallorys and they are still working. That was 15 years ago.
Yeah, you go around the melting point of the electrolyte. With both ways you work on electrolyte and oxide layer.

Take a look here:
http://www.nmr.mgh.harvard.edu/~reese/electrolytics/
http://www.nmr.mgh.harvard.edu/~reese/e ... s/tec1.pdf
http://www.nmr.mgh.harvard.edu/~reese/e ... s/tec2.pdf
tubeswell
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Re: Forming Caps with Variac

Post by tubeswell »

You shouldn't need to form new caps.

Old e-caps need DC to form them (AC fucks them up - but a variac is a handy adjustable power source if you have the AC rectified and filtered). You gradually increase the +ve DC on the positive side of the cap in 'voltage steps' and measure the voltage drop across the series resistor as you go. At each step of voltage, the voltage drop between the DC source and the cap will progressively lessen over time as the dilectric gunk in the cap forms and the cap holds more voltage. Then you increase the DC supply voltage to the next 'step' and wait until the voltage drop across the resistor decreases again, and so on, until your cap is sitting happily at the required voltage you desire to form it to.
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martin manning
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Re: Forming Caps with Variac

Post by martin manning »

Great link, Roberto, thanks for posting!
Last edited by martin manning on Sun Aug 19, 2012 12:27 pm, edited 1 time in total.
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Kagliostro
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Re: Forming Caps with Variac

Post by Kagliostro »

Not to dis his method, but It was suggested to me at one time by a man named John Fouches to put the caps in question in a shoebox with a lid and than into an oven set to 150 degrees for hour or so before installing them in an amp.??? Yeah, sounds kind crazy but I di this with a dozen or so old Mallorys and they are still working. That was 15 years ago.
I received a similar council, only the method was a bit different and consist to put caps in hot water for some time

K
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Re: Forming Caps with Variac

Post by Reeltarded »

rooster wrote:That article by Larry was amazing

If I remember correctly he was using NOS version but also used ones in old amps, too. It would be great if he would catch this thread. If I remember correctly he had a 72 hour variac burn in, running very low voltages for many hours. He said he was very successful at resuscitating these old caps.
Larry uses NOS LCRs and those amps s-PEAK for himself. :)

The Sun angle on me is pretty high this time of year. I like the tanning version better than the scary easy bake thing. That worries me because if it doesn't smell done I might forget there is something cooking ;)
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rooster
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Re: Forming Caps with Variac

Post by rooster »

Man, that was some late nite typing on my part. I am going to edit my spelling mistakes!!!

As with all technology and when it interacts with the manufacturing process, we may look back one day and see that even though it's basically the same product being made today, it's different. Those STR6L6 tubes quickly come to mind, amongst other things. So yes, I think some of those old caps are in the same bag. Not all of them, but certainly Mallorys and a few select others. They were built to serve and perform perfectly for a long time. And this was in the day when things were actually built to last, don't forget.

If they impact the TONE of an amp for the good, well, then in a perfect world I want a lot of them. :) I definitely think making an effort to restore them for use is worthwhile, whatever your method.
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Structo
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Re: Forming Caps with Variac

Post by Structo »

Thanks for the input guys, I really didn't think there was M0jo in old caps but perhaps there is for guitar amps.

I remember Larry saying he takes old electrolytic caps and puts them in hot water (water just below the top) on can caps.

This helps dissolve any crystals that have formed in the electrolyte.

Many ways to achieve the same thing.

The drawing I posted is not quite right, Larry says to use a 100K resistor.

Then wait until the voltage drop is 5v or less before taking the cap out of the circuit.

Here is Larrys actual post at Metro.
http://forum.metroamp.com/viewtopic.php?p=81922#p81922
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Re: Forming Caps with Variac

Post by C Moore »

Posted: Sat Aug 18, 2012 7:21 am Post subject:

--------------------------------------------------------------------------------

I'm pretty sure that you don't need to form new caps that you may order unless they have been on the shelf for years.

NOS caps are a different story depending on their age.

Frankly, I don't see the attraction to using old caps because new ones are built so much better.
Lower ESR and more compacts sizes for a couple reasons.
____________________________
Just kind of paraphrasing Structo above -
Are 30-50 year old caps, that never saw service (NOS) really that desirable.?
Were they not just the "normal" cap of their day, just as an F&T, Xicon, etc etc is today.?
Are they that much better to justify the hassle of "forming" them.?
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Re: Forming Caps with Variac

Post by Cliff Schecht »

The short answer... Yes, it does make *some* difference.

TBH though I'm yet to really see much qualitative data on this other than Solen film caps don't work too well in some guitar amp circuits because they are too "quick". So in that regard, in some of my Fender clones, I have used some pretty damn old (but still ok to use) e-caps, just for the heck of it.

With older capacitor technologies you have higher leakage currents, higher ESR, less capacitance tolerance, etc that all play into how quickly the capacitor can charge and discharge. Because these older caps are actually crappier, they do add certain nuances to how the power supply and power amp deliver power to the speaker. Combine this with just adequately rated power (and IMO underrated output) transformers and you have a power supply that is going to have some inherent "flaws" that happen to add to the musicality of an amp as well (I guess I'm looking specifically at Fender here).

In reality this is probably not the best place to cork sniff anyways. Older caps are more liable to fail under constant heavy loading (even if they reform fine initially) and really they just don't decouple as much noise from the power supplies because the impedance is too high. I definitely wouldn't bother with old e-caps in a single-ended amp, I use the best e-caps I can find there usually.

Also, something to keep in mind, past a certain point electrolytic caps all started being made essentially the same way, using the same electrolyte gel from the same company and same aluminum even in many cases. With most modern capacitors you can almost tell the ESR by the physical size. So be it F&T, Atoms or other similarly large caps, you really aren't going to be able to hear or feel a difference between these. Think about what happens when you recap an old Fender with new e-caps, they go from flabby weak POS's to fire breathing monsters (assuming everything else works alright!).
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boots
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Re: Forming Caps with Variac

Post by boots »

Thanks Guys for all the good input! I have a better feel for the forming process now.

One reason I am contemplating re-forming caps is because it turns out I have a few NIB can caps in my junk box. I am wondering if they are worth trying to use in a build.

They are 16 uF at 600 VDC (perfect!) with date codes of 5-69 (May 1969?). They are obviously NIB.

Whaddya think? Are they worth the trouble to mess with?
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