Using a Variac to form Filter Caps on new Build

[Superlow]

So, I just finished building an 18 watt lite, I will post some pics soon. Anyway, who out there uses a Variac to form their caps on a new build? I assume I attach the amp to a dummy load/speaker and just put the power & standby in play mode, set the Variac on a low setting and bring it up slowly? Yes? No? I have been reading a lot Gerald Weber lately, not sure why, he is one of those, "My way is the only way," kind of people. He has a some decent info at times I suppose. Any and All help is always appreciated.

Thanks guys,

Ed

[prs_matt]

If they are new electrolytics, Just fire it up. There is no need to form new caps. If they are old but still good, that might be a different story.

[Superlow]

That's what I figured, I didn't know if this would add a certain "mojo," to my new build.

Thanks,

Ed

[panhead]

I read what Gerald Weber had to say about forming caps, and it made me wonder. You buy a new cap and it should be formed. I doubt the capacitor manufacturers expect customers to form every electrolytic they purchase. I came to the conclusion that new caps must be pre-formed and ready to solder in place. Obviously old caps are another story, but if I'm just fixing an amp I'm not worrying about keeping every single one of the original components. 40 or 50 bucks to recap an amp with new caps is a hit I'll take. I'm not in the amp repair business. Friends will occasionally bring me something to take a look at and see what I can do with it. I don't have time to change voltage settings every hour for a day. I have to go to work.

[Superlow]

I'm with you Pan. I borrowed a Variac from my local recording studio, just to see if this would make any difference, like you though, I have a day job, I can't watch the voltage setting on a Variac for twenty-four hours. That's Assanine.

[billcreller]

I agree with you Panhead.  Some purist's can find more dam things to fiddle around with.
  I knew a guy who had an old Lincoln that he called his "drag car", and he would turn it over with the starter with no ignition, 'To get the oil pressure up" before lighting it off.      
  Truth was , it couldn't get out of it's own smoke.
  Anyway, if we had to mess with each filter cap, "to get the oil pressure up", it would be a real pain. ;D
      BILL

[HotBluePlates]

I've formed new caps on exactly 1 amp ever. It made absolutely no difference in performance.

This is a hold-over from radio repair practice. Let's say you have a 50 year old radio or piece of military equipment. The caps haven't burst yet, and you're wanting to test the circuit. You "RE-form" the caps that have been sitting around forever by applying low voltage and/or limiting the current to the device under test. The goal is to prevent any bad caps from being a short circuit and taking out something expensive like a power transformer that cannot be replaced (on a 50 yr old piece of original equipment).

Sometimes, in the course of this, the guys noted they got lucky, and the old caps seemed to work when brought up slowly. Sometimes they'd also take dried out caps and heat them in an oven at about 150 degrees, hoping to re-liquify any remaining electrolyte. But all this is about trying to "get away with" using old parts and not buying anything new. At the time this was popular, good quality can caps didn't exist to replace the originals that were so common in these items.

Unfortunately, Gerald seemed to extrapolate this stuff to being needed for new amps. Seriously... I tired it, had the amp on a variac for a day and a half as Gerald prescribed, started with low volts, then dialed it up bit by bit. It made absolutely no difference compared to slapping them in (properly!!) and turning the amp on. I have not formed a single cap since...

The 1 "sorta-exception" is I might turn an amp on the first time using a light-bulb current limiter, to check to see if I made any accidental shorts. This works just as well as a variac for "forming caps" if you so desired, because voltage is what causes the aluminum oxide dielectric to form... it's big current when there is no dielectric that is a problem. So you limit the current, and the cap could form up just as well (assuming it ever needed to).

Also note that electrolytic caps fail quickly if the amp/device sits for decades without being used. They tend to last a very long time (for good caps) if the item is used on a regular basis, as the application of operating voltages "re-forms" the caps...

Unless you've had a bunch of filter caps that have sat in a drawer for the last 10-15 years unused, you don't need to form them.

[Superlow]

Thanks HBP,

Your posts are always incredibly insightful... As I said before I just wanted to see if there was magic behind the myth...

[HotBluePlates]

Thanks for the compliment.

I read that way back when too. I wanted there to be some magic, especially since a variac is *really expensive*, if bought new. I tried it out on the variac in the electronic shop of the school I went to at the time. Like I said, I wish it would have made a difference... I could have fired up that amp 36 hours earlier had I known it wouldn't!!

[PRR]

> set the Variac on a low setting and bring it up slowly?

That's what they do at the capacitor factory.

They start with aluminum foil in a pot of borax and water. At first, everything conducts great (which is bad for a cap). If they put big voltage on, it would boil and explode. Instead they bring the voltage up gradually. The voltage and current forms a layer of "tarnish". If the materials are pure, this tarnish is dense aluminum oxide, a very good insulator. It makes a fairly good and very compact/cheap capacitor.

So for fairly new caps, the factory already did it for you.

If electrolytics are left idle, no-voltage, the AlOx re-dissolves. In the 1930s, caps could go bad in less than a year. Often the symptom was bad hum for a few minutes and then the caps burst. And if you suspected this might happen, you could try bringing it up slow so that the AlOx could re-form at low voltage and low heat.

As materials improved, cap shelf-life got very much better. A 20 year old cap is probably still good. And if it isn't good, it probably won't re-form, it's just rotted beyond all hope.

FWIW: I once bought a 12-pack of surplus military caps. 8 were fine and 4 got hot. I wired up a series resistor to re-form them too slowly to over-heat. 3 stayed bad: just would not stop leaking. One came down in leakage but never good enough to risk putting in the amp and trying to do a show. Promotors really don't like cap explosions when the paying crowd is in the room.

Caps from the early 1960s, maybe there is some sense in re-forming them; my feeling is that they may as well be replaced. Simple Variac or resistor re-forming is NOT as controlled as the way the factory does it.