Just did a cap job on my UL Super Reverb - have a question.

[mike9]

I just recapped my '79 UL super reverb. (my first cap job) It passed the short test and I have a variac. Is it wise to form the new caps with a variac? If so what would the best procedure be? Thanks for any and all advice.

[Structo]

If they are new caps a lot of techs don't think you have to form them.
But it certainly can't hurt anything.

I usually use my light bulb current limiter and bring up the current slowly.

But with a variac I would probably just advance the voltage every 15 minutes or so until you are up 120vac.

I would leave the tubes out and be sure to have the standby off.
If it has a tube rectifier then you have to have that installed of course.

[ToneMerc]

If so what would the best procedure be? Thanks for any and all advice.

here ya ago

http://forum.metroamp.com/viewtopic.php ... ht=forming

[mike9]

Thanks guys - Tom - how do you use your limiter - change lamps in it?

The other way is really confusing so I'll opt to keep it simple. I believe most new caps are factory formed and I got these from Doug Hoffman yesterday.

[Phil_S]

Yes, gradually increase the wattage of the bulb 25-40-60-75-100 but that sort of stepping up is overkill. 40-75-100 should do it. We should all stock up. Pretty soon you won't be able to buy an incandescent. I didn't think I'd see it in my lifetime, but it is here.

I use mine for short testing. I keep a 40W bulb in it.

[rp]

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[Cliff Schecht]

Here's what my dad told me about reforming caps. It's always good to test them for leakage as your testing them, this will give you a clear indication whether the cap is usable/passable.

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).

[mike9]

Well I wrote Doug Hoffman and asked him. To his knowledge caps are not formed at the factory. In all his years of building and repairing he just forms them by putting voltage to them and he's never had an issue. I started with a 40 watt lamp in my current limiter then 75 then 100 watt as suggested. Then I biased the power tubes and it sounds wonderful - tighter bass and less noise than before.

Thanks all for the info -