reforming caps.

[sean k]

I never really took any notice of old caps when I built my first amps and theres two that hum like banshees. The other thats actually usable has a choke and that may be the reason its reasonably quiet.

So I've got a, what are they called, a massive rheostat to dial up wall voltage from 0 - 240 VAC, so how can I reform the power caps in my amps?

The two I'm talking about are quite small and quite cramped so to get into and under the signal wiring would be quite a chore so could I just remove the valves, put a biggish resistor, maybe 50 - 100k from the diodes to the first cap and then just dial up the voltage slowly till I have the same on each end of the resistor? And will this reform the caps?

[gbono]

Mmmm - and these capacitors are how old? Just replace them with new ones - reforming will not bring old dried out caps to life.

[sean k]

Well they're not particularly old, NOS from a surplus shop, or maybe they are but they're not used except for when I power up the amp and remember it's completely noisy.

It's not so easy to replace them and I'd like to know about reforming and conditioning anyways for the next few amps where I use NOS caps...

Though I might start getting small transformers and taking half the winding of to make chokes...

[ThermionicScott]

NOS electrolytics are just as prone to drying out as used ones of the same age. I wouldn't bother, unless you don't need this amp to be dependable.

- Scott

[soundmasterg]

The only NOS caps worth getting are film caps, and not all of them either. The brown ones used in some of the Silverface Fenders, Sunns, many Silvertones, etc., are lousy caps with high ESR and poor tone, though they work fine as a rule even after all these years. The blue film ones that were used in other Silverface Fenders are excellent caps and sound and perform great. Then you have the paper and polyester caps like the blue molded caps that were in the blackface Fenders and they usually always work and sound fantastic, but many paper caps like old Astrons and Bumblebees leak DC and don't work well these days. In all cases though, electrolytic caps have a short shelf life and in operation, will always wear out eventually. NOS electrolytics are a dumb thing to buy as even if you reform them, they may not work, and you will get better performance from new electrolytic caps in all cases. Electrolytic technology has gone in leaps and bounds since the 50's and 60's and new caps are way better than old electrolytics. Moreover, anyone selling NOS electrolytics is a snake oil salesman and is just trying to take your money. Buy new electrolytics....your amps will be happier.

Greg

[Steve Conner]

+1... NOS capacitors are just a plain stupid idea! Having said that, I've used them for prototypes, but when I build the thing up for real, I replace them with new ones.

To reform an electrolytic capacitor you need to slowly charge it up over several hours. If you've already powered it up and it didn't blow your fuses or explode in a goopy mess, then it doesn't need reforming.

If it didn't explode, but it isn't filtering out hum either, then reforming won't help, it's dried out and needs replaced.

The massive rheostat is called a variac, but I prefer to use a fixed high voltage source with a large series resistor. You can just hook it up and watch the voltage on the capacitor, when it gets to the same as the supply voltage you're done. If it never gets there, you can calculate the leakage current using Ohm's law and make a judgment call.

[sean k]

Thanks Steve,
#1, at the time, which was many years ago, I didn't know that old caps were a stupid idea and basically they were the only ones available... but now I know better, that old caps are stupid, but I thught I might be able to cure the old caps.

But because of your concise comments, exactly what I was looking for, I now know that the caps I have aren't worthy... the amps power up and work but are noisy as...

I can get new ones now but back then the NOS were the only stuff available at an even reasonable price. Thanks for all the info everybody...major surgery ahead....

[pontiacpete]

I restore caps that are used in old radios w/ a heathkit IT 11. I bring it up slowly and measure the current leakage across a resistor , like a 1K . If the leakage is reduced to an acceptable level, then I'll leave it in. I use this formula,,, uf x vdc=sqr T/5=uA, for the best possible reforming, or I divide by 2 for the next best.
I don't bother doing this for a guitar amp, i just put new ones in,,, reliability on a gig is important.

[olddawg]

I have only used NOS caps on valuable collectable vintage amps where keeping it original with the exact parts is important. I don't like to do it.