This is for the tech guys from the UK really to answer, if they know. Do you guys use a variac when forming your filter caps after replacing them? In the future i will be doing this but everyone says you have to use a variac to bring the voltage up slowly to form the electrolyte in the caps.

I know nowadays there are pre-formed caps on the market, but by all accounts you don't know what you're getting, as they could have lain on the shelf of a shop untouched for a year and crystallised.

The consensus of opinion seems to be that it's not worth taking the chance with a pre-formed cap as it could fail prematurely. Most techs seem to take the view that you need to do the pre-forming.

Sorry for the tech question but just interested what the guys do on here who work on their own amps.

_________________
Luck won't do it and ignorance can't

George Van Eps.

I'm not an amp expert. (My dad was an EE so I learned the basics from him, but as the old saying goes, "I know just enough to be dangerous".)

Cap forming is a controversial subject. There may be a consensus on some internet guitar amp forums but it's not a universal consensus. And "pre-formed" caps aren't new -- ever since modern electrolytic caps were invented in the 1940's they've been pre-formed, and ever since then their unused shelf life has been rated in decades. There's some tiny risk with electrolytics that haven't been charged in months or years, but most people think re-forming fairly fresh caps is a waste of time. And many people think it's a waste of time even on decades old caps -- lots of people take amps that haven't been used for 40 years and simply plug them in and turn them on without problems.

BTW, the dialectric is what reforms (the thin layer of aluminum oxide on the surface of the plates), not the electrolytic. If/when the electrolytic dries out or crystallizes the cap is dead, un-revivable, trash. It can't conduct electricity ever again.

If you're reviving an old amp that hasn't been used for a long time and don't want to change the caps so as to maintain originality, bringing them up to full voltage slowly is a very good idea. But if you're replacing the caps with fresh ones, most people consider reforming the new caps to be overkill.

Also, there are a variety of ways to gently reform caps. A Variac is just one way to do it (the most expensive way if you don't already own a Variac).

Thanks strayedstrater, there are a lot of different opinions about this as you say, heard about a guy who replaced his caps, didn't form the cap, just changed it and he got hardly any life out the cap, it just blew. This is why they say to people if they have a good amp and they're not playing it at least turn it on and fire it up once in a while, get the caps working or they can crystallize.

Amp guru Gerald Weber, and just about every tech i've read say to not take a chance on pre-formed caps as you don't know how long they've been lying in the shop and to fit them you're taking a chance of them a) sounding really bad b) blowing out on you early on and you getting not a lot of mileage from the caps.

I would like to believe that you can just take the old one's out and slot the new one's in but it sounds as if you're taking a chance with that.

As regards the variac i know you can also make a current limiter by running two single extension cords in series with a 100W lamp but i'm not really sure how to make that.

One guy i read about in a book said he changed his caps didn't pre-form them and the amp's tone sucked really bad, it sounded worse than the old caps! then he did another amp did the pre-form on the caps and it sounded fabulous.

What to believe

_________________
Luck won't do it and ignorance can't

George Van Eps.

Gorgon wrote



Could try a triac, also coloquially called a light switch dimmer to vary the voltage.
Less than a tenner!
Actually that trick with the light bulb is often used to limit current draw when youve built an amp and are powering up for the first time. Gives peace of mind and is a sensible thing to do anyway. To make one just bung the bulb in series with the supply through a switch and start off with a 240v lowish wattage bulb.

...Al

There's a drawing of a "current limiter" in one of Gerald Weber's books that involves a 100w light bulb and a switch to operate it....works like the others mentioned,I use it after working on an amp to see if there could be a short,if the bulb glows dimly it's okay,if it burns bright when the amp is on,something is wrong.

Thanks for the replies guys, that looks the way to go, to build that little current limiter. I've got that book Rebelsoul "a desktop reference" our electrics socket wise are three pin so it's a bit different from the two pin US system but it still must be pretty easy to build that, the drawing looks pretty simple.

_________________
Luck won't do it and ignorance can't

George Van Eps.