Using a variac?

dean · February 13th, 2006, 08:28 PM

I'm going to use a variac to "form" the new caps in my amp, but I need to know the basic procedure for the process. In the past, I've just installed the new filter caps and fired the amp up. Basic questions like: How long should the process take? How high should the volts go? Special concerns?

If you've done this before, I sure would appreciate some advice about what to do. Or could you point me to a website that has the info I need to do this successfully?

I'll be using a Staco 10 amp autotransformer rated at 1.4 kva. The volts guage reads from 0 to 150.

Any advice?

Thanks,
Dean

Dacious · February 14th, 2006, 03:18 AM

If the caps are new manufacture (witin five years), just stick 'em in and fire the amp up. Variacs may be useful to 'reform' NOS caps which may not be real flash.

But old Fender and Marshalls which are twenty years old with original caps in 'em have never been on a variac. No new mass-produced amps are made like this.

Otherwise the guide I read said 10 volts increase per hour up to 115 (US). So 10 volts for an hour, then 20, then 30 etc.

Marky D. · February 14th, 2006, 02:28 PM

I've used the variac method, the "just firing it up" method, and one discussed in on of Gerald Weber's books where a light bulb is connected in series w/ the AC line and absorbs some of the initial current.

They all seem to work ok.. never had any problems. All with brand new caps, though.

z8894 · February 14th, 2006, 03:02 PM

http://www.geofex.com

Select Tube Amp FAQ, then scroll down to maintenance issues. There is cap forming article there.

Below is the text of the article,

Do new caps need to be formed?
There's a lot of controvery on "reforming" replacement caps. Here are a few answers.
Manufacturers of caps design their caps for a ten year working life, and a five year shelf life. That means that the stresses and heat of working in equipment will leave the vast majority of caps functioning OK after ten years of normal operation. After that, it's gravy to the buyer.
They also design them to work OK after sitting on a shelf unused for five years, meaning that the cap should not fail if it's put into operation at rated voltage after sitting unused for five years. As noted above, the caps do slowly un-form without regular use.
If the electrolytic caps you use to fix your amp are over five years old as determined by the date code on them, you ought to at least worry about forming them, and if they're over ten years old (like NOS multisection cans), definitely re-form them. Other than that, put them in and turn it on.
How do I "re-form" electrolytic caps?
You'll hear folks talk about "bringing an amp up slowly on a variac"; this can work but is not particularly good for your tubes. A better way is this:
Pull out all the tubes.
if your amp has a tube rectifier, solder in temporarily some high voltage silicon diodes across the tube lugs to be a rectifier that does not depend on the filament voltages. If your amp has silicon diodes, you can skip this.
open up the wire that goes from the rectifier tube (or solid state diodes) to the first power supply filter stage and solder in series with the wire a temporary 100K 2- 5W resistor. This resistor will limit the current that can flow into the caps and the amount of voltage that is applied to them to safe values that will cause the insulating layer to re-form.
clip your voltmeter across the resistor
button it up. Turn it on (no tubes in it, remember). Watch the voltmeter.
when the voltmeter reading drops to less than 20-30VDC, your caps are formed.
open it back up and pull out those diodes and resistor, putting it back in original shape.
The forming could take hours to days.

February 13th, 2006, 08:28 PM	#1 (permalink)
dean Friend of Leo's Join Date: Mar 2003 Location: Twin Cities, Minnesota Age: 60 Posts: 2,042	Using a variac? I'm going to use a variac to "form" the new caps in my amp, but I need to know the basic procedure for the process. In the past, I've just installed the new filter caps and fired the amp up. Basic questions like: How long should the process take? How high should the volts go? Special concerns? If you've done this before, I sure would appreciate some advice about what to do. Or could you point me to a website that has the info I need to do this successfully? I'll be using a Staco 10 amp autotransformer rated at 1.4 kva. The volts guage reads from 0 to 150. Any advice? Thanks, Dean __________________ "I used to be clueless, but I've turned that situation around 360 degrees."

February 14th, 2006, 03:18 AM	#2 (permalink)
Dacious Friend of Leo's Join Date: Mar 2003 Location: Godzone Posts: 2,623 35 Photos	If the caps are new manufacture (witin five years), just stick 'em in and fire the amp up. Variacs may be useful to 'reform' NOS caps which may not be real flash. But old Fender and Marshalls which are twenty years old with original caps in 'em have never been on a variac. No new mass-produced amps are made like this. Otherwise the guide I read said 10 volts increase per hour up to 115 (US). So 10 volts for an hour, then 20, then 30 etc. __________________ My other Telecaster is a Thinline The Tele Bible, Ch 1, v 10 Love thy Telecaster, covet not thy neighbour's Strat!

February 14th, 2006, 02:28 PM	#3 (permalink)
Marky D. Tele-Meister Join Date: Mar 2003 Location: Minneapolis, MN Age: 47 Posts: 314	3 ways.. I've used the variac method, the "just firing it up" method, and one discussed in on of Gerald Weber's books where a light bulb is connected in series w/ the AC line and absorbs some of the initial current. They all seem to work ok.. never had any problems. All with brand new caps, though.

February 14th, 2006, 03:02 PM	#4 (permalink)
z8894 Tele-Meister Join Date: Mar 2003 Location: Alpharetta, Ga. Age: 43 Posts: 116	Here is a "how to" article on it. http://www.geofex.com Select Tube Amp FAQ, then scroll down to maintenance issues. There is cap forming article there. Below is the text of the article, Do new caps need to be formed? There's a lot of controvery on "reforming" replacement caps. Here are a few answers. Manufacturers of caps design their caps for a ten year working life, and a five year shelf life. That means that the stresses and heat of working in equipment will leave the vast majority of caps functioning OK after ten years of normal operation. After that, it's gravy to the buyer. They also design them to work OK after sitting on a shelf unused for five years, meaning that the cap should not fail if it's put into operation at rated voltage after sitting unused for five years. As noted above, the caps do slowly un-form without regular use. If the electrolytic caps you use to fix your amp are over five years old as determined by the date code on them, you ought to at least worry about forming them, and if they're over ten years old (like NOS multisection cans), definitely re-form them. Other than that, put them in and turn it on. How do I "re-form" electrolytic caps? You'll hear folks talk about "bringing an amp up slowly on a variac"; this can work but is not particularly good for your tubes. A better way is this: Pull out all the tubes. if your amp has a tube rectifier, solder in temporarily some high voltage silicon diodes across the tube lugs to be a rectifier that does not depend on the filament voltages. If your amp has silicon diodes, you can skip this. open up the wire that goes from the rectifier tube (or solid state diodes) to the first power supply filter stage and solder in series with the wire a temporary 100K 2- 5W resistor. This resistor will limit the current that can flow into the caps and the amount of voltage that is applied to them to safe values that will cause the insulating layer to re-form. clip your voltmeter across the resistor button it up. Turn it on (no tubes in it, remember). Watch the voltmeter. when the voltmeter reading drops to less than 20-30VDC, your caps are formed. open it back up and pull out those diodes and resistor, putting it back in original shape. The forming could take hours to days.