Forming Caps No Variac
From kee–(at)–ustin.ibm.com Wed Jan 10 11:12:53 CST 1996
From: kee–(at)–ustin.ibm.com ()
Subject: Re: Forming Filter Caps ?
Date: 9 Jan 1996 19:40:13 GMT
In article <4cu8of$kf--(at)--ark.interport.net>, Jon Steele
> Amp Techs –
> How crucial is this business of gradually ramping up the voltage
> after replacing filter caps? I don’t have a variac.
> Thanks in advance if you find the time to respond.
You will find folks that tell you everything from “it doesn’t matter much”
to “OHMIGAWD!! FORM THEM OR THE WORLD WILL END!!!”.
It’s good to do it if you can.
If you’ve gone to all the trouble to replace the filter caps and have a day or
so to spend forming, you don’t need a variac. Just open the line from the
rectifiers to the first capacitor filter section. Solder in a 100K 2W
resistor in series. Remove all tubes except the rectifier tube.
Put a voltmeter (safely) across the first filter cap section, and close it
back up. Turn the amp on, and watch the voltage. When the voltage on the cap
comes up to 90-95% of the lowest of the rated voltage on the cap or the
normal operating voltage of the amp, the cap is as “formed” as it needs to
This forming up of the caps and the increase of voltage may be instantaneous
for new caps, or may take a day or so for NOS multisection cans. If it never
comes up, the cap is faulty and would soon fail on you anyway.
Shut it down, bleed the caps down, remove the 100K resistor, re-connect the
rectifiers and filter caps, and put the tubes back in.