Forming filter caps...

... isn't also a bad things for new caps and enhances their physically parameters, also when they've been (quick-) formed at the manufacturer.

But for new caps, which have been stocked on the shelf for more than 5 years it's obligation!

I'm describing the steps valid for a 100W Marshall, with the 45W or 50W models you can share some of this steps:

- pull out the power tubes and the preamp tubes

- disconnect the yello wire going from the PT between the first two filter caps, temporarily tape the unsoldered end for safety (100W only)

- unsolder each one lead of of the both 56K bleeder resistors across the both screen's filter caps and tape it temporarily for safety (100W only)

- disconnect the red wire from the rectifier diodes (the rectifier tube) to the first filter cap and swap in temporarily a 100K/1W resistor

Now fire the amp up completely (power & standby to 'on') and measure the voltage drop across the 100K resistor you've swapped in!

You'll notice, that first you'll have a voltage drop of about 200V, which within 15-30 seconds reduces itself to about 30-40V - then further on it is dropping slowly.

If so, all's ok and you can switch off your meter and do your other things, because now the caps require 4-24 hours time to form, depending on the cap's quality and the cap's age.

Only insure, that nobody else could get in contact with your 'open' amp - way the best is, to lock the room, where the amp is sitting and keep the key in your pocket. It's danger to the life for unskilled people!!!

From time to time you can check the process of the voltage drop - and as soon as the voltage drop across the 100K is less than 5V (five volts), your caps are formed.

Switch off the amp - and before you remove the swapped in 100K resistor, implicitly bleed the caps by connecting pin #1 of V1 with an alligator clip to ground for at least 2-3 minutes!

Now reset the temporarily tweaks and put in all the tubes again - you've done Fire up the amp and enjoy its (valorized) sound

Larry


BTW: Also when an amp hasn't been in use for longer than six months, you should form the caps, before you fire up this amp again! If not, the longevity of the installed caps might be significantly diminished!

Larry, is it OK to simply pull the tubes (ALL of the tubes, unless you have a tube rectifier that is) and simply variac the amp? Start low and slowly increase every hour or so?

That's at least better, than not at all forming the caps.

The first problem is, that with the variac you're limiting the voltage to the caps, and not the current - the current only is limited indirectly.

The second problem is, that you can't sense, how slow the caps need it to be formed or how fast you can proceed - you're deciding the process in this case.

The third problem is, that most of the people don't have so much patience, to regulate up the variac every 10-15 minutes a tad further, and that sometimes over a duration of up to 24 hours.

But the benefit of the resistor method is, that the caps are regulating all by itself - and you only have to check the proceeding result.

So why the stress with the variac?

Larry

Thanks Larry

That is really helpful.

A nice simple way of not having to variac an old amp

Just need a secure place to do it so that I do not let the kids fry themselves!!!

right on Larry!

correct me if im wrong,

should i unsolder the red wire that goes from the rectifier to the mains filter cap and unsolder that wire on teh filter cap side? where should i connect the other lead of the 100k resistor? to the ground lug of the mains FC``?

When you disconnect any cable from any solder joint, then suddenly you do have two contacts!

Right? The 1-st one still is the solder joint, the 2-nd one is the disconnected cable end.

And now swap in between both contacts...

Larry

aaaarrrrg let me get this.. ill make drawing here from my amp......


the balck block i painted is how you meant it? sorry im still a bit late here

Can someone edit a 1959 layout, to be sure? I understand disconnect the yellow wire going from the PT between the first two filter caps.

Unsolder each one lead of the both 56K bleeder resistors across the both screens’ filter caps and tape it temporarily for safety (100W only)


Disconnect the red wire from the rectifier diodes (the rectifier tube) to the first filter cap and (swap in temporarily a 100K/1W resistor)

Are you saying to place a resistor between the red wire and the filter (main filter)?

Thanks

The Resistor then is located between the rectifier and the first filter caps - temporarily, until forming is done.

Larry

so the little drawing that i did was correct then huh?

Sorry Larry, I need a visual aid. Retard.

hey neighbour. you do know me right? guess we`re on the same track here mate.

kahelle

HaHaHa

I buyed a batch of "NOS" LCR 50/50 dual caps and reformed in about 4-5 hours with a 22k series resistor.These LCR were from 1990 and never used,the voltage drop were no bad,about 10-15 volt at the start of the process a reduced it very fast.
Before learning to reform the caps,I made a terrible mistake.....I putted one of the NOS LCR in my amp as filter cap (JCM900 style pattern of filtering) and after 5-10 minutes the cap became a bit hot.....not a good thing.....With another cap,but reformed,this thing not happens.