Forming filter caps

[Doug H]

can you rescue a cap that has been run without forming? or is it too late once a cap that needed forming has been wired up and run live.

[gluke]

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

Great stuff.

Just to triple check. This is the red/Yellow wire (center tap) on the regular PT.

[novosibir]

can you rescue a cap that has been run without forming? or is it too late once a cap that needed forming has been wired up and run live.

No prob with newer caps, which also w/o forming it will be within acceptable ESR & leakage current values.
Older caps are raw formed then, no chance to rescue it, except you store the cap again for at least 5 years and then form it. A raw formed filter cap almost always will have remarkable higher ESR & leakage current than caps formed after my suggestion.

Larry

[novosibir]

Just to triple check. This is the red/Yellow wire (center tap) on the regular PT.

I don't know, about which amp and which PT you're speaking! You know it, but not me
By all means the original Marshall (Dagnall) 100W PT does have a yellow wire as the CT of the HT secondary.

Larry

[gluke]

Sorry, Metro 100W kit. The standard PT secondary is two red + red/yellow.

[novosibir]

Sorry, Metro 100W kit. The standard PT secondary is two red + red/yellow.

Ah-ha - american color code, not british

Then red/yellow instead of yellow has to be disconected temporarily.

Larry

[5150loveeddie]

Every resistor does have two leads - or two terminals - and you always have to connect both leads, that a current can flow through the resistor at all. When you now disconnect only one lead of the both, the current flow stops immediately...

... but the current doesn't care about, which of the both resistor leads you've disconnected

If you're still in doubt, then remove both resistors entirely and put it back in place, after you've done.

Larry

Thx Larry!!

[roylfuchs]

Why in the heck doesn't George include this step in his instructions for the kits? I didn't know about this procedure and I've been running the amp ever since without doing it.

[novosibir]

Why in the heck doesn't George include this step in his instructions for the kits?

When you're using brand new filter caps, the benefit of forming it isn't too much of a difference, also depending on the cap's manufacturer. Marshall, Fender, aso. never formed filter caps after the amps were finished... But I recommend forming filter caps by any means, also when it's effect with brand new caps sometimes isn't more than just like to open up the last 10-15 horsepower out of a Formula 1 engine.

Larry

[5150loveeddie]

LArry, the method works great, I've revived a jtm45 from 1965 and a SL from 1971 they all came back to below 5v accross the 100k after 30hrs plus+, only thing I was worried is exceeding some cap(s) maximun voltage specs, specialy on the SL from 71, It had near 500v on some caps and they are allowed 450v so I had to reduce my wall voltage with a variac to about 100-105v to stay below 450v on the highest cap in there, same happen with the JTM45, voltage went up to 470v after a while....

Whitout the tubes in there drawing current you will exceed in lots of cases that max cap voltage in one or two of them in the amp depending there position in the circuit. So I guess we MUST reduce that I suppose!!

[novosibir]

Yes this has to be considered by forming caps, that the higher B+ due to no load w/o tubes doesn't exceed the max. voltage rating of the caps! Almost always a 470K/1W or a 220K/2W resistor right after the rectifier from B+ to ground helps to pull the increasing B+ down enough.

Don't confuse anything here! The 'pull down' resistor has to be connected right after the rectifier diodes, but still behind the temporarily used 100K (current limiter) resistor, so that this one hasn't to draw the additional current of the 'pull down' bleeder

Larry

[5150loveeddie]

Yes this has to be considered by forming caps, that the higher B+ due to no load w/o tubes doesn't exceed the max. voltage rating of the caps! Almost always a 470K/1W or a 220K/2W resistor right after the rectifier from B+ to ground helps to pull the increasing B+ down enough.

Don't confuse anything here! The 'pull down' resistor has to be connected right after the rectifier diodes, but still behind the temporarily used 100K (current limiter) resistor, so that this one hasn't to draw the additional current of the 'pull down' bleeder

Larry

Ok so basicly we could stick a "pull down" resistor in series with the current limiter resistor (100k) making sure the pull down is first after the rectifier B+? .....

[novosibir]

Ok so basicly we could stick a "pull down" resistor in series with the current limiter resistor (100k) making sure the pull down is first after the rectifier B+? .....

No, not in series with the 100K! From still behind the 100K (rectifier side) to ground, like a bleeder resistor.

Larry

[5150loveeddie]

Ah ok I get it, thank you Sir!!!

[rip]

something like this right?