Forming filter caps

[fredcapo]

Just FYI I have continued the reforming process, progressively reducing the series resistance from 100k to 50k to 25k over a period of 6 days. The voltage drop came down to under 5 volts. I restored the circuitry and powered the amp first at 80 percent voltage then dialed up to full. The amp sounds great. I have not left it on for more than 15 minutes at a time. I may make some other measurements of voltage, ripple, etc before I deem it good-to-go. I will be selling the amp for the owner in the near future. Thanks.

[syscokid]

As I'm posting this, I'm forming new filter caps on a vintage Ampeg VT-22 (100 watts). In the partial schematic below, I detached one end of R56 and R57, and there is no centertap. The voltages dropped real fast to just under 55vdc. But now it's really slow...currently showing about 45vdc...30 minutes since at 55vdc.

Do I need a lot more patience?

Or am I missing something?

Should I detach one end of R55 ?

Ampeg VT-22.JPG (114.09 KiB) Viewed 4613 times

[novosibir]

Should I detach one end of R55 ?

No, but one end of the R54

Larry

[syscokid]

Should I detach one end of R55 ?

No, but one end of the R54

Larry

Thanks Larry.

Right before I detached one end of R54, the voltages were at 28vdc. I just got thru trying that, and the voltages jumped to 89...and never moved from there for at least 1 hour. If I bypass it straight, with no resistor...it will blow the fuse...

I reverted by re-attaching R54...and as of this reply it's down to 18vdc! I'm confused...

I'll leave it with this set-up and check the amp after a good nights' sleep.

[syscokid]

After 24 hours, the voltage never went below 16vdc. I reset the amp, and turned it on, and all seems fine. Big difference with fresh caps in there.

I also forgot to test the new caps with my ESR meter (...I keep forgetting I have one!). I'll leave that for another time.

[ckpop1]

Hello, I will be reading this whole thread on reforming capacitors but here is what I am wondering ?

The method Larry uses is for reforming the capacitors in a amp at the same time. He mentioned about getting the measurement 5v or below. With lets say 6 caps all together in a 100watt does that mean the voltage drop becomes less then 1v per cap when they are reformed all together ?

Another question when reforming a 50/50 capacitor by itself out of amp I see several people using different value dropping resistors. For example I have seen others use 10K, 30K, 100k, 470K. How is that value calculated and is there a best value for reforming a individual 50/50uf cap ?

I am just trying to get a better understand, Thanks

[ckpop1]

Hello, If you Reform a 50/50uf cap not in the amp what is the correct value dropping resistor that should be used. I have seen others use 10K, 30k, 100K and 470K ?

[Trainwreck]

Hi,
I´m not a pro, but I think it depends on your voltage you use for. With the usual 50/100 Watt Marshall PT I would take 100k. Thats the way I understood.

Best

Hans-Jörg

[Lefty Lou]

Pardon me one and all, especially if this has already been discussed on one of the currently (18) pages to this thread BUT, wouldn't this work just as well Larry?


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You use progressively higher wattage bulbs to gradually raise the voltage seen at the amp's power source. Always start out with lower-wattage bulbs, i.e., 15-watt, 30-watt, and gradually work up to 60-watt, 75-watt, 100-watt. If the lower wattage bulbs, get bright STOP IMMEDIATELY! This indicates a high current fault within the amp.

[Trainwreck]

Hi,
the caps will not have their own time they need to reform, because you determin how fast when you swap to stronger bulbs. So its not the same as the cap choose its time to be propper reformed.
So - or similar - was the answere somewhere back.

Best

Hans-Jörg

[erigm]

Hi Larry,

Thank you so much for your first post and starting this thread!!! I've read through the whole thread, so I apologize if the answer to this was posted earlier. What percentage of the capacitor's rated voltage should they be held at when forming the capacitors. For example, if the capacitor is rated for 450V, at what voltage should the power supply be (VDC after the rectifier)? Thanks!

Eric

[jbhunn]

I form caps at 10% under the stated voltage rating - so 450v for a 500v cap for example. However - I always use a variac to make sure the voltage across the current-limiting resistor is never higher than 100v and gradually bring it up to 20% below the rated voltage - 400v for a 500v cap. As the electrolyte forms and the voltage drops across the limiting resistor(s) the rectified voltage increases - so monitor it. Then I increase the voltage to the -10% range to finish the process and monitor the voltage across the limiting resistors until the voltages stabilize.

[erigm]

I form caps at 10% under the stated voltage rating - so 450v for a 500v cap for example. However - I always use a variac to make sure the voltage across the current-limiting resistor is never higher than 100v and gradually bring it up to 20% below the rated voltage - 400v for a 500v cap. As the electrolyte forms and the voltage drops across the limiting resistor(s) the rectified voltage increases - so monitor it. Then I increase the voltage to the -10% range to finish the process and monitor the voltage across the limiting resistors until the voltages stabilize.

Thanks jbhunn! I was planning on using a variac as well, so thanks for the tip about the 100V across the current-limiting resistor. I will do that. And the 20% rule, then bump to 10% sounds like a good idea too.

Larry gave us a hint about submerging the caps (to just below the terminals, so the terminals are not submerged) to de-crystalize the electrolytic gel. I was wondering if there isn't a "dry" way to do this … like a real low temp oven, or dehydrator, or something. Ideas?

Eric

[Homebelly]

Hoping some one might jump in and comment on this diagram.
I'm wanting to form the caps in my JTM45/100 dual rectifier EL34 build.
I'm thinking i might need two 100K resistors...

Although, while i write this and look at the diagram i am also wondering if i just need to disconnect the second rectifier at the negative end of the cap's and just use the single 100K at the positive end as shown..?

Any help appreciated.

cap forming by Homebelly, on Flickr

Link to original build thread..
http://forum.metroamp.com/viewtopic.php?f=32&t=38418

[axeman]

I forgot how to form a 12xxx, can't find instructions