Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kwm488 on December 08, 2011, 11:13:01 am
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hi
i plan to build the new marshall plexi 100 amp. the cap is old LCR 50+50uf cap. should i need to reform before i use it? if yes, how to reform?
thank
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To "reform" a filter cap, you bring up the B+ voltage on it slowly (over 24 hrs or so) using a variac on the PT.
But it's a waste of time.
"Reforming" it will not magically make it safe to use, quiet or like new. Get a new can cap. They are not rare or expensive. You can get LCRs or I would suggest F&T or JJ, some folks don't really trust the quality of the LCR but I have no opinion on it personally.
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Hi
Because I believe old LCr or daly 50+50uf should be sound better than new jj and f&t cap.
As the reform, can I do it without variac?
Thank
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I don't think a certain brand of filter cap will "sound better" than another of a given capacitance.
Some parts sellers & manufacturers may express a different opinion, especially when they offer a Sprague for twice the cost of an illinois cap... Some have better quality control standards than others. But I'm sure LCR or Daly will be fine.
But a very old (>10 yrs) electrolytic cap will sound worse than a new one. Reformed or not.
If you must "reform" the old cap, simply apply gradually increasing DC volts to it, up to and a little over its expected operating voltage. Without a variac, I guess you could start with a 9v battery, then connect batteries in series or something...? Again, I think you're wasting your time for a false sense of security.
I "reformed" some old caps (with a variac) in an old Victor stereo one time. Waste of time. It cost me fifteen bucks just to replace the d**n things.
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Hi
I see another method before, mod the Marshall amp(add resistor or so). So the pt will be a variac. I hope to know more of this method.
Also, how can I use 9v battery method?
If I don't reform , will the cap work? Or short the cap life?
Thank
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Hi
I have once more question
What happen if the 50+50 uf LCr cap dead? Because my friend amp have a noise. His amp is real 1968 Marshall superlead which using green color 50+50uf cap
Thank
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Look in here;
http://www.electrojumble.org/reforming.htm (http://www.electrojumble.org/reforming.htm)
I tried it, the leakage current did drop way down after doing this. How long the the cap will now last, I don't know.
Brad :icon_biggrin:
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The green ones are really worth saving.
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Old electrolytic caps are bad news. I think it's ill advised to reform old caps but the procedure is simple enough. You want to put 2 resistors in series so that there is a current flow path. Use something like a 10k and a 1M. Put the cap to be reformed in parallel with the 1M so it will be across the larger voltage. Put your volt meter in parallel with the 1M and cap. Start out with a small voltage, like 10VDC and gradually increase it. When I say gradually, I mean over the course of many hours. Eventually you want toe DC voltage to be = to the rated DC voltage of the cap. When you're done, carefully disconnect the DC supply so that the 1M is still in parallel with the capacitor. It should drain very slowly. 1M at 450VDC should take about 1 minute to drain. If it drops quickly, throw away the cap and call it a lesson learned.
>What happen if the 50+50 uf LCr cap dead?
The first sign of a failing electrolytic cap is hum. This is because the cap is starting to go short. This is putting additional load and stress on the PT. The worse the cap gets, the worse the hum and B+ will start to be lower. Your PT will start to run hot. When the cap goes 100% short, it can take out your PT and rectifier if improperly fused.
I do realize that it is very important to some people that their amp looks original. If you are not afraid nasty ass goo and somewhat talented with a knife, you can open up an old can cap and restuff it with new electrolytics. In a happy power supply, grey caps sound as good as blue, or pink, or even green caps. A more common practice is to leave the old can caps disconnected in place and install new axial caps inside the chassis where they don't show.
Again, old electrolytic caps = bad news. It's a problem just waiting to happen that can ruin a vintage PT. I strongly discourage the use of old electrolytics.
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hi
please forgive me, i really don't know how to check the LEAKAGE of capacitors. do anyone can teach me?
and i own a box of old daly capacitors, i don't want to waste it.
also, my friend marshall amp still using very very old green color capacitors, i forget the brand. so i hope to help him to check.
he get the hum problem now. but i remember 2 month before, his amp no hum problem, and he never used it anymore. yesterday, when he try to turn on and use again, his amp get the hum problem. so i don't sure is the problem come from LEAKAGE of capacitors
(http://i23.photobucket.com/albums/b383/kwm488/IMG_4145.jpg)
(http://i23.photobucket.com/albums/b383/kwm488/IMG_4158.jpg)
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>i really don't know how to check
Remove all of the valves.
Attached volt meter across the 1st filter cap.
Plug amplifier into lamp limiter.
Turn on amplifier and read voltage.
Turn off amplifier and watch voltage. It should drop very slowly. If it decreases quickly, the caps are failing.
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thank you for your help, is it mean i can check the 1st filter cap only? but i hope to check all 6 filter cap
and i am sorry to ask again. if i don't do reform and use the daly cap . will the cap get problem.
thank
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My little test procedure is a quick check to test the overall health of the power supply (all the caps). You would have to pull each one out of circuit and charge it individually to test each 1 by itself. Electrolytics are like car tires, you should change them all before you have a blowout.
In your picture, those caps look used. I see writing on one and some solder flux on another. I wouldn't use them in a customer's amp. If you must use them, you should certainly reform and test them first. Even if they NOS, I think they are from 1975. I wouldn't expect them to last very long. My opinion is do not use them. Sell em on fleaBay and buy some new better capacitors.
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hi
if one of the filter cap is dead, will the votage drop slower or faster after turn off the amp? because my friend have 2 of old green color filter cap in his amp only
thank
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I have a test stand that measures caps. It is a Graham coil tester, but it test caps too. It will charge the cap and measure the leakage. You can find them around vintage auto places. They were originally made to test magnetos and the individual parts inside. Very popular in the 50's and early 60's. Should be able to pick one up for less than 50 buck which is small price considering the cost of caps. They are not like tube testers whereas you have to calibrate them so maintenance cost is no issue.
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If you see any ugly mustard colored yellow caps in there, I'd be glad to dispose of them for you.. You know just a public service, no charge or anything, save the planet and all that..... :icon_biggrin:
Regards,
D.
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If you see any ugly mustard colored yellow caps in there, I'd be glad to dispose of them for you.. You know just a public service, no charge or anything, save the planet and all that..... :icon_biggrin:
Like these old ones I found in the shed? :w2:
(http://i497.photobucket.com/albums/rr333/valvetone/impact/new/mustards.jpg)
:icon_biggrin:
... but none of them are the most important value - .022µF :cry:
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I have four mid 90's LCR 50uf - 50uf @ 500v dual caps that I had bought for a project and never used. I reformed them with my Sprague Tel-Ohmike with current limiting last year. They seem to work fine when I got done with the reforming as the voltage was stable and in an amp they did not hum. I also used one of them briefly in another project and it worked fine for months. I changed it out because I wanted a lower value cap in that application.
If you really wanted some LCR dual caps to use that have already been reformed, then I will sell these if you're interested. I think they would probably work just fine, but as I am sure you understand, I could not sell them with any guarantee or warranty.
If you are interested, then pm me. If not, I will find a use for them at some point with one of my projects.
Greg
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Like these old ones I found in the shed? :w2:
... but none of them are the most important value - .022µF :cry:
Hey show off :icon_biggrin:... What values DO you have?
And a word about old electrolytics.... dump them, buy new.