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Old 17th May 2012, 08:23 PM   #1
oneir is offline oneir  United States
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Default Recapping tube preamp questions

Hello, this is my first post and I have a question or two about replacing the electrolytic caps in a tube preamp (Audio Research SP-10mkII). I should state right off that I don't know a cathode from a commode, but that will probably be immediately obvious from the nature of my questions. The main question has to do with value of a suitable replacement part in the SP-10 power supply. There are different supply rails I guess you would call them in the power supply chassis, (you can view the PS schematic here http://www.arcdb.ws/SP10/ARC_SP10PS_schematic1.gif, and other SP-10 info at this link: ARCDB - SP-10 ) Different kinds of capacitors are used on the different rails: one has Mallory and the others have El Cap. Recently, I noticed that the El Caps in the ps unit, which are smaller than the Mallory and have a shiny aluminum skin, have slightly bulging tops on the positive end and the orangey-ochre material at the top was slightly cracked looking. I don't remember what they looked like originally but I'm pretty sure that slightly bulging and cracked are not good signs. Having decided to replace those caps before they fail I started to look for some replacement parts. The originals are 6000uF 25v axial leaded cans. The closest thing I could find at Mouser.com was a Vishay/Sprague part rated at 6200uF 25v. It has the same axial type construction, so nothing will need to be adapted or changed to fit on the PS board. Is the 200 uF difference in storage capacity going to cause a problem if I substitute this for the original 6000uF part? That is the main question I am looking for help on.

Also, there is another less urgent question. The schematic included in the SP-10 owner's manual indicates a 5000 not 6000 uF capacitor in these positions (C17 and C18 as seen here http://www.arcdb.ws/SP10/ARC_SP10PS_schematic1.gif ). The other rail with the Mallory caps also diverges from the schematic. In the schematic, those capacitors are listed as 200uF at 400v. But the parts in the actual circuit are 180uF at 350v. This preamp has never been worked on before or modded in any way. My family are the original owners, having purchased it from an ARC dealer in 1986, so I know the parts inside are what ARC shipped it out with. The 350v Mallory caps supply a pair of 6L6 tubes in the powersupply. These caps don't show any bulging or other sign of needing to be replaced - but I thought I'd go ahead and replace them anyway as a precaution and since I'm already opening the hood and changing stuff.

I found exact replacement parts for these - 350v 180uF and even showing the same manufacturer's model number. Although the maker is now listed as Cornell Dubilier, the outside of the replacement caps says Mallory (I guess they absorbed Mallory's old stock). Should I use the replacement Mallory caps since they are exactly what was used in the powersupply or should I hunt for what the schematic says ought to be there (400v 200uF caps - which in some pictures appear to be Sprague Atoms)?

Thanks for any insight and advice.

Edit: I meant to include some part numbers. The candidates for replacing the El Caps have a Vishay/Sprague model number:
53D622G025HL6 which can be seen here:
53D622G025HL6 Vishay/Sprague Aluminum Electrolytic Capacitors - Leaded
They are to replace El Cap 6000uF 25v parts marked 8438. I can't find much information about these El Caps

The Mallory replacement caps have the same part number: TCG181T350N3C as seen here:
TCG181T350N3C Cornell Dubilier Aluminum Electrolytic Capacitors - Leaded

Last edited by oneir; 17th May 2012 at 09:52 PM.
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Old 17th May 2012, 09:33 PM   #2
DF96 is offline DF96  England
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Paragraphs might help people find your question, so they are able to answer it.
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Old 17th May 2012, 09:39 PM   #3
SY is offline SY  United States
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Don't worry about the brand too much, try to match the values and voltage ratings. It's OK to go higher in voltage. You'll want the footprints to resemble what was in there so you don't have to do much mechanical modification. Use 105 rated caps. Buy from a reputable supplier, NOT from some guy on EBay. Watch polarities- that's an explosive mistake.

Safety first! If you're not experienced on working with high voltage circuits, get some help from someone who is.
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Old 17th May 2012, 09:42 PM   #4
oneir is offline oneir  United States
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Quote:
Originally Posted by DF96 View Post
Paragraphs might help people find your question, so they are able to answer it.
Sorry about that - I wrote it with paragraphs, but somehow the carriage returns get stripped from the post.
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Old 17th May 2012, 10:10 PM   #5
DF96 is offline DF96  England
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OK. Now its much easier to read.

Big electrolytics caps typically have quite loose tolerances on value +-20% or even more. This means that 180uF and 200uF can be regarded as the same thing. Similarly for 5000 and 6000. Look for similar values (within about 20%) and equal or higher voltage rating, combined with the right shape to fit in the space. Ripple current rating might be an issue, but harder to advise on this.
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Old 17th May 2012, 10:57 PM   #6
oneir is offline oneir  United States
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so if all other things, like the voltage ratings and temperature ratings, were held equal, a 5700uF part would be a safer replacement than 6200uF?

The preamp has a 6000uF part in it, but the schematic says it ought to be 5000uF. The 6200uF part (which I have in hand) is 24% over the figure that is listed in the schematic. The part that ARC actually used is already 20% greater than specified. I ordered the 6200uF part because there was no exact match and it was closer (on the high side) to what I saw in the power supply chassis than the next closest available option on the low side (5700uF). <br><br> Then I started to wonder if I shouldn't be trying to match or stay close to the example of the schematic, instead of taking the SP-10 specimen in front of me as the definitive guide. I had read of the 20% rule before, and so while my primitive mind is biased toward believing more storage is better, knowing there is a divergence between the schematic of the preamp and the physical preamp has caused me to have some second thoughts. Is 6200uF so different from 5000uF (+24% ) that I should put them aside and get something within 20%?
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Quote:
Originally Posted by DF96 View Post
OK. Now its much easier to read.

Big electrolytics caps typically have quite loose tolerances on value +-20% or even more. This means that 180uF and 200uF can be regarded as the same thing. Similarly for 5000 and 6000. Look for similar values (within about 20%) and equal or higher voltage rating, combined with the right shape to fit in the space. Ripple current rating might be an issue, but harder to advise on this.
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Old 17th May 2012, 11:42 PM   #7
SY is offline SY  United States
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More storage may or may not be better, but more ripple current is definitely not. So don't go crazy on capacitance.

Anyway, a good quality cap from 4700u to 6200u should be fine.
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Old 17th May 2012, 11:50 PM   #8
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Okay, maybe I'm the one with a weird way of looking at this, but .....

There is every indication that the original manufacturer wasn't particularly fussy about the value of that cap. Therefor there isn't any reason for you to worry much about it either. I consider 6200 a very reasonable sub for a 5000 and as a replacement for a 6000 it's basically perfect; it's the nearest standard value..
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Old 18th May 2012, 02:18 AM   #9
oneir is offline oneir  United States
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Quote:
Originally Posted by SY View Post
More storage may or may not be better, but more ripple current is definitely not. So don't go crazy on capacitance.

Anyway, a good quality cap from 4700u to 6200u should be fine.
Okay, I am going to stop worrying about it now. In any case, there doesn't seem to be a lot of choice left in axial type caps that could be used. The 5600 uZ Sprague part at this voltage is over three times as expensive at about $40 a piece. So I'm lucky to find identical Mallory parts and some Spragues that will work in place of the El Caps and with the right "footprint", as you say.


The main thing is to preempt any catastrophic failures in the powersupply, but with these parts it should look 'period correct', as well. Hopefully the sound will be unchanged from stock.


There is one last question I have about this project:
Would it be a bad idea or a good idea to "form" these new caps in the powersupply using a variac? And is this best done in the destination circuit where they will be used, or is it better done in some kind of test mule circuit?


I know this forming of capacitors subject sometimes has started spirited discussions but I thought I should seek an opinion. These Mallory and Sprague capacitors are possibly NOS (both Mallory and Sprague appear to have been acquired by other companies) and who knows how long they've been sitting idle. Also, when I look at the PS of this preamp it seems large and complicated to me (but I would need three chances to successfully wire up a flashlight). So I wonder if there might be something in there that makes it not such a good idea to bring the mains voltage up gradually. Could forming a capacitor be done on some kind of test bed circuit?
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Old 18th May 2012, 02:35 AM   #10
oneir is offline oneir  United States
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Quote:
Originally Posted by Ronsonic View Post
Okay, maybe I'm the one with a weird way of looking at this, but .....

There is every indication that the original manufacturer wasn't particularly fussy about the value of that cap. Therefor there isn't any reason for you to worry much about it either. I consider 6200 a very reasonable sub for a 5000 and as a replacement for a 6000 it's basically perfect; it's the nearest standard value..
That's reassuring. Thanks for your reply. It might be the case though that they weren't particularly fussy about updating the schematic they supplied with the owner's manuals. They may have cared about the value of the capacitor but not cared very much about apprising owners of changes to the circuit. This preamp is a MkII version and the schematic might be more accurate as a description for a MkI than a MkII. My ability to read schematics is 99.9% nonexistent - especially when it comes to checking what's in the case vs. what's on the diagram. ARC may have made other changes to the circuit in the MkII that would call for the caps at those positions to be 6000 instead of 5000uF as the schematic would have me believe - and I would be unable to see what the changes were or why they were made. I can't tell whether the base value I should be trying to stick close to is really 5000 or 6000uF. But as long as people who do understand this stuff say that 6200uF makes a decent substitute for either 5000 or 6000, and that it won't explode upon turn-on, (if installed correctly) then I am not going to worry about using it.
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