Electrolytics in series ... question?

[Diverted]

Hi,

Getting ready to recap a Stromberg Carlson AU33 amp. The first two power supply filters are rated at 30uf@525v according to the schematic. Hard to find 525V caps so am considering replacing them with two 60uf@350v caps in series. That should give me 30@700v. Only question is, should I be running distributing resistors in parallel with the series'd caps? And if so, how would I settle on an appropriate value? Thanks!

[jjasilli]

The Manual has them rated @ 500V.  See: http://audiophool.com/MadeInRoch.html  Scroll down to your amp.  You need to download the djvu app to read the file.


Doug has a suitable replacement here:  2X 32uF @ 500V:  http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Capacitors&ORDER_ID=464967223


Antique has at lease 3 suitable replacements.


N.B.  If you go the series cap route (which i don't recommend), then:
1.  The cap can CANNOT BE THE TOP CAPS.  Because the can shell is grounded you'll short out the Power Supply and make the chassis LIVE.  This is true unless you use an isolated cap can mount, but then the can shell will still be live and will need to be covered in insulation.


2.  Yes, use bleeder resistors.  There's a lot of voltage so it should be distributed to the bottom cap in the series chain. 

[Diverted]

Yes, use bleeder resistors.  There's a lot of voltage so it should be distributed to the bottom cap in the series chain. 

Thanks for the tips. Yep, using can caps is a no-no in this kind of application as it won't work.
Anyway, the can on my amp is stamped with 525V as is the schematic pasted to the inside bottom (see pic). I don't want to take a chance using 500V caps.

So getting back to something you mentioned: Why would you not recommend caps in series?

And my original question remains: How do I select the correct dropping resistors if I were to run caps in series?

Thanks!

[SILVERGUN]

From Merlin's site:

[sluckey]

The actual value of the resistors is not critical but they need to be the same value so the voltage will split equally. 220K is a very common value used for this.

[jjasilli]

+1.  These are not dropping resistors, but bleeder resistors.  Not only do they discharge the caps at turn-off of the amp, but also distribute the voltage to the lower cap.  This saves the upper series cap from getting slammed.


Posters should always !!!POST THEIR SCHEMATICS!!!  Pet peeve rant over.  Perhaps there are different editions of this amp. 


I already pointed you to suitable 525V cap cans.


I don't recommend the series caps in this case, because it unnecessarily complicates the matter; and raises the safety issues I mentioned (though this can be safely addressed).

[Diverted]

+1.  These are not dropping resistors, but bleeder resistors.  Not only do they discharge the caps at turn-off of the amp, but also distribute the voltage to the lower cap.  This saves the upper series cap from getting slammed.


Posters should always !!!POST THEIR SCHEMATICS!!!  Pet peeve rant over.  Perhaps there are different editions of this amp. 


I already pointed you to suitable 525V cap cans.


I don't recommend the series caps in this case, because it unnecessarily complicates the matter; and raises the safety issues I mentioned (though this can be safely addressed).

Thanks. Sorry about the "dropping resistor" quote. It was a typo.

Also, thank you for the link. Though you mentioned I could find the correct 525V cap cans there, I couldn't find any; the highest rated that O could find is 500, which doesn't help.

Finally, sorry about not posting schematic. Here is a high res version. Just click on photo to enlarge:

https://imgur.com/a/pcdhu

[Diverted]

From Merlin's site:

Thanks Silvergun and Sluckey. This is just what I was looking for. An added bonus is that the resistors will bleed off the B+ when the set's powered down. Thank you!

[sluckey]

Here's a listing for all the old style "twist-lock" cap cans that AES has. Beware, pricey! ...

https://www.tubesandmore.com/products/capacitors?filters=Type%3DMulti-Section%20/%20Can%20Type%2CBrand%3DCE%20Manufacturing

And these two cans will probably replace both of your cans...

https://www.tubesandmore.com/products/capacitor-ce-mfg-525v-303030uf

https://www.tubesandmore.com/products/capacitor-ce-mfg-450v-20101010-f

These caps will be a drop in replacement if the original cans are 1-3/8" diameter and mount on a plate like this. (Sometimes the caps will mount directly to the chassis without any extra plate, if the chassis has the four tab slots cut.)

https://www.tubesandmore.com/products/mounting-plate-metal-1375-can-capacitor

[sluckey]

One more then I'll shut up. Look at this schematic. It contains a voltage chart. The highest voltage on that chart is 415v. I would not put too much emphasis on using a 525v cap.

http://theused.com/wp-content/uploads/resource/stromberg_au-33_ps-33.pdf

[PRR]

Upgrade to film caps.

Mouser.com " EPCOS / TDK B32778P6356K000 "  35uF 10% 630Vdc
https://www.mouser.com/datasheet/2/400/MKP_B32774P_778P-1220067.pdf

[mresistor]

Dave at justradios.com has electrolytics rated at 600v.  Click on the link and scroll down, middle column.  Store is in Canada fyi.


http://www.justradios.com/cart.html

[mresistor]

This ebay seller also has 600v electrolytic caps.     https://www.ebay.com/itm/QTY-3-New-MIEC-33UF-600V-105C-Axial-Electrolytic-Capacitors/321441865758?hash=item4ad76da01e:g:1W4AAOSw8lBTqIeH

[Diverted]

Thanks for the help, all. Caps ordered from justradios. I appreciate it!

[sluckey]

When you finish this project please tell us what your measured B+ voltage is.