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Old 26th May 2011, 07:23 PM   #1
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Default Recapping old tube amp

I can't find a can type capacitor with 40uf 20uf 10uf and 4uf. I know there are a lot smarter people on here than myself so I ask some assistance on outfitting this thing with new caps. I tested all the tubes and there are plenty of life in them (telefunken EF86, Bugle Boy made in holland 12AU7, RCA 6AX5, RCA 6V6GT's)

http://recordist.com/ampex/schematic...n/ampex612.jpg


above is link to schematic
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Last edited by famousmockingbird; 26th May 2011 at 07:26 PM.
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Old 26th May 2011, 07:29 PM   #2
DF96 is offline DF96  England
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If the cap needs replacing then just use separate components.
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Old 26th May 2011, 07:33 PM   #3
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I was mainly worried about where to mount the caps. Build a board?
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Old 26th May 2011, 07:37 PM   #4
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Those are simply filter caps coming off the PS - so no big deal (please don't start a debate) in terms of affecting the audio. As DF96 suggested I'd just use some suitable subs. Outside of that check on some of the HAM radio sites - those guys can scrounge up almost anything. The problem is the parts are often old and hard to put full faith in. YMMV
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Old 26th May 2011, 07:50 PM   #5
DF96 is offline DF96  England
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Assuming the 40uF and 20uF are for the PSU, you may be able to buy a 40+40 or 50+50 can which will fit. Then use separate ones for the others. They will be light enough to be supported by their own wires so just solder in to an existing tag if possible. If not add a small tag strip somewhere.

NOS caps will need to be reformed - Google for details. This process applies a small current to rebuild the oxide layer.
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Old 26th May 2011, 08:10 PM   #6
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by famousmockingbird View Post
I can't find a can type capacitor with 40uf 20uf 10uf and 4uf. I know there are a lot smarter people on here than myself so I ask some assistance on outfitting this thing with new caps...
Get a three or two section can cap. With sections at least as large. It is easy to find a 40, 40 or a 40, 40, 20. Then get an axial 4uF cap and wire that in point to point.

It's worth upgrading the size of the filter caps EXCEPT do not make the first one larger than 40uF. I would not even go to 47uF without first checking the data sheet for the rectifier tube. Most tubes have a spec for the largest allowable filter cap. But after that first one going up to a larger size does no harm and likely some good so don't spend a lot of time looking for an exact match.
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Old 26th May 2011, 08:21 PM   #7
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JJ Can Capacitor 40F x 20F x 20F x 20F / 500V

So I could use something like this and just not use the last 20uf section? Can I use a 8uf instead of a 4uf?
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Old 26th May 2011, 09:00 PM   #8
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Imho, add 3-4 small tag strips around the chassis and take all signal earths to them using a distributed star distribution with single point chassis connection. Use two tag strips soldered to the existing electro, such that new individual electro's are soldered between the new tags. This keeps the same aesthetic look above the chassis. New electro's are much much better than old NOS, and so much smaller. You've got ton's of room.

Ciao, Tim
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Old 26th May 2011, 09:08 PM   #9
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by famousmockingbird View Post
JJ Can Capacitor 40F x 20F x 20F x 20F / 500V

So I could use something like this and just not use the last 20uf section? Can I use a 8uf instead of a 4uf?
You can use the last section. 20uF is a more reasonable value anyways. You have to remember that these old amps were built to a cost

The advice above to not even use a can cap is good. Can caps are grounded to the chassis. Better to use a star ground. But the JJ cap look to be a drop in upgrade

Last edited by ChrisA; 26th May 2011 at 09:14 PM.
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Old 27th May 2011, 06:25 AM   #10
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Quote:
Originally Posted by trobbins View Post
Imho, add 3-4 small tag strips around the chassis and take all signal earths to them using a distributed star distribution with single point chassis connection. Use two tag strips soldered to the existing electro, such that new individual electro's are soldered between the new tags. This keeps the same aesthetic look above the chassis. New electro's are much much better than old NOS, and so much smaller. You've got ton's of room.

Ciao, Tim
This is good advice. Modern electrolytics on tagstrip is easy and cheap, and sounds better than multiple section types. The multi-sections often have ripple-current ratings of only 200 .. 300mA, which is not enough for larger amps, and short life results.

I recommend the Panasonic TSUP or TSHA or Nichicon KX, and the Nichicon VR for smaller sizes, as 'known good' sounding.

If you care about the sound quality, please buy brand new caps from Farnell, Mouser etc. Electrolytics degrade quite quickly in storage AND use - they are electrochemical liquid soaked in a bit of hemp, stuck in a can with a rubber stopper. Used caps, ebay sales, "NOS" purchases, Rally/Hamfest, nonfranchised distis and the like are a bad choice. They seem to measure OK when they are 10 years old, for C and ESR (I use a proprietory ESR meter, but the difference is easily distinguishable by listening.
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