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Old 7th November 2010, 05:38 PM   #1
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Default Coupling cap replacement in series?

Hey guys I'm trouble shooting a guitar amp conversion.

I want to replace the .047 600V coupling cap(s) that go into my 12AU7 phaze inverter.

I have 3 .2 caps I can series wire to give me the required 600V rating. It works out to a .06 600V cap.

Do you see anything wrong with that?

The leaky cap is on the left, the new ones on the right.

I have a thread in the musical instruments section.

http://www.diyaudio.com/forums/music...mp-thread.html
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Old 7th November 2010, 06:44 PM   #2
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Default .047 600V caps are dirt cheap, but......

I think you would be closer with 4 in series, yielding .05uf and 800V with proper dropping R's across each cap.

You'll need to put balancing resistors across each cap so that they will equally share the voltage. 220K or so is typical.

You need the resistors because the resistance of each cap can vary, and they will not share the voltage evenly.....one can easily exceed it's 200V rating without the R's.

Last edited by boywonder; 7th November 2010 at 06:47 PM.
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Old 7th November 2010, 07:06 PM   #3
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Quote:
Originally Posted by boywonder View Post
I think you would be closer with 4 in series, yielding .05uf and 800V with proper dropping R's across each cap.

You'll need to put balancing resistors across each cap so that they will equally share the voltage. 220K or so is typical.

You need the resistors because the resistance of each cap can vary, and they will not share the voltage evenly.....one can easily exceed it's 200V rating without the R's.
That's a really bad idea for coupling capacitors. Those 220K resistors in series will make a DC voltage divider with the following stage grid DC return resistor, and that will throw off the bias. He'd be better off sticking with the leaky coupling capacitor.

It would be better still to try to obtain a coupling capacitor with the 600V rating. AuriCaps come in 0.47uF / 600V (or very close to it).
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Old 7th November 2010, 07:37 PM   #4
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Ok sounds good.

I did try it real quick, it did "work" and cleaned up the sound a little.

I do have some other older caps that may be working.

Just playing around to see what I need. Making a list for new caps, but as this is a conversion project I got to fiqure out all my changes first.

Thanks guys.
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Old 7th November 2010, 07:43 PM   #5
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Quote:
Originally Posted by Miles Prower View Post
That's a really bad idea for coupling capacitors.
Thanks for the clarification Miles, didn't think about the application..............
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Old 7th November 2010, 10:57 PM   #6
jjman is offline jjman  United States
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The trio looks like "brown turds" like Fender used in the later 60s / early 70s. They have a bad rap but all of mine test strong and do not leak dc.

I'm all for improvising when needed but I would not do that. To scary for me.
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Old 7th November 2010, 11:05 PM   #7
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Ok I removed them.

Is there a slim chance the 600V is not required for the phaze inverter?

I also have some .04 pancake style coupling caps (everywhere in the signal path) I would like to replace, however I don't know what voltage they are. Any suggestions?
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Old 7th November 2010, 11:24 PM   #8
jjman is offline jjman  United States
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Fair chance the PI's voltages are below 400.

400v and 630v versions:

150473J400DB Mallory Polyester Film Capacitors

150473J630EC Mallory Polyester Film Capacitors

May as well go 630v.
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Old 8th November 2010, 02:33 AM   #9
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Ok, well I managed to scrounge up a pair of Sangamo .1 600V.

Seems to be ok, sounds great. I wonder if I should get .1 rather then the .047 that was in there. What do you think?
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