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Cradle22 16th March 2003 01:01 PM

Replacing ceramic caps - do or don't?

Since I'm building my P3A amp right now, I was wondering if I could replace C4 and C6 (100 pF ceramics) with Wima FKP caps, which I also have?

Somebody once told me that replacing ceramics wasn't such a good idea, so I am interested in the pros and cons of such an idea...

And generally speaking, can I replace normal ceramics with glimmer caps? C2 is 220 pF, and I also have some 220pF glimmer caps lying around...?

jean-paul 16th March 2003 01:09 PM

Wima FKP is an excellent choice, certainly compared to ceramics. Rod himself stated that Styroflex was a good choice also.

Don't know about glimmer caps because I don't know what they are :confused:

I am building a P3A too at the moment because of all the positive rumours on this site. Although I am reluctant building class A/B amps with feedback I want to give this one a try and I'm very curious how it will sound. Mailed Rod a few times about the design and my version will be slightly different but he assured that my mods were possible. Differential transistors will be MAT02 and emitter resistors of the power transistors will be 0.22 Ohm 1 % 90 ppm low inductance Dale that were doing nothing in my drawer. Input cap is a BG N series 4.7 uF 50 V. Power supply will be + / - 25 V ( 2 x 18 V transformer ) and output power 30 W otherwise the 0.22 Ohm were not safe.

Wanted this mods cause I had all these parts available and did not want to go out and buy the standard ones. BTW the way Rod treats his customers ( he is always ready to give advice ) is an example for others.

Christer 16th March 2003 01:13 PM

Glimmer = mica

Mica is called glimmer in swedish, so it is most probably the
same in german (since he used the word).

jean-paul 16th March 2003 01:15 PM

Mica is better than ceramics too. I think I would choose the FKP.

Cradle22 16th March 2003 01:39 PM

Thanks. Actually I am glad that you agree on replacing them, because I already did on one side...

I am very eager to try it, but I forgot to buy the heatsinks... and have no case yet...

I will stick to the original design, using MJ15003/MJ15004 as power transistors, and a 30-0-30 500 VA toroid. I haven't yet decided on the rectifier diodes... probably the BYV 28/200 I have lying around are not strong enough? Tomorrow I will also get 8 MRU 860, maybe I will take them...

One thing that was misleading in Rod's building plan, is that for C1 he first gives the value of 1uF (which I actually bought), and later he uses 4.7 uF (which I think I should use, 1uF would be a little bit on the low side, wouldn't it?)

I use MKP, and boy, they get big...
But the caps I use on Rod's Designs are always somewhat bigger than the design on the board, I always have to bend some pins in order to fit them... for example, C+ and C- on P3A... I am using MKP, and while the punctures have the correct distance, both caps are put to near together on the board, so I have to move one a little bit to the edge of the board...

But yes, Rod's help is very good. Sometimes I ask him directly, sometimes (if it is more a "general" question, not specifically on his design) I ask my questions here...

Heating up the soldering iron now...



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