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100uF too much for a 5Y3?


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10/22/2000 1:57 AM
Chris B
100uF too much for a 5Y3?
I have a dual 100uF filter cap for the power supply on my next project. I remember reading somewhere that a large filter cap is hard on tube rectifiers. How much capacitance is enough and how much is too much?  
 
Chris B
 
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10/22/2000 2:03 AM
Jim Jones

Hi Chris,  
 
Yep, if you're using a 5Y3, 100uf is too much right after the rectifier tube. According to the data sheets 32uF is about tops. A lot of the old amp schems have 16 or 20uF after the recto.  
 
Jim
 
10/22/2000 2:21 AM
Chris B
Why is too much capacitance a bad thing?
I'm currently using an amp with an 80uF cap right after the 5Y3. Is this a bad thing? I tried a three section 20uF cap and it didn't filter the supply enough. It still hummed like crazy. With the 80uF the hum is lowered alot but isn't gone completely. I'm not using a choke. I have resistors in the supply.  
 
I have a Silvertone that has a 20uf, 10uF, & 5uF filter cap and no choke, only resistors, and it dosen't hum much at all. Why did the 20uF hum in the homebrew?  
 
Chris B
 
10/22/2000 2:49 AM
Winnie Thomas

I'm currently using an amp with an 80uF cap right after the 5Y3. Is this a bad thing? I tried a three section 20uF cap and it didn't filter the supply enough. It still hummed like crazy. With the 80uF the hum is lowered alot but isn't gone completely. I'm not using a choke. I have resistors in the supply.  
 
I have a Silvertone that has a 20uf, 10uF, & 5uF filter cap and no choke, only resistors, and it dosen't hum much at all. Why did the 20uF hum in the homebrew?  
 
Chris B
 
 
There are other reasons why the homebrew would hum than filter caps. Do you have the filter caps all grounded together? If you ground all but the preamp filter caps together, and then ground all the other grounds to the same, but another spot, the hum will be lessened. I learned this the hard way.  
 
Winnie
 
10/22/2000 3:55 AM
Mark Abbott
Re: The larger caps place great stress on the 5Y3.
As subject.  
 
10/22/2000 4:06 AM
Chris B
Re: Why is too much capacitance a bad thing?
The filter cap that I have in the homebrew is an old twist-lock type cap and the can itself is the ground. With the cap locked to the mounting plate and the plate bolted to the chassis, it's grounded about the best I can do. Would a replacement cap be the answer?  
 
Chris B  
 
P.S. Would the 100uF cap work with SS rectifiers? I believe I have seen Marshall schematics with high value caps used with two SS diodes.
 
10/22/2000 5:58 AM
Peter S

Chris,  
The 100uf cap will work with a ss recto, but it sounds to me like you may have a ground loop problem. Read R.G. Keen's article on star grounding in guitar amps. You probably won't need to completely star ground the amp, but using some of the basic principles for why star grounding works will make your amp as quiet as a Church mouse.  
 
Peter
 

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