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Old 14th February 2003, 12:51 PM   #1
G is offline G  United States
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Default Too much capacitance in power supply?

Hi All,

I have a little SE 6BQ5 amp out of a console that I plan on rebuilding on a new chassis with a 5687 as the gain tube. The little amp uses a 4 section 40uF 350 v capacitor in the power supply. I recently purchased 4 Seimens 330uF/ 385 volt snap in caps that I plan on using in the power supply. The power supply is not a choke input so I think that the new caps will work fine but will the amp sound better or worse with the increase in capacitance? I can just use 2 JJ dual section 40 uF caps. Thanks.

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Old 14th February 2003, 12:53 PM   #2
dhaen is offline dhaen  Europe
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Gavin,

What kind of rectifier?
Is there a choke?

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Old 14th February 2003, 12:59 PM   #3
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I'm torn between a 5AR4 for the slow warmup or a 5Y3GT which is what came with it. There is not a choke in the power supply. What do you think?

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Old 14th February 2003, 01:39 PM   #4
dhaen is offline dhaen  Europe
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Gavin,

You'd better not use big caps like that with valve rectifiers. They won't last long at all.
If you want to use valves for rectification, you'll need to use the small cap option.

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Old 14th February 2003, 01:48 PM   #5
Gunders is offline Gunders  Norway
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If the power supply is using vacuum tubes for rectifing you can't use large capacitance capacitors.
For vacuum tubes there's a limit for the capacitance of the first tube. usually this limit is between 32uF-56uF.
One of the problems with high capacitance capacitors is the start-up current and the current peaks during operating.
Vacuum tubes does often have a quite low max current rating, about 1,5 - 2A
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Old 14th February 2003, 02:00 PM   #6
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Thanks guys. 40 uF it is.

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Old 14th February 2003, 02:04 PM   #7
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Default LC

Hi Gavin,

If you can get at least one choke for your PSU pls. fit it after the rectifier.
In doing so you can than add a decent amount of reservoir caps after the choke.

If you don't you'll end up with a whimpy PSU and a rectifier that gets hit by the caps charging at switch on thereby risking arc over etc.

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Old 14th February 2003, 03:47 PM   #8
Bobken is offline Bobken  United Kingdom
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Hi,

Beat me to it, Frank!

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Old 14th February 2003, 04:00 PM   #9
Joel is offline Joel  United States
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It's not just a question of saving the rectifier, the larger the capacitance, the more VA needed from the power transformer to maintain the charge. Your hum can actually increase with too much capacitance, and you can also shorten the life of your transfrmr in extreme cases.

You can live by this statement: no tube power amp PSU needs any one capacitor to be larger than 40uF.
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Old 14th February 2003, 04:13 PM   #10
Morse is offline Morse  United States
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Hi Gavin;

I'd use something even smaller than 40uF in front (say 20uF) of your choke and finally put in something stouter (e.g. parallel the 40+40uF cap). If money and space in your chassis permits a pair of chokes, you might try a 2pi filter with an even larger value cap as tail end Charlie.

By the way, why not model your PS with PSUD II while you're at it? Depending on your choice of rectifier and the choke(s) you put in you might be able to put in rather more than 80uF after the choke and still not draw too much current from your rectifier.

Also, don't forget current inrush limiting thermistors - they are cheap and they can be switched out of the circuit after turnon, either with a manual switch or a timed relay.

Good luck and all the best,
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