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Old 9th January 2012, 02:33 AM   #1
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Default 5AR4 rectified power supply question

My amp uses a 350-0-350 transformer into a 5AR4 loaded with a 10uF Solen film cap, then Hammond 10H 100mA choke (262-ohm), then a JJ 500uF 500V cap. It looks like the previous owner added the 500uF cap since the amp schematics call for a 47uF + 47uF Elna Cerafine in this location.

Is it bad for my 5AR4's to have this much capacitance in the power supply?

Would it be worthwhile to replace the JJ cap with, say, a Mundorf M-lytic HV 50+50 or 100+100 in parallel with a good film cap?

Thanks all!
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Old 9th January 2012, 10:48 AM   #2
DF96 is offline DF96  England
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The 5AR4 does not see the 500uF cap, so that is OK. The potential problem may be that the PSU could have a poorly damped subsonic resonance - bigger caps are not necessarily better, but some people seem to imagine they are. Calculate the resonance frequency and Q for 10H, 262 ohms and 500uF all in series.
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Old 9th January 2012, 12:44 PM   #3
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Peak repetitive current will be OK, since that's primarily due to the 10 uF input cap. I think it'll be OK on inrush too due to the slow warmup of the 5AR4 -and even a fast-heating rectifier like 5Y3 would probably be OK due to the resistance of the choke.

Increasing capacitance decreases the Q - damping can be a problem if output capacitor is too small.
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Old 9th January 2012, 01:03 PM   #4
DF96 is offline DF96  England
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Yes, in this case the resonance is so low (2.25Hz) that choke resistance provides enough damping. For higher resonance frequencies one would have to rely on the PSU load adding some damping too.
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Old 9th January 2012, 01:56 PM   #5
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DF96,

Can you show the equation used for calculating the "resonance frequency and Q for 10H, 262 ohms and 500uF all in series"

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Old 9th January 2012, 02:03 PM   #6
DF96 is offline DF96  England
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f = 1/( 2 pi sqrt ( LC ) ) - usual LC resonance formula.

then Q = 2 pi f L / R

In reality the situation is complicated by the finite effective resistance of the rectifier and the equivalent resistance of the reservoir cap being discharged and recharged at twice mains frequency. Then you need to consider the load resistance too.
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Old 9th January 2012, 02:25 PM   #7
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Thanks
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Old 9th January 2012, 04:24 PM   #8
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When I model this power supply in PSU Designer, I get a warning that the rectifier IFRM of .75A is exceeded with a value of 1.61A at time 0.168. Does the program not account for the slow turn-on of the 5AR4?

Also, since the JJ cap is the output cap of the power supply, would a higher quality cap result in improved sound?
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Old 9th January 2012, 04:42 PM   #9
DF96 is offline DF96  England
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I don't think PSUD2 knows about valve heaters.
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