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Old 9th June 2010, 05:58 PM   #1
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Default capacitor value

my question refering to the schematic is whether or not it is necessary for the "optional" capacitor to be 630 volts.

i can find 500v and 450v ones easily

thanks
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Old 9th June 2010, 06:35 PM   #2
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Don't have much knowledge where valves are concerned but I would guess so, because the voltage at the output of the transformer will be higher than it is after the resistors (because they drop a bit of voltage). Do remember though, if you can't find 630 volt caps easily, it's always O.K. and sometimes better to go for a higher voltage.
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Old 9th June 2010, 06:43 PM   #3
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Yes the higher voltage will work. However, when you put it in the B+ will increase a little and there will be higher peak currents through the rectifier and the transformer.

If you don't need the extra voltage, leave it out. The 33k resistors show that the load is very light, so that capacitor will give no advantage.
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Old 9th June 2010, 06:45 PM   #4
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Quote:
Originally Posted by jackand08 View Post
Don't have much knowledge where valves are concerned but I would guess so, because the voltage at the output of the transformer will be higher than it is after the resistors (because they drop a bit of voltage). Do remember though, if you can't find 630 volt caps easily, it's always O.K. and sometimes better to go for a higher voltage.

i measured the voltage and it is 350v before the first cap so i dont understand why it is listed that the value should be 630v when 450v is more than adequate

anyway thanks for the reply
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Old 9th June 2010, 06:51 PM   #5
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Originally Posted by bob91343 View Post
Yes the higher voltage will work. However, when you put it in the B+ will increase a little and there will be higher peak currents through the rectifier and the transformer.

If you don't need the extra voltage, leave it out. The 33k resistors show that the load is very light, so that capacitor will give no advantage.
im getting hum and i attribute this to dc ripple. am i wrong?

thanks
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Old 9th June 2010, 07:10 PM   #6
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The hum could be for a number of reasons, it could be DC ripple or it could be an incorrect grounding setup. Just check your grounding and that (if it's an old amp) no capacitors are dead as dead caps will not filter out any hum. You can try lifting the ground in various places (such as the inputs or volume control0 but make sure that you NEVER disconnect the mains safety earth.
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Old 9th June 2010, 07:51 PM   #7
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Show us the diagram so we can help you better.

If your dc meter reads 350V you must add to that the peak ripple, then add another 10% or so to account for power line variations. That could bring the peak voltage up as high as 430V as an estimate. So a 450V capacitor would be marginal but probably okay.
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Old 9th June 2010, 07:53 PM   #8
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Quote:
Originally Posted by jackand08 View Post
The hum could be for a number of reasons, it could be DC ripple or it could be an incorrect grounding setup. Just check your grounding and that (if it's an old amp) no capacitors are dead as dead caps will not filter out any hum. You can try lifting the ground in various places (such as the inputs or volume control0 but make sure that you NEVER disconnect the mains safety earth.

awesome. thanks!
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Old 9th June 2010, 08:15 PM   #9
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Quote:
Originally Posted by bob91343 View Post
Show us the diagram so we can help you better.

If your dc meter reads 350V you must add to that the peak ripple, then add another 10% or so to account for power line variations. That could bring the peak voltage up as high as 430V as an estimate. So a 450V capacitor would be marginal but probably okay.

the diagram is posted above

thanks for the informative response. how do you calculate pp ripple

Vr=I/fC?

Load current = 2400mA
f=60hz
C=33uF

=1.2V?

i dont know lol
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Old 12th June 2010, 10:48 PM   #10
cbdb is offline cbdb  Canada
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Calculating ripple is not that straight forward (especially after the 3 stage low pass filter in your diagram). It depends on how much current the transformer puts out, as well as the size of the caps and the load.

The voltage rating of the caps (if they are the same capacitance) will not effect the voltages or ripple current. They can effect the caps life span.
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