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Old 5th November 2006, 11:24 PM   #1
kff322 is offline kff322  United States
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Default Power supply delima

I just finished constructing a power supply for a tube amp.
The B+ transformer is 510-0-510 but I am getting 700+ when all I want is around 500V
setup:
transformer > Full Wave > Cap > Choke >Cap
The transformer is a hammond 717 Specs
Is there any way to regulate this simply?

Thanks
Keith
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Old 6th November 2006, 12:09 AM   #2
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Have you put a load on it?
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Old 6th November 2006, 12:21 AM   #3
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getting a higher voltage than 500V makes sense, seeing as it's a cap input filter... 510*sqrt(2) = 720. if you wanted better regulation, you could expect around 90% of the voltage, or 460.
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Old 6th November 2006, 12:46 AM   #4
kff322 is offline kff322  United States
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Yes I have put a load on it and the voltage is still way to high.
How exactly could I regulate the voltage?
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Old 6th November 2006, 12:54 AM   #5
anatech is offline anatech  Canada
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Hi kff322,
Remove the first cap. Provide a minimum load on the supply so the choke does not "drop out". That should do it for you.

-Chris
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Old 6th November 2006, 01:40 AM   #6
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Kff322,

Chris is correct. To get the rail voltage where you want it, you should use a (tweaked) choke I/P filter. What is the value of the choke you are currently using? The critical current draw (in mA.) needed for good regulation is approx. V/L. Please observe that larger inductors require a smaller minimum draw for proper performance.

Are you using SS diodes? If so, protective measures are in order. Inductive "kick back" spikes that are not taken into account can DESTROY SS diodes with HIGH PIV ratings.

Older editions of the ARRL handbook teach that the way to execute a choke I/P filter is LCLC. The 1st inductor takes a BEATING and must be robust. The 2nd inductor can be of "ordinary" construction.
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Old 6th November 2006, 03:55 AM   #7
Jeb-D. is offline Jeb-D.  United States
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Remember transformer voltage ratings are in RMS which means your rectified DC voltage will be 1.41 x transformer voltage rating. Then subtract (idle current X DC resistance of the choke). The Most common way to reduce voltage in a tube amp is to put a resistor followed by a decoupling cap.
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Old 6th November 2006, 12:15 PM   #8
kff322 is offline kff322  United States
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I am using solid state diodes, The Choke is 3H at 500ma and 35 Ohm resistance.
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Old 6th November 2006, 04:06 PM   #9
Jeb-D. is offline Jeb-D.  United States
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That's a massive choke
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Old 6th November 2006, 04:48 PM   #10
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excuse me for this intervention.
on datasheet hammond trasfo i read CCS & ICAS
What does it mean CCS and ICAS ?

tnx !
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