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Old 14th January 2009, 08:12 PM   #1
wicked1 is offline wicked1  United States
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Default Rectifier Capacator (voltage adjust)

I've got the tubelab se. I've got a hammond 350-0-350 transformer. c4 on the schematic, (http://www.tubelab.com/AssemblyManua...matics_TSE.htm) the capacitor immediately after the rectifier, was 47uf. I was getting 410 volts b+

I decreased the value of that capacitor to 15uf, and now I'm getting 412 volts!

I thought a lower value capacitor would lower the voltage.

Am I wrong about that?
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Old 14th January 2009, 08:26 PM   #2
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try modelling it in this

http://www.duncanamps.com/psud2/download.html
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Old 14th January 2009, 08:53 PM   #3
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Indeed, modeling it shows what I see in real life. Smaller cap = higher voltage.

Strange.. Everything I read says larger cap = larger voltage.
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Old 14th January 2009, 08:55 PM   #4
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depends what design you use. if CLC then reducing input cap reduced voltage.
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Old 14th January 2009, 08:59 PM   #5
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But, that is the design I used...

47uf (now 15uf) cap, choke, 100uf cap
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Old 14th January 2009, 09:14 PM   #6
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drop it lower, to say 6uf, it should then drop
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Old 14th January 2009, 09:21 PM   #7
wicked1 is offline wicked1  United States
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Actually, I didn't have my model completely set up. Now that I have completed it, it does go down w/ lower value cap. But, thats not what I'm seeing on my actual amp. I went as low as a 4.6uf and still get 410v out.
Guess I have some testing to do.....
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Old 14th January 2009, 11:03 PM   #8
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Hi

Simple, maybe superfluos question: how much current are you drawing from the supply?
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Old 15th January 2009, 02:42 PM   #9
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I am curious about that as well. I can easily test the current the entire amp is pulling, but my clip around inductive meter is at a friends house right now.
But would that help? Or should I test only what b+ is pulling?
Unfortunately this is on a PCB, so it will take cutting a trace, or pulling a component to test.

Something is wrong, though. Or, at least I think there is.
I've used everything from 4.7uf up to 47uf and its ALWAYS 410 volts (+/-3v w/ power fluctuations)
Last night I was looking for failed components and ended up replacing all the caps, other than the nice paper/foil/oil coupling caps. While the caps were out I measured the resistance of all of the resistors I could.. everything measured fine. Put in all new caps, and still the same results!!!!!!! I've basically rebuilt this amp.
The only components that are questionable (no way for me to test them) are the IC's in the tubelab design, but and IC should either work or not, right?
The amp still works. I have been noticing way more distortion at mid/high volume levels, but thats up for debate as to whether something in the amp has failed, or if Im just getting better at hearing distortion.
I've also tried different output tubes w/ the same results.
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Old 15th January 2009, 03:03 PM   #10
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I asked about the current being pulled as a choke input PS only works if a minimum current is pulled: if you only had a bleeding in the circuit pulling 1mA a choke input PS will act like a capacitor input, increasing the voltage dramatically.

From your description I understand that you have the amplifier playing, so it is pulling the expected current. If you want to have it working more or less as a 'choke input PS' you will have to decrease the value of the first capacitor 'way more'. Ideally a choke input PS doesn't even have one, though a small value (less than half a uF) will not affect the 'choke input function' and even help stabilizing the whole thing. Start with about 220nF and keep adding more (I would say a 100 to 200nF a time) until you reach the desired voltage. This way you can play with your output voltage, adjusting it between 0,9x VRMS and 1,4x VRMS.

Most of these recommendations are taken from Valve amplifiers: I would seriously recommend you to read his extensive description of choke input PS.
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