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Old 8th December 2014, 09:32 AM   #1
Benji90 is offline Benji90  Italy
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Default Tube preamp psu

Dear Everyone,
I'm building a Kondo m77 clone following this scheme (Kondo - Audio Note M7 Tube Pre-amp Clone | DMS Audio). I would like to build it a tube psu with a full wave ez81 rectifier, 10h choke and 360-0-360 transformer (I already had at home this components) with a classic clcrcr(and so on) scheme. I'm finding it hard to understand which would be the load and current of this line stage. The b+ voltage is ~250V.
Best regards
Benjamin
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Old 8th December 2014, 11:30 AM   #2
SY is offline SY  United States
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You can get this from Ohm's Law. The first stage has 113V on the plate, a 250V rail, and a 120k load resistor. So that stage's current is I = V/R = (250-113)/120k = 1.14mA. Since the cathode of the second stage will have a voltage within a volt or two of the grid voltage and the grid is connected to the first stage plate, the cathode resistor (120k) will have about 113-115V across it. Let's say 115V. Again, I = V/R = 115/120, or very slightly under 1mA (this, by the way, is very poor design practice!). So for each channel, the total load current is slightly over 2.1mA.
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Old 8th December 2014, 11:47 AM   #3
Benji90 is offline Benji90  Italy
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Thank you for your reply!
I would imagine that the current through each stage would change when I apply a signal and the grids are "open". Is the assumption correct? Do i have to take it into account while designing the ps?
Excuse me if my questions seem obvious but I am only a beginner
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Old 8th December 2014, 11:54 AM   #4
SY is offline SY  United States
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The grids should not draw current unless the tube is bad, so you're right about that. Yes, the current modulates with signal (though it is, on average, constant), so a low power supply impedance is more important than when CCS loading is used. That change in current is largely cancelled between the two stages- as the current reduces in the first stage on a positive signal swing at the plate, the current increases in the second stage to keep the overall current relatively constant.
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Old 8th December 2014, 04:42 PM   #5
JMFahey is offline JMFahey  Argentina
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Quote:
(this, by the way, is very poor design practice!)
How dare you disagree with Kondo-San , the creator of the silver wire transformer and the silver foil cap?

May the Audio Gods in the Nirvana/Olympus/Mt. Fuji send lots of rays to fulminate you
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Old 8th December 2014, 05:00 PM   #6
Benji90 is offline Benji90  Italy
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Thanks for the reply!
So if both channels have together a current load of 4,2mA they should have a combined resistance of ~60k (250/0,0042). Correct?
I tried to design a psu with psu designer II. It' only an experiment to see if I understood today's lesson, but could it work? How do I decide how many Rc sections do I need? the more the better or there are some disadvantages in adding to many RC sections?
I'm sorry for the bad quality pictures but I am unable to export the files in a more popular format
Thanks again!
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Old 8th December 2014, 05:08 PM   #7
SY is offline SY  United States
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That's correct. You can also use the constant current sink option for the load.

Don't go too crazy on the multiple filter sections- in the real world, the layout and actual ground wiring are most important. With a simple CLC, you should be able to get any ripple more than low enough. Keep the input cap reasonably small (say, 50-80uF) to limit charging currents, then use something larger on the output (say 100-300u). If you want to go crazy, add a separate RC section for each channel, but you're already at the point where it's unlikely to affect the actual circuit performance.
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Old 9th December 2014, 09:17 PM   #8
Benji90 is offline Benji90  Italy
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Today I builded the circuit posted above. On psu designer 2 it apparently works fine, but that's not the case with the prototype that I've build. On the real one I get a higher voltage (around 50V more). For example on the first capacitor the simulator states that I should get around 440V whether in reality I get close to 490V. Somebody knows where the problem could be? Thanks!
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Old 9th December 2014, 10:46 PM   #9
SY is offline SY  United States
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Do you have the power supply loaded? if not, the voltage will be high.

Did you input the correct resistance for the transformer?

Either way, too high is easier to fix than too low!
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Old 10th December 2014, 07:15 AM   #10
Benji90 is offline Benji90  Italy
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Hi SY, thank you for your help!
Yes, I loaded the circuit with a 68k resistor. I have on each anode a 270 Ohm resistor. The resistance from anode to anode taking into account the wiring of the transformer (I hope I have explained myself) in now ~750 Ohm. I actually updated these data in the psu designer but still there is a ~50 V difference in reality. I tried the rectifier on it's own without anything connected to the cathode and I get 350V which should be correct. As soon as I connect just the first 47uF capacitor the voltage goes up to 490 and it stays there all the time no matter what elements are attached afterwards. I tried also to change th capacitor but it doesn't change anything at all.
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