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Old 27th January 2010, 06:16 AM   #1
cgian is offline cgian  Greece
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Default Question for B+ in Simple SE

Hi to all.
I just built a Simple SE amp with EL34 ,12AT7 and 5U4G tubes.The power transformer has a high voltage secondary of 700 (680V with multimeter, (223VAC primary)) and 180mA.Although the voltage after rectifier (after c1) is 438 VDC when i check with a multimeter the voltage on c2 i get 348VDC.I use a choke 10H 150 Ohm, 180mA and a supplementary cap 1uF 630V.I dont know why there is so big voltage drop.What can i check to find where is the problem?
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Old 27th January 2010, 12:29 PM   #2
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Originally Posted by cgian View Post
...the voltage after rectifier (after c1) is 438 VDC when i check with a multimeter the voltage on c2 i get 348VDC. I use a choke 10H 150 Ohm
Something does not seem correct. To go from 438 VDC to 348 VDC is a 90 volt drop. To lose that much voltage across your 150 ohm choke would require a current draw of 600 mA.
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Old 27th January 2010, 01:18 PM   #3
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With power off, double-check the resistance on that choke. Or, you may have some leakage in C2? Or the supplementary cap 1uF/630V? To test this, remove those caps one-by-one from the circuit while measuring the voltage at the 'cold side' of the choke. This is rare, but, there could be a high-resistance fault in the choke? With power on, measure from B- to the case or core.
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Old 27th January 2010, 02:39 PM   #4
cgian is offline cgian  Greece
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I checked the resistance of the choke while power was off and it is 130 Ohm.I removed the supplementary cap the voltage was 342 to 348 VDC at cold side.I cannot remove C2.What can i do as an alternative solution?
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Old 27th January 2010, 03:01 PM   #5
rknize is offline rknize  United States
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Either something is not wired correctly or there is something wrong with that choke. For the C2 cap to eat that much current, I would expect it to explode and it doesn't sound like your power transformer could ever supply that much current anyway.

Are you using two meters to take this measurement? If so, have you tried swapping the meters? I assume you are using the same ground reference for both measurements?

If you have a 5W or larger power resistor in the 100-150 ohm range, try swapping it for the choke. Then measure the voltage directly across the resistor.
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Old 27th January 2010, 03:10 PM   #6
cgian is offline cgian  Greece
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I'm using one meter and the same ground reference.Unfortunately i dont have a resistor to use instead of choke.
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Old 27th January 2010, 03:51 PM   #7
rknize is offline rknize  United States
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I'm hoping you used 500V caps because you are running a lot of B+. So you need to be careful here, but what happens if you pull the EL34s? If you still see a big drop, try disconnecting the output transformers primaries from the PCB. Specifically, the side that connects to the B+ rail.
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Old 27th January 2010, 04:19 PM   #8
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A 90-volt drop across 130 ohms would be a 62-watt dissipation! (P=e^2/r) That choke would be rather warm if it were dissipating 62 watts. So, assuming it's not, methinks that 438V measurement is wrong. Is this a full-wave or half-wave rectifier? Your meter may not be measuring correctly because of significant AC mixed with the DC at C1. Try and measure the DC current between C1 and the choke or C2 and the choke. (You'll have to break the circuit and insert your ammeter in series.) Or, if you can measure the total AC current this baby is drawing, do that first. That would give you a quick reality check.
Oh, did you check to see if the choke is leaky? If it's isolated from the chassis, just measure the DCV from B- to the choke core.
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Old 27th January 2010, 05:17 PM   #9
rknize is offline rknize  United States
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Good point about the noisy DC...my good meters handle these pretty well but some of the cheaper ones do not. Repeating the measurement with less load (no EL34s, for example) should yield more accurate results because the switching noise will be less prevalent.
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