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Old 30th August 2013, 09:09 PM   #1
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Default Mighty Midget Problem (One channel is louder)

I recently built the mighty midget amplifier. I have two problems:

1- First both channels did not work. I realized that the binding posts were shorted to the ground, hence no sound. I fixed that issue and now both channels work. But one channel is louder than the other. I tried swapping the tubes with no luck. So it must be either a cap or a resistor. Is there a way to track it without a scope, only with a multimeter.

2-The other problem is that my edcor transformers are designed for 220v but I run the amp at 110V, and I can get clean sound from both channels, although one channel is louder. I got transformers for 220v because I will be moving to europe soon. Anyways, when I connect my amplifier to a 1000W rated transformer (240V output) the fuses blow. Is it possible edcor made a mistake and sent me 110V transformers, and how can I check my power transformer. Regards,

Emre
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Old 31st August 2013, 06:57 PM   #2
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Just a quick update. I measured the secondary voltage for my power transformer and found out that edcor actually sent me the wrong transformer (the 110V to 250V instead of the 240V to 250V one). So if you live in North America and order an European mains power transformer (if not a dual secondary) call them to make sure they get the order right. So problem 2 is solved.

But I still have the uneven channel output. I found that today one of the leds is shorted. Could that be reason of the problem? I wont be able to replace it till tuesday. So any suggestion is appreciated.

Emre
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Old 31st August 2013, 07:28 PM   #3
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Is that a bias LED?
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Old 31st August 2013, 08:38 PM   #4
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Here is the schematics from Mr. Millett's website: http://www.pmillett.com/file_downloads/midget_sch.pdf

I cannot find out what those leds are for with my electronics knowledge. Led numbered D8 is shorted.
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Old 31st August 2013, 09:37 PM   #5
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D8 is a rectifier diode not an LED, do you mean D3 ?
If one of the LEDs is shorted then it needs replacing. This will alter the the operating point of the input pentode which will alter the current passed and the gain of the stage. Fix it and report back.

Shoog
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Old 31st August 2013, 10:15 PM   #6
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Yes sorry my bad it is actually a rectifier diode, D8. I will fix it and report back.
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Old 31st August 2013, 10:49 PM   #7
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If its the recifier diode it can't be the cause of uneven loudness. You need to check elsewhere for that.
If D8 is not working then I would expect a loud hum to be bleeding through.

Shoog
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