DCPP - "Engineer's Amplifier" : Need a bit of help - diyAudio
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Old 29th December 2012, 11:58 PM   #1
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Question DCPP - "Engineer's Amplifier" : Need a bit of help

I just finished my amp.

I plugged it... power... no flammes and tube being light was a good point!

But i tried to adjust bias, ok on one side but the other don't go over 280mv

And i noticed that the mosfet are not getting hot at all but the R21 resistor (big black one rated 6.5W) is hot as hell (i actually burn my finger on it).

I tried to check sound output (jack 3.5 to rca with my phone) but no sound is goind throught the speakers

any info on what could be wrong? I can post a really high res picture of the board if needed; i'm dying to ear the sound it produces!

Thanks!
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Old 30th December 2012, 01:16 AM   #2
dheming is offline dheming  United States
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It's always a good call to include pics. That way people can look for any mistakes that might have been made.
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Old 30th December 2012, 05:42 PM   #3
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Ok, here are the two picture (220V connector) and the board+cables:

http://www.binard.biz/tmp/DSC_3240.jpg
http://www.binard.biz/tmp/DSC_3241.jpg

sorry, the files are heavy...
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Old 31st December 2012, 11:56 AM   #4
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It might be best if you post the circuit diagram, the two pictures don't reveal much.
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Old 31st December 2012, 02:09 PM   #5
DUG is offline DUG  Canada
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C22, C26...Do they not have polarity marking on the board or are they in backwards...I couldn't really tell.
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Old 31st December 2012, 11:19 PM   #6
SandyG is offline SandyG  United States
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This looks to be Pete Millet's

DCPP Amp
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Old 1st January 2013, 12:01 AM   #7
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Did you check all the voltages on the schematic?
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Old 1st January 2013, 01:36 AM   #8
roline is offline roline  United States
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R21 is the HV feed resistor to the screen regulator. Check your power supply section to ensure the proper voltage on the input and output of the mosfet. With the 3 50v zeners, you should have 150v on the screens. If you were to pull all 4 output tubes out, the resistor should not get hot. There are several electrolytics on this reference, be sure they are not backwards or shorted.
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Old 1st January 2013, 02:04 AM   #9
roline is offline roline  United States
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Check C26, I could not tell in the photo. The MosFet could be shorted to GND...
Pull the output tubes and free the Mosfet from the chassis. There is little current 2ma or so for the bleed resistors. If you have your voltages, verify no short at the MosFet mount to chassis. Or Ohm out the S and D to the chassis for low resistance-short.
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File Type: jpg DCPP_PCB_SCREEN VOLTAGE.jpg (646.2 KB, 606 views)
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Old 1st January 2013, 03:19 PM   #10
6L6 is online now 6L6  United States
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Look closely at the schematic (and Pete's photos) for the output wiring - the white is the output +, the Yellow or Orange is the output - and you need to ground the output to the pcb. Just use one speaker - jack and transformer wire at this time (Yellow or Orange)

The 6.5W resistor gets hot. That's normal - it dissipates about 150v into heat.

Have you checked the voltages on the schematics? There are DC voltages printed all over it. Start at the PSU and move on from there, they all need to be verified. Fix your output wiring and then do that.
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