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Need help with noise troubleshooting
Need help with noise troubleshooting
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Old 1st August 2016, 04:47 PM   #1
login721 is offline login721  Viet Nam
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Question Need help with noise troubleshooting

I made a copy of Marshall jcm800 , with a little modify at output stage (use 12ax7 as power tube).
My problems are ;
1. it very noisy .the noise goes up as i turn up the master volume and gain , even in clean channel.
2. All my fuses are blown .The power transformer rating is : input 100v ,ouput 200v 80ma and 6.3v 2A.Output transformer is 5w primary 10k ohms, secondary 8ohms.Pre amp tube are 12ax7 x3 , 12au7 x1. I used 250v 100mA fuse for F2 and 125v 300mA fuse for F1.Every i connect the amp to AC the fuses are blown , but if i use cooper wire instead it works but very noisy .

Can anyone help me with troubleshooting please !
Thanks in advance , and sorry for my bad English !
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Old 2nd August 2016, 06:34 AM   #2
login721 is offline login721  Viet Nam
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This is my schematic
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File Type: jpg scan-001.jpg (800.8 KB, 40 views)
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Old 2nd August 2016, 06:44 AM   #3
29285 is offline 29285  Europe
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Your ac-powered filament circuit should be connected to GND.
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Old 2nd August 2016, 09:52 AM   #4
DF96 is offline DF96  England
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What do you mean by noise? Random noise, crackling, hum, buzz, oscillation?
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Old 2nd August 2016, 11:49 AM   #5
login721 is offline login721  Viet Nam
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Quote:
Originally Posted by voltwide View Post
Your ac-powered filament circuit should be connected to GND.
Thanks! What do you mind by ac-powered filament? (sorry for my bad English) .I used diode bridge to convert AC to DC , i uploaded the power transformer schematic below .

Quote:
Originally Posted by DF96 View Post
What do you mean by noise? Random noise, crackling, hum, buzz, oscillation?
this is the audio file i recorded : https://drive.google.com/file/d/0B17...ew?usp=sharing
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File Type: jpg schmematic.jpg (210.3 KB, 34 views)
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Old 2nd August 2016, 01:07 PM   #6
29285 is offline 29285  Europe
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I meant the 6.3V~ heating circuit has to be grounded somewhere, not floating as depicted.
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Old 2nd August 2016, 01:16 PM   #7
login721 is offline login721  Viet Nam
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Quote:
Originally Posted by voltwide View Post
I meant the 6.3V~ heating circuit has to be grounded somewhere, not floating as depicted.
Thanks !
i did as you dais and the noise is completly gone . <3
i have one more question : can you help me choose value for the fuse F! and F2 please !
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Old 2nd August 2016, 04:03 PM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by login721
Thanks! What do you mind by ac-powered filament? (sorry for my bad English) .I used diode bridge to convert AC to DC , i uploaded the power transformer schematic below .
Your circuit shows AC heaters, and a rectifier bridge for the main DC supply rail.

Heaters, whether AC or DC, must have a reference voltage (which may be zero i.e. circuit ground).
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Old 2nd August 2016, 09:55 PM   #9
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by DF96 View Post
Heaters, whether AC or DC, must have a reference voltage (which may be zero i.e. circuit ground).
I usually elevate my heater voltage referenced to 45VDC.
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Old 6th August 2016, 02:14 PM   #10
login721 is offline login721  Viet Nam
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Thank guys!
I finally completed it .
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