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Old 1st January 2013, 05:36 AM   #1
diyAudio Member
Join Date: May 2012
Default Hum problem through one channel

I'm having a problem with very audable hum through just the right hand channel from my amp. I've had a good look through past threads but haven't found an answer yet.

My Amp is a Mingda integrated MC34-A06, I have included a circuit diagram and photo of the interior.

I've done a little bit of trouble shooting already based on the other posts:
- It hums even when nothing is plugged in (obviously power and speakers are)
- Hum volume stays constant as I play with the volume control
- Swapping the 12AX7B and 12AU7 over to the other channel does not stop the hum
- The mains input wires run well away any of the right channel parts.
- I've poked everysolder joint and they all seem good
- The amp is about 1 year old so all capacitors should still be good, I think.

p.s. yes, I am a complete begginer to all this, but I have read up on how to stay safe and promise I am being over cautious. Also I have just started reading morgan jones's Valve amplifiers 4th ed so if anyone has a page referance they think would be useful that would be interesting.
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File Type: jpg DSCN1114.jpg (1,012.9 KB, 56 views)
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Old 1st January 2013, 06:32 AM   #2
diyAudio Member
Join Date: May 2012
I've made some progress, but potentially uncovered a different problem.
I started swapping round the EL34 valves and descovered that the problem follows one of the valves... so time for a new set I suppose.
it's the second time I have had a valve go bad in that position on the amp. last time it became much worse with the valve creating worrying 'pops'... might this be a problem with the amp or have I just been unlucky with valves? All the biasing is set up fine, so thats not causing the problem.
any Ideas? or should I just go ahead and replace.
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Old 1st January 2013, 07:13 AM   #3
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azazello's Avatar
Join Date: Aug 2008
Location: Bulgaria
You can clean contact inside of sockets of output tubes and tube pins.
/first- switch off amplifier/
/I had the same problem with Marshall guitar amplifier/

Last edited by azazello; 1st January 2013 at 07:36 AM.
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Old 1st January 2013, 07:46 AM   #4
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azazello's Avatar
Join Date: Aug 2008
Location: Bulgaria
Bad pins cause disballance of opposite currents in shoulders in output transformer and summer ripples aren't zero, it maybe cause hmm.
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