I´ve made a Mu stage with 6888 and 6C45... - diyAudio
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Old 20th August 2006, 06:56 PM   #1
Ryssen is offline Ryssen  Sweden
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Default I´ve made a Mu stage with 6888 and 6C45...

And it sounds so nice with headohones.The current through the 6C45 is about 30mA at the moment.But it is this with the 6888,there´s a little hum in one channel and if I switch the 6888 in the chanels the hum moves to the other channel I have 2 more 6888 and they all hum,1 is deadquite.what to do?
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Old 20th August 2006, 08:43 PM   #2
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Default Heater-Hum

Try biassing the heaters to around 40V Positive in relation to the Cathode. You may need to use separate heater supplies for each 'Half' of the Mu-Stage This Usually reduces Heater-based hum to practically nothing assuming the tubes are in good order...
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Old 20th August 2006, 10:29 PM   #3
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Yes the tubes are in good condition,I have separate heater supplys,I will try bias to 40v.
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Old 21st August 2006, 07:46 PM   #4
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I biased it to 130v,and it didn´t help..
more suggestions?
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Old 21st August 2006, 09:28 PM   #5
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Ryssen,

What's the value of the grid resistor ? and before is there an other one input to ground ?

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Old 22nd August 2006, 04:00 PM   #6
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This is the one I´m building from..
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Old 22nd August 2006, 05:23 PM   #7
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Unless you specifically Need a High input impedance, I would try lowering the input/Grid resistor from 470K down to say 100K or even 47K.....
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Old 22nd August 2006, 05:51 PM   #8
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When I connect the source it´s 133 ohm....
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Old 22nd August 2006, 09:35 PM   #9
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Update the hum i not comming from the heaters it is at the anode,when i switch on it humms from both channels after a while it dissapears in one channel,but not in one.
The HT is regulated.
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Old 23rd August 2006, 12:31 AM   #10
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Ryssen,

With a scope could you check the pentodes ; see if there is anything wrong there

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