I´ve made a Mu stage with 6888 and 6C45...

Ryssen · 20th August 2006, 06:56 PM

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?

Alastair E · 20th August 2006, 08:43 PM

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...

Ryssen · 20th August 2006, 10:29 PM

Yes the tubes are in good condition,I have separate heater supplys,I will try bias to 40v.

Ryssen · 21st August 2006, 07:46 PM

I biased it to 130v,and it didn´t help..

more suggestions?

Alain Dupont · 21st August 2006, 09:28 PM

Ryssen,

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

Regards.

Alain.

Ryssen · 22nd August 2006, 04:00 PM

This is the one I´m building from..

Alastair E · 22nd August 2006, 05:23 PM

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.....

Ryssen · 22nd August 2006, 05:51 PM

When I connect the source it´s 133 ohm....

Ryssen · 22nd August 2006, 09:35 PM

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.

Alain Dupont · 23rd August 2006, 12:31 AM

Ryssen,

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

Regards.

Alain.

20th August 2006, 06:56 PM	#1
Ryssen diyAudio Member Join Date: Dec 2003 Location: Sweden--> Here	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? __________________ Regards Åke

20th August 2006, 08:43 PM	#2
Alastair E diyAudio Member Join Date: Feb 2004 Location: South Wales	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... __________________ Das Beste Oder Nichts

20th August 2006, 10:29 PM	#3
Ryssen diyAudio Member Join Date: Dec 2003 Location: Sweden--> Here	Yes the tubes are in good condition,I have separate heater supplys,I will try bias to 40v. __________________ Regards Åke

21st August 2006, 07:46 PM	#4
Ryssen diyAudio Member Join Date: Dec 2003 Location: Sweden--> Here	I biased it to 130v,and it didn´t help.. more suggestions? __________________ Regards Åke

22nd August 2006, 05:23 PM	#7
Alastair E diyAudio Member Join Date: Feb 2004 Location: South Wales	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..... __________________ Das Beste Oder Nichts


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21st August 2006, 09:28 PM	#5
Alain Dupont diyAudio Member Join Date: Sep 2003 Location: Paris - France	Ryssen, What's the value of the grid resistor ? and before is there an other one input to ground ? Regards. Alain.

22nd August 2006, 05:51 PM	#8
Ryssen diyAudio Member Join Date: Dec 2003 Location: Sweden--> Here	When I connect the source it´s 133 ohm.... __________________ Regards Åke

22nd August 2006, 09:35 PM	#9
Ryssen diyAudio Member Join Date: Dec 2003 Location: Sweden--> Here	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. __________________ Regards Åke

23rd August 2006, 12:31 AM	#10
Alain Dupont diyAudio Member Join Date: Sep 2003 Location: Paris - France	Ryssen, With a scope could you check the pentodes ; see if there is anything wrong there Regards. Alain.

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