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

[Ryssen]

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]

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...

 

[Ryssen]

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

 

[Ryssen]

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

 

[Alain Dupont]

Ryssen,

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

Regards.

Alain.

 

[Ryssen]

This is the one I´m building from..

 

[Alastair E]

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]

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

 

[Ryssen]

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]

Ryssen,

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

Regards.

Alain.

 

[ac6y6]

While my circuit was not a mu stage, just a pentode driver. I had the same problem when building an e-linear amp simular to Pete Millett's:

http://www.pmillett.com/file_downloads/KT88_SCH.PDF

I added a small (150pf) cap from the pentode's plate to ground to stop the oscillation. The hum went away.

Sven

 

[Alastair E]

Hmm,-

You may have an earth-loop problem...

These can be a real PITA to find. Often layout is the cause... Try earthing chassis at one-point Only, and return all earths of the amp to one point in the PSU, and Not to the chassis

Check for ocillation as has been said, and worth checking something silly isnt happening to the supply-rails from the PSU too.

Another quick dodge, is to try reversing the main secondary +B connections on the mains Tx, Sometimes this can have good results..Even worth trying with the heaters too...

Are you using a twisted pair of cables for the heater-supply? Does the hum go away if you disconnect the heaters while the set is running? Was there Any difference when biassing to Plus 40V the heater supplies? Is the current through the Mu stage around about correct for those tubes? Does the hum go away if you disconnect the source? Worth remembering, that most sources have caps in, so definately worth changing that 470K to 47K, even though the source Impedance is 130 ohm-( This is the cause I personally think.....) The Data sheet mentions that the Grid-Resistor to earth shouldnt be higher than 150K for that tube too.... Is the Grid-Stopper (1K) mounted On The Valve-Base Pin?-Good for preventing oscillations and other weirdness...

 

[Ryssen]

I tried a couple of small caps from anode to ground,no difference.And if I remove the filiament while running-no difference!I use twisted cables.
No difference with biasing the filliament.Ive tried it with 47K input imp.
no difference.I think it was a little less hum when turning the power cord around,how do I go on from there?Will try some difference groundings.

 

[Ryssen]

Update: No succes with different eaths,In about a month I will be getting some 6AG7 I will try them,just have to change one pin.Or I will have to liv with a little hum in one channel,I can´t hear any hum while playing music,but when its not playing.