Humming Sovtek EF86

[nilrog]

Hi,

Has anyone here experienced humming problems from the filament supply with Sovtek EF86 tubes?

I have recently built an amplifier which has EF86 as input tube. And when I turn on the amplifier one channel has a very noticable humming sound. The humming comes from the filament which is powered by AC. The other channel is almost dead quiet.

I have traced the problem to the tube itself because the humming moves to the other channel if I swap tubes between the channels. Yesterday I also exchanged the humming Sovtek tube with an Ediswan tube and the humming was gone.

The strange thing is that the same thing happened to a friend of mine who also built an amplifier and we ordered the tubes together. That means that 50% of our Sovtek EF86:s is bad He has however converted his filament supply to DC so he has no longer any problem with filament hum, but I don't want to do that with my amplifier. Especially since my Ediswan tubes are quiet in the same amplifier

Is this a "known" problem with Sovtek EF86 or have we just been very unlucky to get two humming tubes from a batch of four?

Regards
/Roger

 

[Family_Dog]

Hi Roger,

Just a thought, you have ensured that the AC wiring to the filament is tightly twisted to pins 4 & 5 of the valve base? Also, that you have earthed the centre spigot of the base together with Pins 2 & 7 of the valve base to a good central earthing point? Does the filament tapping of your mains transformer provide a centre-tap point for earthing?

-Eric

 

[EC8010]

I can't comment on the Sovtek, but even Mullards commonly have hum problems due to poor heater/cathode insulation. I suspect that this is because the helical heater design makes maintaining good insulation much more difficult. You can prove whether it's the valve or not by swapping in the EF86 from the quiet amplifier. If the hum goes away, then the other valve is faulty.

 

[Sheldon]

Hi Roger,

Just a thought, you have ensured that the AC wiring to the filament is tightly twisted to pins 4 & 5 of the valve base? Also, that you have earthed the centre spigot of the base together with Pins 2 & 7 of the valve base to a good central earthing point? Does the filament tapping of your mains transformer provide a centre-tap point for earthing?

-Eric

Good suggestions. However rather than earthing the heater, do as schematic has pointed out in another thread: http://www.diyaudio.com/forums/showthread.php?postid=593191#post593191 Instead of referencing the heater to ground, raise it to about 10-20volts higher than the cathode voltage. I did this on my 6ck4 amp and it worked very well. As the "grounding" point, I made a voltage divider from B+ to ground and referenced at that point. The schematic is here: http://www.diyaudio.com/forums/showthread.php?postid=622719#post622719

You can make the divider to any voltage required and it serves as a bleeder for the caps too.


Sheldon
Sheldon

 

[nilrog]

Just a thought, you have ensured that the AC wiring to the filament is tightly twisted to pins 4 & 5 of the valve base? Also, that you have earthed the centre spigot of the base together with Pins 2 & 7 of the valve base to a good central earthing point? Does the filament tapping of your mains transformer provide a centre-tap point for earthing?

Yes, the wiring is twisted and pins 2&7 are grounded. But I haven't grounded the centre spigot, but I will look in to that too. There is no centre-tap on the transformer but the filament is referenced to ground with two 120 ohm resistors.

You can prove whether it's the valve or not by swapping in the EF86 from the quiet amplifier. If the hum goes away, then the other valve is faulty.

That is what I did. Replacing the humming Sovtek with the other "hum-free" Sovtek (or another tube branded Ediswan) and the humming goes away. And when I put the humming Sovtek in the channel which was not humming before I have humming there too.

Good suggestions. However rather than earthing the heater, do as schematic has pointed out in another thread: http://www.diyaudio.com/forums/showthread.php?postid=593191#post593191 Instead of referencing the heater to ground, raise it to about 10-20volts higher than the cathode voltage. I did this on my 6ck4 amp and it worked very well. As the "grounding" point, I made a voltage divider from B+ to ground and referenced at that point. The schematic is here: http://www.diyaudio.com/forums/showthread.php?postid=622719#post622719

You can make the divider to any voltage required and it serves as a bleeder for the caps too.

I haven't tried this yet since "the rest" of my amplifier has been quiet enough. But now that I have replaced the tube I think I will try it just to see if the little hum that is left can be reduced too.

Thanks for all the suggestions!
/Roger