12AX7 Tube hum problems in DAC only not in amp.

[sleepyguy24]

Greetings all

I was reviewing all the tubes I've acquired and was trying to weed stuff out. I was trying various 12AX7 tubes with my version 1 Jolida Glass FX Tube DAC and Jolida 502P amp. I noticed something with new issue Mullard 12AX7s and Svetlana 12AX7s in the tube DAC.

When I used the Mullards and Sveltanas in my Jolida JD-502P amp the tubes sounded good and there is no loud hum coming from my speakers. When I use these tubes in my Jolida FX Tube DAC there is a hum coming from my speakers. It only occurs when I use the Mullards of Sveltanas in the DAC.
 
Whenever I've used other 12AX7 tubes (NOS or other new issues Tung Sol, JJ, EH) with the Jolida FX Tube DAC there has been no hum.

Has anyone encountered anything like this? I contacted Jim McShane as I purchased the Mullards from him and he stated this:

"...it would appear the DAC has the tubes in a strong hum
field, so it would be wise to either use shields in those
positions or use a tube with a low hum construction."

When I contacted Jolida in MD they stated that the issue is with the pins and contact area in the sockets. There could be issues there with the varying thickness of the pins of the various tubes I've been rolling in and out.

Any other thoughts or ideas on this are appreciated. You may see some Mullard 12AX7s and Svetlanas up for sale soon.

Thanks

[richidoo]

I think pins are OK. New Mullards are made in same factory as Tung Sol and EH, likely use same pin wire.  If the tube has good grip in the socket then it's probably OK. Wiggle it around to clean the pins and socket contact. I have heard of power tubes having small pins, but not signal tubes, and never new Sensor tubes, they are an American owned company, they have awesome quality control.

So it could be EMI from PS transformer getting into a tube. 12AX7 has high gain, so it's sensitive. Tube covers would seem like a reasonable thing to try.

Here's a tube comparison that notes the NOS Mullard is sensitive to hum:
http://www.tgpwebzine.com/?page_id=83
So maybe it is a weakness of the internal structure of the Mullard design, which is copied in the New Sensor reissue.

The EMI may be stronger at the 12AX7 location in the DAC than it is in the amp. If it is EMI, then it is likely 60Hz magnetic fields from the PS transformer, especially if they use C core, not toroids which have very low EMI emissions. The field strength of the magnetic field is reduced exponentially with distance, so a couple inches more distance could make the difference and explain why it hums in one and not the other, if it is EMI cause.

[sleepyguy24]

Hi Rich. Thanks for the help. I think you are right about the pins, shielding and EMI. I tried the reissue Tung Sol 12AX7s, Svetlana 12AX7s as well as the Mullard reissues again last night in the DAC and they all caused a hum in my system. No hum using various NOS 12AX7 type tubes. The JJs I have are a more mellow 12AX7 to me and I don't believe I heard the hum. Will test some of the newer Chinese tubes tonight.

Here is an internal pic of the Jolida FX Tube DAC.



It seems like where the tubes are it should be far enough away from the PS but I guess not. Is there another thing to use other than tube shields? I can't figure out a way to apply those kinds of tube shields to the Jolida Glass FX DAC with how the tubes are situated.

Top



Front.



Well Thanks again for the help. I was also thinking since the 12AX7 is so sensitive and my speakers are sort of efficient I may try 5751 type tubes.

[richidoo]

It looks more like the tranny would pollute the circuit more than the tubes. I agree that tubes look far enough away from tranny. But there are a lot of wires and circuits within hum distance of that transformer.

But circuit EMI from tranny doesn't explain why your old tubes have less hum, unless they have better internal shielding - possible.

Unless your tube sockets are metal and can ground a twist-on  tube shield I don't know how effective a shield would be. I have seen them slid onto tubes using ceramic sockets. I think an aluminum/copper tube shield would not be effective against a LF magnetic field anyway.  They are more intended for blocking out radio station interference.

From wikipedia "Mains hum":
In vacuum tube equipment, one potential source of hum is current leakage between the heaters and cathodes of the tubes. Another source is direct emission of electrons from the heater, or magnetic fields produced by the heater. Tubes for critical applications may have the heater circuit powered by direct current to prevent this source of hum.

Are your heaters AC? Probably regulated DC judging by the complexity of the circuit. That still doesn't explain why some tube are quiet and others have hum.

Could it be a ground loop?  Try the worst humming tube in the DAC in another system, like HT receiver.

Some other ideas here: http://www.geofex.com/article_folders/stargnd/stargnd.htm

That's all I got... 

[sleepyguy24]

Morning Rich

All you got was more than enough to help figure stuff out as well as educate me. I also saved that article for later reading.

It turns out the problem was a ground loop hum that was accentuated by the Mullard and Tung Sol 12AX7 tubes in the signal chain. What I found was that when I plugged the Jolida DAC straight into the socket that I know was properly grounded there was no hum. When I plugged the Jolida back into the Iso transformer I was using previously the hum was back there. It turns out the Isolation transformer was not properly grounded internally. A week or so ago I got brave and replaced the original captive cord with a new hospital grade power cord that looks like the Iron Lung Jellyfish cords on the Iso transformer. I didn't connect the ground wire as securely as I had thought.

Again. Thanks for all the help.