tube mic cable weirdness

[pucho812]

I just finished building a pair of tube mics. I have both mics working and I have both power supplies working. Here is where it gets weird of the 2 X multipin cables, I have one that works without issue and one that causes hum when connected.  I swapped power supplies and mics and both mics and PSU's work with the one cable but not the other. I have been over each cable, and  all I can see is they are identical. I have them wired pin for pin using the same color  of  conductors on the  gotham tube mic cable I used. I do not measure any odd Ω between conductors and pins, I measure low Ω when I measure the cable  from connector to connector.  I am wondering what else I can be looking for. I don't see or feel any break or weirdness in the wire. 
I am pretty sure I am missing something obvious. Any ideas?

 

[MagnetoSound]

Gotta be a grounding thing. What sort of hum? Like 60Hz in the room or 120Hz rectification?

 

[moamps]

Maybe shells of the connectors are not connected to shield/gnd on both cables.

 

[mjrippe]

Maybe shells of the connectors are not connected to shield/gnd on both cables.

That was my first thought as well. 

 

[pucho812]

Well  lets see if I can be a bit more clear..

We have PSU-A, MIC-A and Cable A, then PSU-B, MIC-B, and Cable B.    I can run both PSU A&B and MIC A&B with Cable A and it's quiet and works sounding like you would expect a mic to sound like.  It's when I try Cable B that is an issue where  I have hum.  In fact I have a second pair of cables C&D for another project and  Cables B, C, and D all do  the hum thing while cable A is fine.  I can touch the mic and the PSU case when using cable B, C or D and  the  loud hum drops. So much like you guys are thinking I would say has to be a grounding thing. I am wondering if the shells are mating properly into their respective receptacles on the mic and PSU for cable B and that would explain things a bit. Perhaps I  have assembled the connectors incorrectly although I do not see how. It's a 7 pin binder connector on the mics and cable which although they contain a lot of parts I can't see that as an issue.

 

[Bo Deadly]

Open a mic and a supply and measure resistance of each conductor from the inside. It could be that one of the contacts is just not quite connecting properly.

 

[pucho812]

Open a mic and a supply and measure resistance of each conductor from the inside. It could be that one of the contacts is just not quite connecting properly.

hmmmm,  I'll give that a whirl and see but  that points me back to either the cable itself having an issue and or the connectors on the cable, which hopefully it is the cable and not the connectors, those guys are pricy compared to the cable.

 

[Tubetec]

I read a pin in the plug should also be connected to ground ,as you shouldnt rely on the plug shell for ground contact on a tube mic .

Have you used high quality 7 pin XLR's ?  ie Neutrik
Low grade xlr's can tend to melt while being soldered , leading to poor connections and bad pin geometry. I usually mate the connector before soldering ,that helps keep things stable while hot .

Ive used the Gotham Gac7 Tube mic cable on a few of my mics , there are a pair of thicker conductors destined for the heaters .

 

[pucho812]

I read a pin in the plug should also be connected to ground ,as you shouldnt rely on the plug shell for ground contact on a tube mic .

Have you used high quality 7 pin XLR's ?  ie Neutrik
Low grade xlr's can tend to melt while being soldered , leading to poor connections and bad pin geometry. I usually mate the connector before soldering ,that helps keep things stable while hot .

Ive used the Gotham Gac7 Tube mic cable on a few of my mics , there are a pair of thicker conductors destined for the heaters .

I am using GAC& cable this time and I am using binder 7 pin connectors

 

[pucho812]

OMG you guys are good, real good.  While I will test this out and confirm, after several back and forths  between testing the good cable and the funky cables,  I notice on the good cable ground  has continuity with the binder  connector shell.    For me on the good cable this was achieved by having the shield exposed and screwing down  the cable strain relief/holder on each side.  On the funky cable(the one with hum) and the other  cables, I noticed I didn't have the ground exposed. My heat shrink was too long and was covering the area where it screws down the strain relief/cable holder  thus preventing ground connection to the shell. A few cuts with an exacto knife and I now have ground connection to the shell.  lets see where the hum goes now.  time to test

 

[pucho812]

And yes that was it. No ground contact to shell. Now both  cables work. You guys are the best.