Tube Microphone Intermittent Electrical Interference

[Torchlight]

I am hoping that someone has had this problem and can help me out. During a recording session last night I was experiencing and intermittent "buzz" sound coming from the tube mic I was using. In trying to discover the source of the buzz I tried the following:

Replaced/swapped: 7-pin cables, XLR cables, power supplies, microphones, snake cabling, and preamps. I even changed out the AC chord that plugs into the mic's power supply and ran the power through a Furman Power Conditioner. Nothing corrected the intermittent problem. Of course each time I made a change I had to wait for a few minutes before the buzz would return. When it did, I tried something else. I even moved the power supply and mic to different locations in the tracking room. What's weird is that I used these tube condenser mics a couple of weeks ago in a session and had no problems. What could be different?

I started to think that the interference could be coming from something in the house (home-based studio). Are tube condenser mics prone to electrical intereference? I removed cell and house phones from their chargers, as well as other appliances that sit in chargers. Nothing corrected the problem! What am I missing? Could the fact that it was raining outside have anything to do with the buzz? I know I am grasping for straws with that one.

When I couldn't get a noise-free tube mic setup I switched to a FET condenser and of course the noise was gone. I am wondering if there is someting inherent about tube mics that leaves them vulnerable to electrical interference. The fact that the noise is intermittent is also interesting, suggesting that this could be the result of charging and discharging? Perhaps like a capacitor somewhere. I am pulling my hair out over this one because I love my tube condensers and use then quite often, especially on vocals.

Please tell me there is someone out there who has experienced this type of interference. Oh yeah, and the buzz is most definitely getting "printed" so this is not "monitor path" type interference. It's in the "channel path".

Thanks for any advice you can give me.

Torchlight

[Craig F]

well what mic is it and how old is it?

an old/bad tube would be my first suspicion

[albee1952]

Or a dirty tube socket. That happens in guitar amps quite a bit.

[Torchlight]

Hi Craig. This is actually happening to two different tube mics and that is why I didn't think it was mic related. For the record though these are the mics:

Cascade M-56 Tube Condenser
Nady TCM-1050


These are not world class Neumans or the like but they both do a very nice job. I have never experienced problems with them before. is it possible that the tubes themselves could create the noise. I believe they are both 12AX7's.

Torchlight

[Torchlight]

Hi Albee. Dirty tube sockets on both mics? What are the odds of that happening on two different mics at the same time?

[Torchlight]

Forgot to tell you how old the mics were. The M-56 is 5.5 yrs old with original tube. The Nady TCM 1050 is about 4.5 yrs old with original tube. How often shoud I be replacing tubes?

Torchlight

[Torchlight]

Where can I buy new tubes if I need them?

[Craig F]

12AX7s are vary common.

If you are comfortable opening them up and looking at the tubes I'll bet that both mics used cheap Chines tubes
Your local music store that sell/services guitar amps (or the dreaded GC) should have 12AX7s for sale, Grove Tube seems to have good quality control on there tubes
You might try buying just 1 and checking it in both mics to see if the problem goes away, if not you have a spare for an amp if you need it at 2am

[albee1952]

I didn't realize when I was offering ideas that it was happening with 2 mics. I would certainly try at least 1 new tube and hope for the best. Tube life is a big variable. Tubes in some devices last a year and go ten years in other devices. Should I assume that each mic has its own power supply? Have they been tested? Just wondering if any kind of line surge might have caused some damage there. I only have a single tube mic and it still works so I have not had to chase this one yet. I guess it wouldn't hurt to try running a long power cord to another outlet on a different circuit as an experiment.

[Torchlight]

Hi albee:

Yes each tube mic has it's own power supply. How should I test the power supplies? They appear to work but I have not run any diagnostics. Here is an email I sent to an engineer friend of mine. It describes what I have done to try and resolve the issue......

I spent quite a few hours yesterday trying to get this resolved but still to no avail. This problem almost seems like a moving target! At first I thought I heard the "buzz" even when using the AKG C451B’s so I replaced them with Shure SM-81’s (all FET condensers). Still heard the buzz (not as prominent as with the tube mic though) with the SM-81s but realized that the tube mic was still connected in the channel path even though I may have turned off it's power supply. This is very puzzling. I even shut each circuit breaker (house breaker box) off one at a time but that was unrevealing. Keep in mind that I had to wait for the buzz to start up again as it was intermittent. When it did, I started turning off breakers. Then finally before going to sleep, I tried just hooking up a large cap FET condenser (Marshall Electronics MXL V67G) and running the two SM-81s at the same time. I listened for more than 15 minutes and there was no buzz. Keep in mind that when I hear the intermittent buzz there is no pattern to when it starts and when it stops. However, it generally stops and starts within a few minutes. I also believe that the buzzing doesn't really get started for at least 30 minutes after the tube mic has been activated indicating that this could be temperature related. I guess that I am still leaning toward the tube mics being the source of the problem, but they may not be acting alone. They may be acting like an antenna for some unforeseen interference. I was actually starting to think that with the 7-pin cable still connected to the tube mic (although disconnected from the power supply), it was acting like an antenna while the tube was discharging. I don't know how long a tube takes to discharge once the power supply is decativated.

Needless to say I am perplexed to say the least. I feel a bit uneasy about having this happen while I have a paying client in the studio. I have to get this figured out.

Thanks,

Torchlight