Staticy noise in a mic even after cleaning

[midwayfair]

Got an ELA251 clone (Dany's board) with a Tim Campbell capsule, and lately it's started making a bit of static noise, at first intermittently and then most of the time. I've cleaned off the capsule with a barely-damp microfiber cloth (and then a dry one), which fixed it the last time it started doing this. (I actually thought I had started a thread, but I couldn't find it.) I thought the noise sounded a little like a pulse instead of a constant scratching sound, but it's really just that the static cuts in and out.

I cleaned off all the components inside and inspected the PSU and there's no damage or anything.

I'm not sure what else to check, or what components might have gone bad that would cause this noise. My best guess since it's not hum is that it's somewhere in the polarization circuitry. The noise is not constant but rather something a bit more like a pulse. It's not hugely loud but loud enough that I can hear it a little while tracking.

Output seems to be normal, and it still sounds right as near as I can tell.

Any suggestions for things I haven't thought of or places to poke?

 

[kingkorg]

Try changing cathode resistor, it was a source of noise for my build once. I changed it for 1/2w.

 

[Gus]

I would first change the tube, are you using a NOS or a new production tube?

 

[midwayfair]

I would first change the tube, are you using a NOS or a new production tube?

I've tried new production EHX and TAD 12AY7s with no change. I have a NOS tube somewhere that I took out of it a while ago. (Also, I've tried all the tubes in another mic.)

I'm actually surprised R1 wasn't already a half watt, I guess I can swap that out, but it doesn't appear to be damaged or measure incorrectly.

I wish he power supply was compatible with some other mics, I would like to eliminate it as a variable, but even the chord for this one doesn't fit the other finished tube mic I have.

 

[midwayfair]

Well, I swapped out the cathode resistor (I had to put a pair of 1/4W 3k3 in parallel since I didn't have a half watt 1k8 on hand) and there is definitely a lot less static.

Also noticed my B+ was a little high after making the change -- and since I was pulling more current with the pair of 3k3, that means it might have been a couple volts higher with the original resistor in there. I guess it's possible it was high enough that the capsule was happy. Either way, back in business. Thanks for your help as always!

Ugh, looks like the output was a little suppressed, too. I might have to redo some vocals from the other day now.

 

[Gus]

Did the cathode resistor replacement fix the issue?
If so that is a little odd there is a bypass cap across the cathode resistor.
Did the removed resistor measure bad?
Cathode resistors can go bad but this does not seem to be as common as a plate resistor going bad.


To add to kingkorgs post
I often use older 1/2watt carbon films for plate and cathode resistors.

 

[midwayfair]

Did the cathode resistor replacement fix the issue?
If so that is a little odd there is a bypass cap across the cathode resistor.
Did the removed resistor measure bad?
Cathode resistors can go bad but this does not seem to be as common as a plate resistor going bad.


To add to kingkorgs post
I often use older 1/2watt carbon films for plate and cathode resistors.

The static is gone and it now has what I would consider to be a normal level of environmental noise for an active mic (not noticeably better or worse than most of the mics I have). Honestly, though, I redid the B+ voltage before listening to it, and I can't definitively say that it wasn't just a slightly high B+ voltage making the capsule unhappy.

The pulled 1/4W measures 1k7 and is banded for a 1k8, so I don't think it went bad. There's a small blister or possibly a bit of flux on the outside of it. But I agree with you: There's a big honking 22uF strapped across it in circuit, it's very strange to me that replacing it should be a cure for what I was hearing. I was originally expecting to find a damaged electrolytic in the B+ or other capsule voltages.

 

[Pip]

Heat equals noise. So if that resistor was getting hot it could really have contributed to the noise floor.

IMO a 12ay7 though electrically in essence the same tube is not a 6072A. The 6072A designation often times is different (depending on manufacturer) internally larger geometry plates come to mind if my memory serves me also much quieter usually. Remember no matter which tube you are using you are only using one half of it, there are actually two tubes in the envelope. So with some minor circuit mods you can access the other side instead.

 

[kingkorg]

Heat equals noise. So if that resistor was getting hot it could really have contributed to the noise floor.

In my case it did. Removed, it measured great.