C37a: Noise only on "M" frequency response

[mkiijam]

There is a slight popping / rumbly / noisy noise on this mic. It only appears to be in the "M" position of the Low Frequency response control. According to the manual, this is the "flat" position. I have attached the schematic. Any thoughts or experience on what might be causing this?

 

[radardoug]

The other positions are a high pass filter, so will cut rumbly noises. Depending what level the noises are at, they may be normal.
We would need more information. Like, set up the mike in a quiet room, tell us how much gain is on your preamp, and what the signal to noise is.
If you live near a motorway, dont be surprised to get low rumbly noises.

 

[fazer]

Sounds like tube noise to me. Try multi 6au6 tubes till you find a Quiet one.

 

[mkiijam]

Replacing the tube solved it.