I have an odd problem with microphony...

Occasionally my system makes a hum that sounds almost like a musical note. Two days ago it started up and, as usual, when I switched the source switch it stopped but would start up again if I tapped the preamp or the phono pre. It seems to react more strongly to the pre than the phono but I will also hear a similar sound if I tap the TT. The most odd aspect is that yesterday I played the same record and there was no sign of this hum. I have a Lamm LP2 Deluxe and a Lamm LL2 Deluxe, both tube amps, and I recently added Herbie's Tube Dampers to try to cure the problem. Any ideas out there?


Sadly, that is very common behavior for noisy small-signal tubes in high-sensitivity slots (preamps). Especially for older tubes with bigger plates. Phantom whistling, rustling leaves, comes & goes, etc. You either have to move the tube to a lower sensitivity slot (for example, move it from an input position to the output / follower slot, or move it to a power amp if it uses the same tube type), or replace it with a more tightly screened low noise tube. New production tubes (like Russian Gold Lions) are fantastic for tricky slots because they can be easily selected for very low noise and tight matches.

The Herbie’s halos look really cool but in my experience do NOTHING for this problem. Don’t even bother. Same for isolation bases and all manner of such tweaks.

Before you start replacing tubes, I suggest you pull the tube(s) and clean the contacts. I had a similar experience with a Brimar NOS 12AU7. It was so bad that walking around the room triggered noise. But after using high quality contact cleaner and spending some time wigging the tub in its position all is working good again.