Last night I was testing tubes for the $5500 AKG C 12 VR microphone. Hundreds of these mics have been built since I started supplying the tubes in 2001. AKG is kind enough to send the users directly to me for replacement tubes. They are very nice people and I am, needless to say, proud of supplying these tubes.
://www.akg.com/site/products/powerslave,id,210,pid,210,nodeid,2,_language,EN.html
I had to make a special test set-up and have them confirm my measurements were repeatable by them in their lab. I have to test these by hand, everything is carefully shielded and I have a scope connected so I can judge the character of the noise by how it looks. I'm very good at that.
The tubes are very sensitive to microphonics around 60 and 120 Hz. and I kept noticing that ifn some tubes the noise was dominated buy those frequencies. Although the power supply is 3 feet away it is on the same table. When I picked up the test fixture ( 12 x 8 x 4 aluminum chassis) the 60 wave on the scope disappeared. This noise was clearly not electrical but mechanical. I considered a 2" block of foam that was out in the shed but close-by was a box containing those "sealed air" pillows that are about the size of a sub roll. A section of 4 was about the size of the chassis and I put them underneath. The results were stunning. All the 60/120 Hz "microphonic hum" was gone.
I'd like to hear back from anyone trying or having tried this. You will have to move the pillow around to balance the weight and pull of the cables which you might try supporting on a few other pillows. It's best to have the connected pillows under the whole chassis rather than one at each end. We want the air pressure to be low in the pillows, not high.
