Recording Tips 02: Microphonic Vacuum Tubes (part 1)

A lot of you have probably heard the term “microphonic tubes.” But have you ever heard one in action? They actually turn into little microphones. (Hence the name). So, this week I decided to replace the input tubes on my Manley Massive Passive with some NOS Telefunkens that I purchased from a reputable tube dealer. I’m about to start a project when I noticed that input spectrum display responded when I bumped my mastering desk. At first I thought it was an accident, but I found that I could repeat the response over and over. Uh oh. Nothing was playing out of the mastering computer. The tape machine wasn’t on. There were no microphones connected to the system (don’t ask), so I was vexed. Instead of hitting the desk I tried clapping and noticed the system still responded. I isolated it to the Massive Passive. I knew it was the new tubes. It bears mentioning that the response was -50 dB so it wasn’t super loud - heck that’s pretty low -- but you just can’t have the audio you’re playing through your speakers feeding back into the chain. Even if you had the volume down what if you were talking while mastering? (Some people talk to themselves. It doesn’t mean you’re crazy or have something wrong with you. Not at all). I swapped out the tubes and heard total silence. Excellent. However, before I did, I recorded a little something for you to hear. The graphic below is the recorded source boosted by 50 dB. Note the claps sound like bells - that’s the sound bouncing off of the tube glass. Of course, you can still make out the voice - an homage to commercial some of you will recognize for the product Mr. Microphone. Now you’ve heard what microphonic input tubes can do in your gear. For more information on vacuum tubes please check out my BEHIND THE GEAR interview with Groove Tubes founder Aspen Pittman. You can find it in Tape Op No 56. Nov/Dec 2006