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

Check out the audio:
MicrophonicTube.mp3

4 Comments

by mofolotopo

on August 26, 2009

For a second there I thought I was listening to a Congotronics track.

by http://openid.aol.com/grahamspice

on September 2, 2009

so did you send the tubes back to the reputable tube dealer? Seems they could have saved you some trouble by testing them before they shipped. This is always the rub with buying NOS tubes, it would seem.

by Larry Crane

on September 28, 2009

Testing tubes won't always point out if they are microphonic. Values may be perfect, electronically, but the filaments still pick up sound.

by Garrett Haines

on October 31, 2009

I did buy these from a reputable vendor. I think for other purposes, they might be great tubes, but for mastering not so much. A prominent tube gear manufacturer wrote to me but did not want to be personally cited so as to not appear like they were hawking their brand on the blog space. I think their point is buy audio tubes from people who really know about audio engineering and audio production, not just playback. But here is the quote I received:

"We go through tubes and reject ones that are too microphonic. That's why the tubes you got from us worked better than some random whatevers with fancy labels that were probably already rejected by somebody if they were't painted chinese tubes or something (seen it all!))"

Recording Tips 02: Microphonic Vacuum Tubes (part 1)

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