Microphonics

[Grainger49]

A buddy had a tube that rang like a triangle when the mute relay picked.  Seriously, it went Pingggggggg.  The decay took about 5 seconds.  That was a seriously microphonic RCA 6DJ8, not known to be microphonic (that I'm aware of).

In my experience I have never been aware of microphonics in my equipment, I went tubed in the 70s.  I don't think that noise during changing the selector switch is a serious problem, not to me.

What I may not be aware of is the sound of microphonic tubes playing music. What does this do to the music and how would I recognize it?  Have I been living with microphonics for years without knowing?

[Doc B.]

6DJ8s are pretty notorious for being microphonic. There was even an article about it in Glass Audio years ago.

[Grainger49]

You missed the point.  As I said, the 6DJ8 wasn't microphonic that I was aware of.  So Now I know that they are notoriously microphonic. 

Then I asked questions about recognizing microphonics that are the crux of the post.

[Paul Birkeland]

You would recognize excessive microphonics by the feedback howl that could ensue if the speakers were adequately coupled to the microphonic tube.

[Grainger49]

Thank you Paul.  I have never experienced this.  In 40 years I just haven't run into tubes that microphonic.  I keep reading about Bottleheads with this problem.  We didn't have that problem with Paully's Bee Pre.

[Doc B.]

You can see the ringing on a scope. So the best bet is to try some different tubes to find some that ring and some that don't. Then listen to both and see if you hear any difference. Different types of tubes will have different resonance frequencies and different decay rates, so there is not really a definitive sound that you should listen for. The 6DJ8 rings in the mid to high treble. The Eastern European tubes with the ribbon filaments tend to ring down in the lower midrange. The 300Bs we use have a main resonance in the midband, but there are also higher overtones.