12AU7 Microphonic In One Channel

[eozen]

Novice valve enthusiast here! I completed the base Crack kit and am thoroughly enjoying music through it, but noticed one thing. When I touch the chassis near the 12AU7 such as when I reach to turn the volume pot, or lightly tap the tube, I can hear it in the right channel. Additionally, occasionally, I can hear what I assume is the tube resonance in the right channel. When this happens, touching the tube stops the ringing.

Are 12AU7s prone to microphonics? Is this something that is covered by the Bottlehead Guarantee? If not, perhaps I want to be looking into getting a replacement tube.

For my understanding, what might cause this to only be present in the right channel, and not in the left channel? Is it possible for half the tube to be bad?

Thanks!

[Paul Birkeland]

Microphonics aren't all that abnormal in vacuum tubes.  In fact all 6C45 tubes that we use in the Mainline will do this.  The best solution is not to tap the tube.  No harm will come to your amplifier from using a microphonic tube. 

[eozen]

Paul,

Understood. The amp is currently sitting on my computer desk. Perhaps I need to move it onto a separate surface to mechanically isolate it from vibration. I sometimes hear what I think is the tube resonance even when not touching the Crack.

Can just half of the tube be bad? The left channel is dead quiet.

[Paul Birkeland]

You might also be dealing with this issue: https://forum.bottlehead.com/index.php?topic=11676.0

It's important to clarify that a microphonic tube wouldn't be considered a bad tube, it's just microphonic.  It isn't all that abnormal to have a difference between halves in something like a 12AU7.  For other tube types like 300Bs or 6C45s they all seem to be similarly microphonic.

[eozen]

Funny you mention that. I actually had that issue and found my way to your post with the diode trick. That one was really quite handy. I was hearing a ton of ground noise from my computer-especially when moving the mouse around. That one was very rewarding to implement. I found some suitable NTE125s at a local electronics store. (https://www.nteinc.com/specs/100to199/pdf/nte125.pdf)

Unless you think it might be otherwise, I am fairly convinced that the ringing I am hearing is a consequence of hammering away on my keyboard or moving otherwise shaking my desk as I use it. It's a very faint ringing that decays by itself in ~5 seconds or so if I don't touch anything and is independent of the position of the volume potentiometer. Under normal use, probably nearly imperceptible, but I probably will want to move the amp off my desk to try to mitigate it.

Will follow up with results.

[eozen]

Moving the amp off the desk and onto a different surface quiets the tube since I can't excite the mechanical resonance. Glad to hear that this isn't abnormal. I won't fret.

For anyone interested—when I was hearing it in the right channel, the slight ringing was somewhere between C6 / Db6, (~1100 Hz).

EDIT:
It seems that

not to tap the tube

was the best solution to prevent microphonic ringing. Even with the amp sitting on my desk. Go figure. 

[eozen]

Hello,

I've been listening to the HD-650 + Crack combo every day for several months now and have been holding off on doing the Speedball upgrade. I wanted to come back around to this post because I noticed that the ringing I was only faintly hearing months ago has gotten worse.

My original hypothesis was that this was related to the tube microphonics, as I stated in the original post, however I came to realize that if I get near the amp, I can hear the same pitch faintly being generated in the amp itself, so perhaps that was not a complete picture. Is there any component that would be liable to make a squealing, quivering, sinusoidal-esque. It is not always present. It can come and go and occurs with varying volume, but is reliably there. Without music playing through the system, I have to take off my headphones.

Perhaps I should go and reflow all my solder joints...

[Paul Birkeland]

You could be experiencing interference from your environment.  I would put some additional separation between any wireless devices and the Crack.

[Mucker]

I have experienced first hand what PB is saying but not so much with a Crack. My Parks Audio Budgie tube phono preamp will sputter, pulse, and have steady intermittent white noise if I don't disable the 2.4 Gig band on my wireless router. They are both in the same room and when I toggle the router 2.4 band on and off it's easy to realize the problem.

I have eliminated the 2.4 band from my home because of this and have switched all devices to the 5 GHz band. All of the noise is gone.

[Happy Ghost]

. I wanted to come back around to this post because I noticed that the ringing I was only faintly hearing months ago has gotten worse.

have you tried cleaning the tube pins and socket? I had a similar problem with one of the supplied 12AU7 tubes.. Cleaned everything and it works beautifully now... One more way to check this would be to try another 12AU7. They are not expensive and pretty easy to procure.. Just my $0.02...

[eozen]

Paul, I've played around with moving the amp further away from any source of 2.4 GHz, this did not seem to help, and the sound was not distinctly that of wireless or other radio communication.

Happy Ghost, I went ahead and tried cleaning my tube sockets and tube pins. This seemed to solve the problem.

My understanding of the physics here is showing, but I want to say that my working hypothesis is that a bad tube connection was somehow creating mechanical vibration that was being picked up by the tube (the same kind of electrical noise you can hear emitted from some PC components when moving the mouse around for example). I would love to understand if this is a plausible explanation.

Thanks everyone