Tube vs solid state microphones

[Seeker]

This is a very interesting thread, I’m working on some solid state mics that will use an external psu, not meant as tube replacements, just working toward an engaging 3D sound, and being free of phantom power restrictions.

it’s very interesting to consider that microphonics might be part of the tube sound…

I have a limited knowledge of tubes, and Im fairly ignorant to the inner workings, so I wonder how microphonics could change the response of a tube. I.e. is there something going on in an electronic sense, an electrical resonance etc, or is the effect purely from vibration?

If the effect of microphonics is purely from vibration it seems hard to believe that the tube would retain a “special” sound with a soft source, a quiet singer etc, esp when the tube is buried in a mic body and possibly physically dampened as well.

My thinking is that if there is something special about microphonic tubes, perhaps tubes that are microphonic might behave a little differently electrically… does this make any sense?

 

[Tubetec]

Through working on guitar amps Ive seen plently of microphonic tubes . Typically when microphony becomes an issue ,the tube electrodes vibrate at a particular frequency , when the guitar player strikes a note in the same region as the resonance it sounds horibly discordant and possibly breaks into oscillation if theres enough gain/volume from the speaker . Theres definately a difference between a combo amp and one with a seperate head , the Marshall amp head traditionally have vibration absorbing feet that can make a difference if you have a wall of 4x12 behind you , the combo version of the same amp tends to have some extra resonances , just little peculiarities in the frequency range , certain notes that brown out a bit , specially in the low end . Difference in speaker cab may also play a part of course . All of these effects due to vibration normally only happen on the peaks , where the acoustic levels involved excite the electrodes in the valve momentarily , in the case of a tube thats become unusable because of microphony a shake or a light tap of the tube close to the ear will reveal mecanical noises , they go in reject bin .

The same action I described in amps above , to some degree effect tube mics as well , you have resonances due the the case/headshell , resonances in the tube electrodes and the effect of the performance itself , subtle things happen on the peaks .

If you look back at live footage of vintage hard rock bands from the early 70's you noticed sometimes the guitarist hits a note and the camera image starts aliasing , its probably no coincidence that they used camera tubes back in the old days in TV .

Ive only ever seen one brand of gear specified for microphony , Bruel & Kjaer .
The calibration procedure of the 2203 details the limits for interference , the capsule is replaced with a capacitor of equivalent value to ground and a maximum deflection on the SPL meter is specified from a source of vibration at a given gain on the unit .More details can be found in the documents section under B&K .

 

[Tubetec]

https://groupdiy.com/attachments/2203-manual-pdf.86889/
From page 41 on deals with the effects of vibration on the unit .

Use of submini tubes largely gets rid of the problem of resonances in the tube itself , EF86 are usually pretty good , earlier metal types are likely to be much more microphonic , the U47 does pay special attention to the mounting of the Vf14 to avoid any chance of vibrations reaching the valve itself .

I've used flexible motor mounts on pre-amp valve sockets and connected the socket terminals with short lenghts of headphone style litz , for the heaters I snipped out a couple of 'pig tails' ( the wire on a loudspeaker connecting the coil to the terminals ) , now the solid core heater supply wires ends just before the tube socket and the flexible pigtail prevents any vibrations entering the tube itself .

I had another preamp where I used very thick screened mains cable to supply HT , heaters and ground , if you happened to thread on the cable the physical vibration was clearly audible in the pre output , a more appropriate grade of cable solved that issue .

Susceptabillity to vibration in sound equipment is generally something you want to try and avoid , then again in the case of combo tube amps at full tilt boogie its all part of the equation and adds a unique sparkle at the edge of the note , must be something similar going on in old tube mics especially with loud sound sources .