We may sometimes say or you may have read about 'microphonic valves' on a particular amp or recent repair. Is it something to be worried about, will it cause problems or should i consider singing into it?
So a microphonic valve is as it sounds, it is where the vibrations in air or through contact cause the valve to amplifier the vibration. As a technical description yes it is a microphone, it converts vibration to electrical signals, though often not as useful as an actual microphone.
How does it happen?
You need to consider that everything in that little glass tube is all suspended on long metal wires often hanging in a glass vacuum, sounds amazing huh? well no not really it's a sheltered life of electrons travelling from the centre heater element (the bit you see glowing orange) through the grid acting as a set of traffic lights allowing the smaller signals to cross the grid and control the big super highway of electrons passing through to the end plate. Spaced fractions of a mm apart all these parts are tightly packed in, any change in voltage/ current changes the electron flow, but hang on, if we keep the electron flow constant and move a fraction of a hair distance the parts inside this changes all the dynamics and the electron path will vary as a result. This is is microphonic.
What valve are affected?
Both preamp valves and power valves are prone to becoming microphonic. Preamp tubes are generally more prone to being microphonic, why? super high gains (high Mu) often amplify even the smallest fluctuation in electron paths, you may read about Mu which is the unit for Voltage gain.
Is it bad?
Generally not however consider 2 possible issues. 1) knowing that something is able to move / vibrate inside the valve is a little worrying as it's really easy for bits inside to short out and this could be very bad! 2) in worse case instances you'll be able to get a howle of feedback from your combo amplifier where the microphonic valve is amplifying the vibration that is then going to the speaker, that is then travelling through the chassis and oh my going back through the valve and, now back through the speaker... you get the idea!