This short article describes something called microphony in guitar amplifier valves.
Microphony particularly occurs in the preamplifier valves, or rather it’s more accurate to say that microphony is more noticeable in preamplifier valves. That’s because these valves are at the start of the amplification chain and so any noise due to microphony is very greatly amplified and hence more noticeable.
For the record, you can most definitely get microphony in the power stage valves too.
What is Microphony in valves?
I’ll talk about microphony in the context of guitar amplifiers. Put simply, microphony is the mechanical vibration of the valve by loud sound waves emanating from the speaker. Remember, the speaker in the average guitar combo is located very close to the valves. Sometimes it’s only inches away!
Here’s what happens with a typical microphony setup. You play a loud chord or note on the guitar. The speaker reproduces this and sends out a high pressure sound wave. This sound wave hits the microphonic valve. The elements inside the valve rattle. This induces an electrical signal into the valve which become subsequently amplified and manifests as either a distortion or a horrible, resonant ‘ringing’ on certain notes or chords.
Why Are Some Valves Microphonic?
Valves are quite simple devices. They consists of little bits of metal (elements) suspended on fine wire stalks (brought out to the pins on the valve base) all surrounded by a glass envelope. The current through a valve is HIGHLY dependent on the spacing of these elements. One thousandth of a millimetre change is definitely noticeable as a current change. So, if you waggle the elements around inside the glass envelope, you’ll get an electrical current variation corresponding to your waggling! Amplify this, and you would definitely hear it. That’s called microphony.
Are All Valves Microphonic?
The short answer is yes. Try the following experiment. Turn your amplifier up reasonably loud, then tap the first preamp valve with a biro (etc.) You will definitely hear the ping.
But microphony in valves is only a problem if … well … if it’s a problem! In other words if you can hear nasty noises when playing. Fortunately it’s easily rectified by replacing the microphonic valve. A particular valve will cause problems if its metal elements are just slightly more prone to vibration. It can also be a problem if their resonant frequency of vibration is at a noticeable and annoying frequency. This all depends on the stiffness of the elements and this varies massively from valve to valve.
99/100 times just swapping the valve will cure it.
Not sure which valve is microphonic still?
Get a pal to play the offending chord or note. Then, using a tea towel or similar, grip the preamp valves on by one, hard, using the towel to stop getting burned. The objective is, of course, to muffle or acoustically isolate the valve from the speaker output. When you grip the microphonic valve, the nasty noise will instantly go away.
How to Locate a Microphonic Valve.
Again, fortunately this is simple. Turn the amplifier up fairly loud. Now just tap each preamp valve (the smaller ones!) with a biro or similar until one give a noticeable acoustic ping through the speaker. Swap that valve and job done.
Microphony in Power Tubes.
It’s not uncommon to get microphony in the power tubes, even though the induced signals are hardly amplified (as the power tubes are so late in the chain).
The reason for microphony in power tubes is the same as for preamp tubes – the elements are ‘waggling’ (vibrating) when hit by a loud acoustic sound wave. But the power tubes and elements are so much larger and hence much more likely to be vibrated.
The symptoms are horrible distorted harmonics when playing loud chords or notes. If you think it’s a power amp valve, try the tea towel test as above (but be more careful not to get burned as the power valves are HOT compared with the preamp valves.
Again, swapping the tubes for a new set will cure the problem.
Please note that if your symptoms are horrible distortion on loud chords, microphony is just one of the possibilities. Eliminate this first as it’s easy to do. There are two other possibilities. The first is a slightly blown speaker. The second is a dry joint inside the chassis, particularly on power resistors. Again, these vibrate when the speaker is active and this makes/breaks the bad joint hundreds of times a second and makes an awful noise!
I hope you have found this article on valve microphony interesting.