On observing changes in electric charge at a distance with an electron tube, grid at space potential, by observing the ac signal produced itself

At your risk.
Take all safety precautions.
Protect hearing by use of limiting, small sound pressure level etc.
A suitable fuse must be used in series with any battery to avoid fire hazard.


(Later addition:
By today's investigation it turns out that it may not be the Tempur mattress that causes the low frequency thump
It seems to be an indirect effect.
gently touching the mattress makes some wire vibrate. Change of capacitance is read by amplifier and sound is created
It remains to find out why gently touching hair produces sound through this setup.)


This is some thought on little understood yet audio microphonic effects while testing the Pleiades V6 1H4 battery powered front end amplifier on top of a Tempur mattress.


Such amplifiers are supplied with deliberately low anode voltage, 12V in the case of medium μ 1H4 but it can be as low as 1.5V in in other electron tubes. This is in order to avoid secondary electron emmision effects etc. Heaters are usually underheated in order to avoid for example secondary emmision of electrons from an electrode by radiation (photons of light from heater sticking an electrode). These amplifiers give the impression of great sensitivity or low noise figure. For references and relevant papers see older euroelectron posts.


See yesterday's observations.


A possible explanation of such high sensitivity at a distance is electrostatic variation captured by the electron tube grid which is left unconnected ie at space potential.


In fact the apparatus may be thought of as a condenser capsule with associated amplifier but the electrostatic capsule is the varying capacitance of nearby objects, ie the capsule is the environment itself.


It is well known that electrometer tubes or electron tubes used as electrometer tubes can deflect a micro ammeter needle according to a detected minute change in charge. But the needle time constant effectively measures a DC component ie a detected component.
Yesterday while taking hearing safety precautions the output of the amplifier was connected to the microphone input of another amplifier with its audio limiter on. Audio level could be adjusted at the output.


Even the gentlest touch on the Tempur mattress produced a very low frequency thump lasting a few seconds. This is assumed to be time it takes the mattress to get back to its shape but the minute acoustic mechanical wave produced.


Other strange very sensitive effects included caressing human chest hair (2 ft from the front end amplifier) and hearing the resultant sound through the apparatus.


See yesterday's post.


PS
The Tempur mattress is on top of a double bed. On top of Tempur mattress is a water resistant sheet, then an ordinary cotton sheet.