At your risk.
Take all safety precautions.
A suitable fuse must be used in series with any battery for fire hazard protection.
1st prototype was made on bread board.
This is the 2nd prototype made inside a white small Legrand electrical box which holds 3 Finder octal bases with 8 screw terminals each. These nice looking cyan colored octal bases are for the input transformer, the 1H4 electron tube and the output transformer. Electrical connections are much firmer this way and less contact noise or other noise is expected.
An alkaline 1.5V Fujitsu battery is used for heater. A 12V miniature Fujicell battery is used for the plate supply.
Grid is open ie at space potential.
At such conditions anode (plate) current is 50 micro amperes.
Schematic is based on the Pleiades V6 with the 12V battery added to the anode circuit.
Signal path:
Pleiades V6 1H4 with Altec magenta 15095 output transformer wired to 10:1 - Sony TC-D5 PRO at XLR mic in, limiter on, mono mode - Sennheiser HD 580
A Tung-Sol 1H4 electron tube is used.
It happened that 1st time of operation is on top of a bed with a Tempur mattress.
Nothing is connected to grid except one terminal of the input capacitor with nothing connected to the other side of this capacitor yet.
This amplifier is extremely sensitive.
Headphone potentiometer level on Sony is at 3.
When the mic input level is advanced the electron flow noise can be heard but it is very low.
At the same time the slightest moment or very gentle taping by finger on the Tempur mattress creates a loud sub sonic sound with a few seconds of sustain!
The other side of capacitor was short circuited to ground in order to investigate where all those strange noises come from. They disappeared except the glass noise of hitting gently the electron tube with imdex fingernail.
This indicates that the electron tube is picking very effectively electrostatic noise.
Just touching the headphones! cable could be heard. It is suspected that this is electrostatic noise too of hand rubbing the rubber cable.
Even gently rubbing finger to chest hair produced a sound at the headphones.
It has just been confirmed by having everything switched off that the sound is not originating from acoustic transmission from cable to headphones.
If the white box is lifted up from one of its sides by hand, operation of the amplifier halts for about 4-5 seconds. A possible cause may be electrostatic change by plastic deflection (piezoelectric effect?) charging negatively the grid, grid goes to cutoff and then comes back to space potential after a release time of these few second.
Nothing else is connected yet, not even the input transformer.
Notes:
The ground pin 8 of the Altec output transformer had been accidentally connected to anode rather than to Vb trough fuse. When connections were reversed the effect was less apparent but connections are left this way and the effect can be comfortably observed.
How would such a sensitive grid sound connected to a condenser or electret condenser microphone capsule, piezoelectric microphone, or a dynamic microphone connected to an input impedance step up transformer?
Μίου...
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