An Interesting source of crack noise.
This is a picture of a VAIC 52B Reference amplifier. It had a crackling tube noise, which was socket related. Remove the tube, sometimes helped, but the noise would come back. What we found was a mosquito, catched between the anode pin and grid pin. The voltage difference is +600Volt DC. So locally, the static electric field is very high, and it attracts light weighted objects like dust pieces, a thin hair, or like here... a Mosquito! As you can see, for the poor animal the electrostatic force was so high, it stretched his legs, and then got dried in this position. Obviously the 600Volts was not enough to burn the Mosquito, it was fully intact. Yet also crackling current through the mosquito in the range of (say) 50nA will give a signal of 50mV, when the grid resistor of the 300B is 1 Mega Ohm. After amplification by the 300B you get 200mV at the tube anode.
The output transformer transforms down 1:20 so reduces that to 10mV at the speaker outputs. The human hearing limit for a crack noise is below 1mV at the speaker outputs.
You can imagine what happens if insects find their way into the inner part 300B socket. So the part on the tube itself. That would give a crackling tube also.
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