My grid dip: how does this electron tube technology work?
Here is a video of the AN/PRM10, and here is a schematic (I believe the image can be zoomed):
As most commercial grid dip meters produced in the past, this one possesses several functions in addition to the grid dip function. One of these is the "absorption type wavemeter", that allows to measure the frequency of an incoming signal. Here is what they say in the doc:
I'm interested only in the tuning head part (the right box in the above schematic). In fact, I am only interested in understanding the "absorption wavemeter" function, that is, whenever switch S-103 is in diode position (the grid dip function is easier to understand and is not the subject here). Basically, the electron tube acts as a diode, which together with the network formed by the Z-201 coil and the tuning cap, forms a basic resonating detector. But I don't understand how the current flows and how this works.
two important points that took me a while to understand:
in diode position, switch S-103 connects the top wire to the earphone jack, which is in fact a connection to ground: only when the earphone is plugged in, the jack disconnects from ground (by mechanical pressure) and then connects to the the 4k earphone impedance. Anyway, the earphone is not needed for the normal absorption wavemeter function: only micro ammeter M-201 is used. So, basically the top wire is connected to ground and is not powered. Only the filament of the electron tube is powered.
I was told that the grid of the electron tube is able to self biase (to a negative potential) whenever the filament of the tube is warmed.
N.B: The whole documentation of the grid dip can be found Here.