Post History

The video helped explain a few things, but it's still not clear what the overall purpose of this device is.  You showed it clearly producing a signal, but the documentation you quote talks about an incoming signal.  The meaning of the current shown on the meter is unclear, and so is the function of the diode/osc switch.  Because of all these things, I'm only going to answer very generally.

At first glance, this circuit seems to be using tube V-201 in a most unusual way.  It's a JAN-955, which I'm not familiar with and didn't look up.  Perhaps this tube is specifically designed for this unusual mode of operation.

It seems what is happening is that the thermionic effect is being used to create a current, which in fact seems to be the only power source for the oscillator.  When you heat something to high enough temperature, electrons at the surface move so fast that they sometimes fly off.  If they then hit something colder, so they won't fly off again from there, a net stream of electrons flows from hot to cold.

Electron tubes work on this principle by easily allows electrons to flow from the hot cathode to the cold anode, but making it pretty much impossible for electrons to flow in the other direction.  That's a basic diode.

In a normal amplifier tube, forward current is encouraged by holding the anode at a positive potential relative to the cathode.  The grid voltage can reduce this current by decreasing or even inverting the E field near the cathode.  Since the grid is usually held negative relative to the cathode, it doesn't draw any current (the boiled-off electrons won't land on it since they are repelled by the grid's voltage).  The result is power amplification.  A few volts change at high impedance on the grid can control a substantial anode current.

Your device is using the temperature drop from cathode to anode as a power source.  Electrons boil off the hot cathode and land on the cold anode, causing a current.  Note that since electrons are negatively charged, the current is actually flowing in to the anode and out the cathode.  The anode in your circuit is actually held at negative voltage, powered by the tube itself.

The grid still works to modulate this current.  Note that a parallel tuned tank circuit is connected between the plate and grid.  This feedback, together with the power provided by the tube itself, forms an oscillator.  The tuning knob adjusts the capacitance part of the LC resonant circuit, and the inductance part is changed by selecting different plug-in modules.  As a result, this oscillator can be tuned over a wide range of frequencies.

The meter seems to be showing some measure of the plate current.  Note that the + side of the meter is tied to ground.

The cathode of V-201 is tied to ground via an inductor.  That makes the power supplied by the tube high impedance at high frequencies.

With the high impedance, and the oscillator coil deliberately external to the case, small external signals at the oscillator frequency will have significant and measurable effects on the oscillator.  It appears that is the point of this device.  To measure external fields by how they perturb this very sensitive tuned oscillator.