CathodeK
Control gridG
Plate (anode)P
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Controls
Heater power75%
Hotter cathode, stronger thermionic emission (≈ T² e^(−W/kT))
Grid voltage Vg−4.0 V
A negative grid repels electrons — this is the “valve”
Plate voltage Vp180 V
The plate’s high positive voltage attracts the electron stream
Plate current
Oscilloscope
Grid voltage (input) Plate current (output)
A few volts of swing on the grid produce milliamp-scale changes in plate current; across a load resistor that becomes an amplified, inverted voltage output — this is amplification. The jitter on the cyan trace is genuine shot noise: electrons arrive one at a time.
How it works
Thermionic emission. The heater warms the cathode
until electrons gain enough energy to escape its surface into the vacuum.
Space charge. The escaped electrons form a negative
cloud near the cathode that repels latecomers — a self-regulating reservoir
of electrons.
Plate attraction. The plate’s high positive voltage
pulls electrons out of the cloud and across the vacuum. Current flows one way
only — remove the grid and you have a diode.
Grid control. The sparse helical grid sits directly
in the electrons’ path. A small negative bias is enough to throttle the current,
cutting it off entirely near Vg = −Vp/μ.
Tiny input, large output — this is how the triode amplifies.
Ip ≈ K·(Vg + Vp/μ)3/2 — the Child–Langmuir three-halves-power law; here μ = 18