The final result is that a negative average voltage equal to the carrier’s amplitude
develops across the triode’s input. If the carrier is amplitude modulated, the average
voltage follows the modulating signal. What is interesting here to note is that in the
absence of a carrier the control grid-to-cathode voltage is almost zero. After carrier
detection, the average voltage turns negative, so there will be a drop in the average plate
current. If the carrier is amplitude modulated, the average plate current will decrease in
more or less degree, following the modulation.

It should be clear that an amplified version of the modulating signal exists at the tube’s
output in the form of low-frequency variations of the plate current. An RF choke
prevents the high-frequency components of this current from circulating through the
high-impedance headphones, which are acting as an audio load .

However, RF-choking action is not always 100% efficient, and some RF energy may
leak into the headphones or following AF stages. This could give rise to some very
strange effects, such as low-frequency motorboating at some value of the throttle
capacitor, or the receiver breaking into oscillation when the throttle capacitor’s vanes
are fully unmeshed. Usually, selecting an RF choke with enough inductance will keep
these problems away. A decoupling capacitor may also help. In Fig, capacitor C3 bypasses
any residual RF components, leaving only AF currents flowing through the
headphones. For grid-leak detection to work properly, the grid-circuit time constant:

t = Rg x Cg = 1M x 100pF = 0.0001 sec = 1E-04 sec = 100 μsec

should be much greater than the carrier’s period T. For example, for the MW AM
broadcast band, the lowest tuned frequency is 530kHz. For this case, T = 1.887E-06 sec,
or 1.887 μsec. Clearly, t >> T. However, t can not be made to be too large, or failureto-
follow distortion will occur in the recovered audio. If TA is the period of the highest
modulation frequency, then the inequality TA > t should be accomplished. Let 3kHz be
the maximum modulation frequency component. Then, TA = 3.33E-04 sec = 333 μsec,
which is greater than t = 100 μsec. Thus, the selected values for Rg and Cg are correct.
Due to an important property of the voltage-clamp circuit, the tuning tank sees a parallel
load approximately equal to Rg/3, or 333.33k ohms in our case.