Grid-leak detector
Encyclopedia
A grid-leak detector is a combination diode
rectifier
and audio amplifier
used as a detector
in vacuum tube
AM
radio
receivers.
The grid leak is the small current
through the grid of a vacuum tube
caused by the small negative EMF
which is present in all vacuum tubes as a result of a space charge
within the envelope of the tube.
The heyday for grid-leak detectors was the 1920s, when battery-operated, multi-dial tuned radio frequency receiver
s using low-mu triode
s with directly heated cathodes
were the norm. The Zenith Models 11, 12, and 14 are examples of these kinds of radios. When indirectly heated cathodes and AC
powered receivers were introduced in 1927, most manufacturers switched to plate detector
s, and later to diode detectors.
Although the regenerative
grid-leak detector was one of the more sensitive detectors of its day, its ability to radiate radio frequency energy when improperly adjusted limited its use in urban settings where multiple receivers
would be operated in close proximity. The RCA Radiola III and IIIa and the Crosley Model 51 are examples of regenerative receivers from this period.
connected from grid to ground, typically as the only bias
component. Thus grid leak current
is used to bias the valve. Resistors in the region of 1 megohm were common.
When the grid leak stage had a transformer
input, the valve grid connected to one end of the transformer secondary, and the other end of the secondary went to ground via the grid leak resistor and a small bypass capacitor.
The use of grid leak bias competed with use of a so-called C battery for providing a fixed negative voltage on the grid. When vacuum tubes came to be constructed with cathode
s electrically isolated from their filaments, it became possible to used cathode bias
ing. This involved use of a resistor
from the cathode to ground, which provides better control of anode current through negative feedback. This method also requires a relatively large capacitor
across the cathode resistor to avoid losing AC gain (through negative feedback), and the cost of a sufficiently sized capacitor was an issue in the pre-war years, as capacitors had low capacity per volume and per cost.
receivers. The grid leak detector stage is a non-linear valve amplifier
. The grid bias produced by the grid leak across the biassing grid leak resistor, plus the typically non-linear characteristic of the valves chosen, result in a non-linear rf amplifying stage. The stage performs two, or more often three functions:
In simple one-valve sets, the grid leak detector drove the headphones, since it produced audio frequency output as well as radio frequency. In better sets, the grid leak detector's output fed an audio amplifying valve, or occasionally more than one.
or IF
transformer through a capacitor (100 pF to 330 pF, with 250 pF being typical). A resistor of a few 100kΩ - 10MΩ is connected either in parallel with this capacitor, or from the detector's grid to ground. The resistor-capacitor combination, in concert with the nearly unidirectional current flow of the detector's grid-to-cathode (or grid-to-filament, if it's directly heated) circuit, forms a clamp circuit. The incoming modulated radio frequency signal voltage swings both positive and negative with respect to ground as it emerges from the final RF or IF stage. The detector's grid-to-cathode circuit will only conduct current when the grid is positive with respect to the cathode. The resulting current flow charges the capacitor to maintain a voltage which biases the detector grid negative with respect to its cathode. The amount of bias varies based on the received signal strength. The grid leak resistor - capacitor circuit is designed to have a time constant which is slightly slower than the rate of change of the audio program and much slower than the rate of change of the carrier.
Because the bias varies depending on signal strength, plate current will rise on very low signals, and operation near cut-off of the tube will cause distortion. On receivers with battery powered heaters, the heater power supply could be used to bias the tube with a small positive voltage.
The nearly unidirectional current flow of the received, modulated signal results in a separate audio signal that is said to be detected or demodulated. The triode detector tube also amplifies this audio signal, thereby providing two processes in one tube. Regenerative detectors often used grid leak, especially in the detection of amplitude modulated signals.
The grid leak detector circuit output, often taken from the plate of the detector tube, may require additional filtering to remove remnants of the carrier portion of the received signal. RF chokes and/or capacitors may be employed.
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
and audio amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
used as a detector
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
in vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
receivers.
The grid leak is the small current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
through the grid of a vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
caused by the small negative EMF
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
which is present in all vacuum tubes as a result of a space charge
Space charge
Space charge is a concept in which excess electric charge is treated as a continuum of charge distributed over a region of space rather than distinct point-like charges...
within the envelope of the tube.
History
Tuned radio frequency receiver
A tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned...
s using low-mu triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
s with directly heated cathodes
Hot cathode
In vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. In the accelerator community, these are referred to as thermionic cathodes. The heating element is usually an electrical filament...
were the norm. The Zenith Models 11, 12, and 14 are examples of these kinds of radios. When indirectly heated cathodes and AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
powered receivers were introduced in 1927, most manufacturers switched to plate detector
Plate detector (radio)
A plate detector is a vacuum tube detector circuit used in A.M. radios. This circuit employs a tube with an indirectly heated cathode, typically a medium-mu triode, or a tetrode or pentode with a sharp cut-off control grid. Rectification of R.F. signals occurs in the plate of the detector tube. ...
s, and later to diode detectors.
Although the regenerative
Regenerative circuit
The regenerative circuit or "autodyne" allows an electronic signal to be amplified many times by the same vacuum tube or other active component such as a field effect transistor. It consists of an amplifying vacuum tube or transistor with its output connected to its input through a feedback...
grid-leak detector was one of the more sensitive detectors of its day, its ability to radiate radio frequency energy when improperly adjusted limited its use in urban settings where multiple receivers
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
would be operated in close proximity. The RCA Radiola III and IIIa and the Crosley Model 51 are examples of regenerative receivers from this period.
Grid leak bias
A valve/tube has a resistorResistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
connected from grid to ground, typically as the only bias
Bias
Bias is an inclination to present or hold a partial perspective at the expense of alternatives. Bias can come in many forms.-In judgement and decision making:...
component. Thus grid leak current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
is used to bias the valve. Resistors in the region of 1 megohm were common.
When the grid leak stage had a transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
input, the valve grid connected to one end of the transformer secondary, and the other end of the secondary went to ground via the grid leak resistor and a small bypass capacitor.
The use of grid leak bias competed with use of a so-called C battery for providing a fixed negative voltage on the grid. When vacuum tubes came to be constructed with cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
s electrically isolated from their filaments, it became possible to used cathode bias
Cathode bias
In order for a vacuum tube to operate in a fairly linear region of its characteristic curve, the grid element must be maintained at a bias voltage more negative than the cathode. One method for accomplishing this is "cathode bias."-Early techniques:...
ing. This involved use of a resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
from the cathode to ground, which provides better control of anode current through negative feedback. This method also requires a relatively large capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
across the cathode resistor to avoid losing AC gain (through negative feedback), and the cost of a sufficiently sized capacitor was an issue in the pre-war years, as capacitors had low capacity per volume and per cost.
Grid leak detector
The grid leak detector was in common use in the 1920s and 30s in radioRadio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
receivers. The grid leak detector stage is a non-linear valve amplifier
Valve amplifier
A valve amplifier or tube amplifier is a type of electronic amplifier that makes use of vacuum tubes to increase the power and/or amplitude of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and...
. The grid bias produced by the grid leak across the biassing grid leak resistor, plus the typically non-linear characteristic of the valves chosen, result in a non-linear rf amplifying stage. The stage performs two, or more often three functions:
- Detection: The nonlinearity of the stage produces an audio frequencyAudio frequencyAn audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...
detected component in the anode load. - RF amplification: The stage is an RF amplifier as well as a detector, thus it gives much better sensitivity than a diodeDiodeIn electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
detector. - Reaction: In most cases feedback was added to give reaction (aka regenerationRegenerative circuitThe regenerative circuit or "autodyne" allows an electronic signal to be amplified many times by the same vacuum tube or other active component such as a field effect transistor. It consists of an amplifying vacuum tube or transistor with its output connected to its input through a feedback...
) to boost RF sensitivity, selectivity and often output volume.
In simple one-valve sets, the grid leak detector drove the headphones, since it produced audio frequency output as well as radio frequency. In better sets, the grid leak detector's output fed an audio amplifying valve, or occasionally more than one.
Operation
In the circuit, the grid of the detector (usually a low-mu or medium-mu triode) is connected to the secondary of the final RFRadio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
or IF
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
transformer through a capacitor (100 pF to 330 pF, with 250 pF being typical). A resistor of a few 100kΩ - 10MΩ is connected either in parallel with this capacitor, or from the detector's grid to ground. The resistor-capacitor combination, in concert with the nearly unidirectional current flow of the detector's grid-to-cathode (or grid-to-filament, if it's directly heated) circuit, forms a clamp circuit. The incoming modulated radio frequency signal voltage swings both positive and negative with respect to ground as it emerges from the final RF or IF stage. The detector's grid-to-cathode circuit will only conduct current when the grid is positive with respect to the cathode. The resulting current flow charges the capacitor to maintain a voltage which biases the detector grid negative with respect to its cathode. The amount of bias varies based on the received signal strength. The grid leak resistor - capacitor circuit is designed to have a time constant which is slightly slower than the rate of change of the audio program and much slower than the rate of change of the carrier.
Because the bias varies depending on signal strength, plate current will rise on very low signals, and operation near cut-off of the tube will cause distortion. On receivers with battery powered heaters, the heater power supply could be used to bias the tube with a small positive voltage.
The nearly unidirectional current flow of the received, modulated signal results in a separate audio signal that is said to be detected or demodulated. The triode detector tube also amplifies this audio signal, thereby providing two processes in one tube. Regenerative detectors often used grid leak, especially in the detection of amplitude modulated signals.
The grid leak detector circuit output, often taken from the plate of the detector tube, may require additional filtering to remove remnants of the carrier portion of the received signal. RF chokes and/or capacitors may be employed.