Pentagrid converter

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Basic heptode-based self-oscillating pentagrid converter circuits.
Top: Indirectly-heated variant
Bottom: Directly-heated variant, which requires the cathode to be grounded Heptode Pentagrid.png
Basic heptode-based self-oscillating pentagrid converter circuits.
Top: Indirectly-heated variant
Bottom: Directly-heated variant, which requires the cathode to be grounded
Grids of a 12SA7GT pentagrid converter, showing all five grids 12SA7GT GridDetail.jpg
Grids of a 12SA7GT pentagrid converter, showing all five grids

The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

Frequency mixer nonlinear electrical circuit that creates new frequencies from two signals applied to it

In electronics, a mixer, or frequency mixer, is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals are applied to a mixer, and it produces new signals at the sum and difference of the original frequencies. Other frequency components may also be produced in a practical frequency mixer.

Contents

The pentagrid was part of a line of development of valves that were able to take an incoming RF signal and change its frequency to a fixed intermediate frequency, which was then amplified and detected in the remainder of the receiver circuitry. The device was generically referred to as a frequency changer or just mixer.

Intermediate frequency frequency to which a carrier wave is shifted as an intermediate step in transmission or reception

In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave 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 heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final detection is done.

Origins

The first devices designed to change frequency in the manner described above seem to have been developed by the French, who simply put two grids into what would otherwise have been an ordinary triode valve (the bi-grille). Although technically a four-component electrode device, neither the term tetrode nor the tetrode valve as it is known today had yet appeared. Each grid was able to accept one of the incoming signals, and the non-linearity of the device produced the sum and difference frequencies. The valve would have been very inefficient, but, most importantly, the capacitive coupling between the two grids would have been very large. It would therefore have been quite impossible to prevent the signal from one grid coupling out of the other. At least one reference claims that the bi-grille was self-oscillating, but this has not been confirmed.

Triode single-grid amplifying vacuum tube having three active electrodes

A triode is an electronic amplifying vacuum tube consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode, the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer the sound of tube-based electronics.

Electrode electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte or a vacuum)

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. The word was coined by William Whewell at the request of the scientist Michael Faraday from two Greek words: elektron, meaning amber, and hodos, a way.

A tetrode is a vacuum tube having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate. There are several varieties of tetrodes, the most common being the screen-grid tube and the beam tetrode. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.

In 1918, Edwin Armstrong used only triodes when he invented the superheterodyne receiver. One triode operated in a conventional oscillator circuit. Another triode acted as a mixer by coupling the oscillator signal into the mixer's cathode and the received signal to the grid. The sum and difference frequencies were then available in the mixer's anode circuit. Once again, the problem of coupling between the circuits would be ever present.

Superheterodyne receiver common type of radio receiver that shifts the received signal to an easily-processed intermediate frequency

A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. It was invented by US engineer Edwin Armstrong in 1918 during World War I. Virtually all modern radio receivers use the superheterodyne principle.

Anode Electrode through which conventional current flows into a polarized electrical device

An anode is an electrode through which the conventional current enters into a polarized electrical device. This contrasts with a cathode, an electrode through which conventional current leaves an electrical device. A common mnemonic is ACID, for "anode current into device". The direction of conventional current in a circuit is opposite to the direction of electron flow, so electrons flow out the anode into the outside circuit. In a galvanic cell, the anode is the electrode at which the oxidation reaction occurs.

Shortly after Armstrong invented the superheterodyne, a triode mixer stage design was developed that not only mixed the incoming signal with the local oscillator, but the same valve doubled as the oscillator. This was known as the autodyne mixer. Early examples had difficulty oscillating across the frequency range because the oscillator feedback was via the first intermediate frequency transformer primary tuning capacitor, which was too small to give good feedback. Also keeping the oscillator signal out of the antenna circuit was difficult.

Autodyne Wikimedia disambiguation page

The autodyne circuit was an improvement to radio signal amplification using the De Forest Audion vacuum tube amplifier. By allowing the tube to oscillate at a frequency slightly different from the desired signal, the sensitivity over other receivers was greatly improved. The autodyne circuit was invented by Edwin Howard Armstrong of Columbia University, New York, NY. He inserted a tuned circuit in the output circuit of the Audion vacuum tube amplifier. By adjusting the tuning of this tuned circuit, Armstrong was able to dramatically increase the gain of the Audion amplifier. Further increase in tuning resulted in the Audion amplifier reaching self-oscillation.

Transformer electrical device that transfers energy through electromagnetic induction

A transformer is a passive electrical device that transfers electrical energy between two or more circuits. A varying current in one coil of the transformer produces a varying magnetic flux, which, in turn, induces a varying electromotive force across a second coil wound around the same core. Electrical energy can be transferred between the two coils, without a metallic connection between the two circuits. Faraday's law of induction discovered in 1831 described the induced voltage effect in any coil due to changing magnetic flux encircled by the coil.

Antenna (radio) electrical device which converts electric power into radio waves, and vice versa

In radio engineering, an antenna is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves. In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.

The invention of the tetrode demonstrated the idea of screening electrodes from each other by using additional earthed (grounded) grids (at least, as far as the signal was concerned). In 1926, Philips invented a technique of adding yet another grid to combat the secondary emission that the tetrode suffered from. All the ingredients for the pentagrid were now in place.

Secondary emission a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles

Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emission of electrons when charged particles like electrons or ions in a vacuum tube strike a metal surface; these are called secondary electrons. In this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. If the secondary particles are ions, the effect is termed secondary ion emission. Secondary electron emission is used in photomultiplier tubes and image intensifier tubes to amplify the small number of photoelectrons produced by photoemission, making the tube more sensitive. It also occurs as an undesirable side effect in electronic vacuum tubes when electrons from the cathode strike the anode, and can cause parasitic oscillation.

Pentagrid

Circuit symbol of a heptode Heptode.svg
Circuit symbol of a heptode

The development of the pentagrid or heptode (seven-electrode) valve was a novel development in the mixer story. The idea was to produce a single valve that not only mixed the oscillator signal and the received signal and produced its own oscillator signal at the same time but, importantly, did the mixing and the oscillating in different parts of the same valve.

The invention of the device at first sight doesn't seem to be obscure, but it would appear that it was developed in both America and the United Kingdom, more or less at the same time. However, the UK device is different from its American counterpart.

It is known that Donald G. Haines of RCA applied for a patent for the pentagrid on 28 March 1933 (subsequently granted on 29 March 1939) under US patent number 2,148,266. The pentagrid also featured in a UK patent (GB426802) granted on 10 April 1935. However, the Ferranti company of Great Britain entered the valve business with the first known UK-produced pentagrid, the VHT4, late in 1933 (though it must have been in development, and would certainly have existed as a prototype well before that time).

The pentagrid proved to be a much better mixer. Since the oscillator circuit was more or less self-contained, good feedback for reliable oscillation across the frequency range was easy to obtain. Some manufacturers that had adopted the autodyne mixer converted some, if not all, of their designs to pentagrid mixers.

What was the goal to develop a reliable self-oscillating mixer? The reasons were to differ from the UK to America. The UK radio manufacturers had to pay a royalty of £1 per valve holder to the British Valve Association to cover use of their members' patent rights. Further, they dictated that not more than one electrode structure could be contained in a single envelope (which would have evaded the royalty - at least in part). The Americans appeared to be driven by the desire to produce a low-cost 'every expense spared' design which was to lead to the All American Five. By making the mixer self-oscillate, the necessity of providing a separate oscillator valve is avoided. The All American Five was to use a pentagrid converter from when it first appeared in 1934, right up until valves became obsolete when transistors took over.

In the UK, the five grids operated thus. Grid 1 acted as the oscillator grid in conjunction with grid 2 which acted as its anode. Grid 4 accepted the incoming signal with the remaining two grids, 3 and 5 connected together (usually internally) which acted as screen grids to screen the anode, grid 4 and grid 2 from each other. Because grid 2 was a 'leaky' anode in that it allowed part of the modulated electron stream through, the oscillator was coupled into the mixing section of the valve. In fact, in some designs, grid 2 consisted of just the support rods, the actual grid wire itself being omitted.

In America, the configuration was different. Grid 1 acted as the oscillator grid as before, but in this case, grids 2 and 4 were connected together (again usually internally). Grid 2 functioned as both a screen and the oscillator anode; in this case the grid wire had to be present to provide the screening. Grid 3 accepted the incoming signal. Grid 4 screened this from the anode, and grid 5 was a suppressor grid to suppress secondary emission. This configuration limited the oscillator design to one where the oscillator 'anode' was operated from the HT+ (B+) rail. This was often accomplished by using a Hartley Oscillator circuit and taking the cathode to the tap on the coil.

It will be noted that the UK version would have had significant secondary emission and would also have had a tetrode kink. This was exploited in providing the non linearity necessary to produce good sum and difference signals. The American devices although having no secondary emission due to the suppressor grid, nevertheless were able to get the required non linearity by biasing the oscillator such that the valve was overdriven. The American version was also a little more sensitive because the grid that accepted the signal was closer to the cathode increasing the amplification factor.

The pentagrid converter in either guise operated extremely well, but it suffered from the limitation that a strong signal was able to 'pull' the oscillator frequency away from a weaker signal. This was not considered a major problem in broadcast receivers where the signals were likely to be strong, but it became a problem when trying to receive weak signals that were close to strong signals. Some short wave radios managed quite satisfactorily with these devices. Special high frequency versions appeared after World War II for the 100 MHz FM bands. Examples are the 6SB7Y (1946) and the 6BA7 (1948). The pulling effect had a beneficial side effect in that it gave a degree of automatic tuning.

Another disadvantage was that in spite of the presence of the screen grids, the electron beam, modulated by the oscillator electrodes, still had to pass through the signal grid, and coupling of the oscillator into the signal circuit was inevitable. The American Federal Communications Commission (FCC) started requiring radio manufacturers to certify that their products avoided this interference under Part 15 of their rules. In the UK, the Postmaster General (who was responsible for radio licensing), laid down a set of stringent rules concerning radio interference.

Hexode

Circuit symbol of a hexode Hexode symbol.svg
Circuit symbol of a hexode

The hexode (six-electrode) was actually developed after the heptode or pentagrid. It was developed in Germany as a mixer but was designed from the start to be used with a separate triode oscillator. Thus the grid configuration was grid 1, signal input; grids 2 and 4 screen grids (connected together - again, usually internally) and grid 3 was the oscillator input. The device had no suppressor grid. A major advantage was that by using grid 1 as the signal input grid, the device was more sensitive to weak signals.

It was not long before the triode and hexode structures were placed in the same glass envelope - by no means a new idea. The triode grid was usually internally connected to the hexode grid 3, but this practice was dropped in later designs when the mixer section operated as a straight IF amplifier in AM/FM sets when operating on FM, the mixing being carried out in a dedicated FM frequency changing section.

The UK manufacturers were initially unable to use this type of mixer because of the BVA prohibition on multiple structures (and indeed unwilling to use separate valves because of the levy). One UK company, MOV, successfully enforced the cartel rules against the German Lissen company in 1934 when they attempted to market a radio in the UK which had the triode-hexode mixer.

Following pressure from the UK manufacturers, the BVA were compelled to relax the rules and the UK started to adopt triode-hexode mixers. The Mullard ECH35 was a popular choice.

One company, Osram, made an ingenious move. One of their popular pentagrid converter designs was the MX40, initially marketed in 1934. They put on sale in 1936, the X41 triode-hexode frequency changer. The clever bit was that the X41 was a direct plug-in pin-compatible replacement for the MX40. Thus a pentagrid radio could easily be converted to a triode-hexode without any other circuit modifications.

America never really adopted the triode-hexode and it was seldom used, even though the 6K8 triode-hexode was available to manufacturers in 1938.

In some designs, a suppressor grid was added to produce yet another heptode design. Mullard's ECH81 became popular with the move to miniature nine-pin valves.

Octode

Octode-based pentagrid converter circuit Mixer oktode.png
Octode-based pentagrid converter circuit

Although not strictly a pentagrid (in that it has more than five grids), the octode (eight-electrode) nevertheless operates on the pentagrid principle. It resulted simply from the addition of an extra screen grid to the UK version of the pentagrid heptode. This was done mainly to improve the antenna/oscillator separation and to reduce the power consumption for use in radio sets operated by dry batteries that were becoming increasingly popular.

In North America, the only octode manufactured was the 7A8. Introduced by Sylvania in 1939 (and used mostly by Philco), this valve was the product of adding a suppressor grid to type 7B8, which was the loctal version of type 6A7. Adding the suppressor allowed Sylvania to lower the current of the 6.3-volt heater from 320 milliamperes [1] to 150 milliamperes [2] while maintaining the same conversion transconductance (550 microsiemens). This allowed Philco to use this valve in every line of radio throughout the 1940s.

The Philips EK3 octode was designated as a "beam octode". The novel part about the design was that grids 2 and 3 were constructed as beam-forming plates. This was done in such a way that Philips claimed that the oscillator electron beam and the mixer electron beams were separated as much as possible and thus the pulling effect was minimised. [3] No information is available as to the degree of success. The manufacturer's information also notes that the valve's high performance comes at a cost of a high heater current of 600 mA – double that of more conventional types.

Pentode

The use of a pentode would seem an unlikely choice for a frequency converter because it only has one control grid. However, during the Great Depression, many American radio manufacturers used pentode types 6C6, 6D6, 77 and 78 in their lowest priced AC/DC receivers because they were cheaper than pentagrid type 6A7. In these circuits, the suppressor (grid 3) acted as the oscillator grid, and the valve operated in a similar manner to a true pentagrid.

One UK company, Mazda/Ediswan, produced a triode-pentode frequency changer, the AC/TP. Designed for low-cost AC radios, the device was deliberately designed to allow strong signals to pull the oscillator without the risk of radiating the oscillator signal from the aerial. The cathode was common to both sections of the valve. The cathode was connected to a secondary coil on the oscillator coil and thus coupled the oscillator into the pentode mixer section, the signal being applied to grid 1 in the conventional manner. The AC/TP was one of the AC/ range of valves designed for low-cost radios. They were considered durable for their time (even the AC/TP frequency changer, which was normally problematic). Any AC/ valves encountered today are likely to be brand new as service shops stocked up on spares which were seldom required.

Nomenclature

In order to distinguish between the two versions of the heptode, manufacturers data often describes them as "heptode of the hexode type" for a heptode without a suppressor grid, and a "heptode of the octode type", where a suppressor grid is present.

Examples

True pentagrids

Octodes (operating on the pentagrid principle)

Triode/hexode types (not operating on the pentagrid principle)

This list is by no means exhaustive.

See also

Notes

Related Research Articles

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Thyratron Gas filled tube, electrical switch, rectifier

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Control grid vacuum tube electrode

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In the years 1942-1944, the Radio Manufacturers Association used a descriptive nomenclature system for industrial, transmitting, and special-purpose vacuum tubes. The numbering scheme was distinct from both the numbering schemes used for standard receiving tubes, and the existing transmitting tube numbering systems used previously, such as the "800 series" numbers originated by RCA and adopted by many others.

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References