Pentagrid converter

The pentagrid converter was a radio receiving valve (vacuum tube) with five grids used in the frequency-converter (mixer) stage of a superheterodyne radio receiver.

Its origins are now clouded by some obscurity, but 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 mixer

There are a few requirements for an efficient frequency changer. It has to be able to accept an incoming received signal and a locally generated signal from a local oscillator. The device has to have a pronounced non-linear characteristic. If this latter requirement is met then the device produces two further signals from the incoming pair, one at the sum of the incoming frequencies, the other at the difference. If the 'mixer' were perfectly linear, no sum and difference frequencies would be produced.

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 electrode device, neither the term tetrode nor the tetrode valve as we know it 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 the writer has been unable to confirm this.

When Edwin Armstrong invented the Superhet Receiver in 1918, although the tetrode had been invented a couple of years earlier, he nevertheless employed only triodes in his design. Armstrong employed a pair of triodes as his mixer stage. One operated in a conventional oscillator circuit, but he was able to employ the other 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 (or plate) circuit. Once again, the problem of coupling between the circuits would be ever present.

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.

The Pentagrid

The development of the pentagrid or heptode 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, but actually produced its own oscillator signal at the same time.

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 to its American counterpart.

It is known that Donald G. Haines of RCA applied for a patent for the pentagrid on the 28th March 1933 (subsequently granted on the 29th March 1939) under US patent number 2,148,266. The pentagrid also featured in a UK patent (GB426802) granted on the 10th 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 existed as a prototype well before this time).

What was the goal to develop a 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 than 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 oscilator was coupled into the mixing section of the valve.

However, 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. 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 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 ater World War II for the 100MHz 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 Communication 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.

The Hexode

It may come as a surprise that the Hexode 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 they put the triode and hexode structures 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 separate valves because of the levy). Indeed 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 be easily converted to a triode-hexode without any other circuit modifications.

It is interesting to note, that 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 9 pin valves.

The Octode

This discussion would not be complete without mentioning the octode. The octode resulted simply from the addition of a suppressor grid to the UK version of the pentagrid heptode. This was done mainly to reduce the power consumption for use in radio sets operated by dry batteries that were becoming increasingly popular.

One octode design worthy of mention was the Philips EK3 Octode. This 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. No information is available as to the degree of success.

The Pentode

Are you quite sure? - Ed. Absolutely. One UK company, Mazda, 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 part of the AC/ range of valves designed for low cost radios. They were as tough as old boots (even the AC/TP frequency changer, normally the Achilles heel of any design). Any AC/ valves encountered today are likely to be brand new as service shops stocked up on spares which were seldom, if ever, required.

Nomenclature

In order to distinguish the two versions of the heptode or pentagrid, manufacturers data often describes them as 'heptode of the Hexode type' for a heptode without a screen grid, and a 'heptode of the octode type' where a screen grid is present.

The Transistor era

The early decades of the transistor era were very much a step backwards. The only transistor type available was a three electrode device analogous to a triode valve. The oscillator signal was either coupled in through the emitter (analogous to the cathode) with the signal input to the base (grid), or both signals were connected to the base. The transistor’s inherent non linearity enabled the sum or difference frequency to be recoverable at the collector (anode). Transistors were specifically produced for this duty with greater non linearity than would be the norm. For broadcast receivers this was entirely adequate. Some designs successfully managed to make the first transistor function as both the oscillator and the mixer. For more demanding applications special versions of the Field Effect Transistor appeared that had two gate connections (and was thus analogous to the original bi-grille valve mentioned in the opening paragraphs).

Of course, in the 21st Century, the mixer is more than likely a small square lump of plastic with several pins known as an Integrated Circuit. One pin accepts the signal input and another, the oscillator input. Who knows what goes on inside?

Significant Types

2A7 and 6A7 – The first of the RCA pentagrids 1933
VHT1 – Ferranti pentagrid 1933.
MX40 – Osram pentagrid 1934.
X41 – Osram Triode Hexode 1936 – Plug in replacement for MX40
ECH35 – Mullard Triode-Hexode
ECH81 – Mullard Triode-Heptode (of the hexode type).
6K8 – American Triode-Hexode 1938.
6SB7Y and 6BA7 – Pentagrids for VHF use 1946
1LA6 and later 1L6 – battery pentagrid for Zenith Trans-Oceanic short wave radio.
7A8 – the only octode produced in America by Sylvania 1939
EK3 – Beam octode produced by Philips.

This list is by no means exhaustive.

The All American Five used a number of valve types in its history and the reader is referred to that article for a listing of the different types used.

External links

References

Sibley, Ludwell, "Tube Lore", 1996

Stokes, john W, "70 Years of Radio Tubes and Valves" 1997

Thrower, Keith, "History of the British Radio Valve to 1940."

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