Pentagrid converter

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 or bi-grid).

[1] 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 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.

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 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).

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).

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.

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.

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.

Special high frequency versions appeared after World War II for the 100 MHz FM bands.

The American Federal Communications Commission (FCC) started requiring radio manufacturers to certify that their products avoided this interference under Part 15 of their rules.

It was developed in Germany as a mixer but was designed from the start to be used with a separate triode oscillator.

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).

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-cell batteries that were becoming increasingly popular.

Any AC/ valves encountered today are likely to be brand new as service shops stocked up on spares which were seldom required.