Pentagrid Converter - The Hexode

The Hexode

It may come as a surprise that 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 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.