PENTAGRID and ELECTRIC MOTORS

The nearly ubiquitous "all American five" tube lineup for AM superhet radios in the US included a 12BE6 pentagrid converter. Other tubes were 12BA6 (IF amp), 12AV6 (detector and first audio amp), 50C5 (audio power output) and 35W4 full wave rectifier. The filament voltages added up to 121 volts in series, so no transformer was required to run off US line voltage.

These radios were so inexpensive to make that if they failed to work at the end of the production line a ram came down and crushed the defective unit tubes and all. I saw this in a Motorola factory near Chicago in 1962.

My memory may be fading a bit after all this time but, as I remember it, the pentagrid was the very first of the special purpose frequency changer tubes.

The first two grids`were used as grid and plate of the local oscillator.

The fourth grid Was the signal input grid mounted between the third and fifth grids tied together. These acted as screen grids to prevent unwanted coupling to G2 and plate.

This made it the single cathode equivalent of a separate triode pus screen grid (tetrode) frequency changer.

Later (Heptode) versions added one more grid (the suppressor grid) between G5 and plate to turn it into the equivalent of a separate triode plus pentode frequency changer.

This was all clever stuff but the use of a single cathode needed too many design compromises and both types were later replaced by separate cathode, triode plus hexode frequency changer designs.

The Phantastron was invented at TRE Malvern England. The name came because, until confronted with a working example, nobody could believe that such a simple single valve circuit could form such a versatile circuit building block.

It was based on a slightly modified high frequency pentode.

The normal third grid was wound with a much finer pitch so that it could act as a second control grid. A positive input to this grid resulted in a DECREASE in G2 currrent. This meant that, with suitable coupling components, these two grids formed a versatile bistable element.

The valve still functioned as a high gain pentode. A miller effect feedback capacitor between plate and G1 enabled it to produce a tightly controlled linear sawtooth.

With suitable interconnects between the basic functions the circuit could operate a time base generator, an accurately controlled delay generator or pulse generator or as a simple bistable.

Most of the UK applications used the special purpose CV329 pentode. I've no knowledge of American versions. They may well have used a modified pentagrid.

My apologies for straying far beyond the usual R.C.M remit but I expect there will still be a few contributors whose memories reach back to this era.

Jim