Pentagrid Converter - The Octode

The Octode

Although not strictly a pentagrid (in that it does not have 5 grids), the octode (eight-electrode) nevertheless operates on the pentagrid principle. It 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.

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 to 150 milliamperes while maintaining the same conversion transconductance (550 microsiemens). This allowed Philco to use this valve in every line of radio throughout the 1940s.

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.