06-06-2021, 02:28 AM
As far as I know the X78 came about as a consequence of Osram’s decision to go with the B7G base for its post-WWII miniature domestic receiving valve range, and to eschew the Philips B8A base. At the time, the B9A base was still in the future, generally unexpected and more-or-less slipping in from left-field when it did arrive. At the time, the American valve-makers were still using the octal base for new releases that required more pinouts and/or a larger envelope than offered by the B7G, the RCA 6SB7Y and Tung Sol 6S8GT being examples.
Anyway, the Osram “77” series of B7G valves included such as the W77, Z77, D77, etc. The designated frequency changer was the X77, a clone of the American 6BE6, and therefore of the pentagrid type. Given that the pentagrid was viewed unfavourably by many, perhaps most European setmakers, a triode-hexode/heptode was required. Osram had the X81 triode heptode on the Loctal base, but it was not a miniature. Thus where a miniature was required, the only option at the time was to put essentially the same innards into a B7G bottle, and use a common heater and cathode pinout (not the only time that that was done when pinouts were in short supply).
Once the B9A base became available, Osram put the same innards into a B9A bottle, as the X79, and with separate pinouts. In the UK, the X79 was probably overshadowed by the later ECH81 (X719). But very early on it was taken up by AWV Australia, who in 1950 obtained its American registration as the 6AE8. AWV was RCA-oriented, so that its main miniature series was B7G based with the 6BE6 pentagrid frequency changer. But it also had a connection to Osram. Given that some Australian setmakers followed the European preference for triode/hexode/heptode frequency changers, the X79/6AE8 was very welcome and as far as I know was widely used. (At about the same time Philips Australia had introduced its ECH80/6AN7, the Innoval version of the ECH42.)
Anyway, the Osram “77” series of B7G valves included such as the W77, Z77, D77, etc. The designated frequency changer was the X77, a clone of the American 6BE6, and therefore of the pentagrid type. Given that the pentagrid was viewed unfavourably by many, perhaps most European setmakers, a triode-hexode/heptode was required. Osram had the X81 triode heptode on the Loctal base, but it was not a miniature. Thus where a miniature was required, the only option at the time was to put essentially the same innards into a B7G bottle, and use a common heater and cathode pinout (not the only time that that was done when pinouts were in short supply).
Once the B9A base became available, Osram put the same innards into a B9A bottle, as the X79, and with separate pinouts. In the UK, the X79 was probably overshadowed by the later ECH81 (X719). But very early on it was taken up by AWV Australia, who in 1950 obtained its American registration as the 6AE8. AWV was RCA-oriented, so that its main miniature series was B7G based with the 6BE6 pentagrid frequency changer. But it also had a connection to Osram. Given that some Australian setmakers followed the European preference for triode/hexode/heptode frequency changers, the X79/6AE8 was very welcome and as far as I know was widely used. (At about the same time Philips Australia had introduced its ECH80/6AN7, the Innoval version of the ECH42.)
Cheers,
Steve
