Introduction of Series 7330

In 1953, Tobu became the first of all private railways in Japan to order a new commuting fleet with large (20-metre long) carbodies. It was called Series 7330, or Series 7800 as re-numbered only a year after introduction.

As the initial classification of "7330" clearly shows, this fleet was the modification from the Series 73, with Tobu's original car body design (called "half-streamlined" front view), which looked obviously better than the National Railway's. Nevertheless, it was a conservative-looking fleet, with so-called windowsills and window headers (the "ledge" on the outer side of the car body), and the wooden floor painted with antiseptics.

The technical specifications of Series 7330 were of basically the same concept as Series 73, with only minor modifications. They were capable of multiple working with Series 73 as well.

The traction motors had the rated capacity of 142kW. Series 73 came with MT40, which were of National Railway's design, and had exactly the same capacity. There was no document stating whether the electrical characteristics of Series 7330's motors were the same as MT40, but it would not be far apart if not equal. The gear ratio of the drive was also the same as Series 73, with 1:2.87. Because the original Series 63 came with the pre-war design, we can conclude that the new Series 7330 was also based on the same pre-war concepts.

The traction controller MMC-H-10E was one of Hitachi's camshaft controllers. It can be compared to Japanese National Railway's CS-10 applied to Series 80 emus for the Tokaido Line south of Tokyo (dubbed Shonan Densha, probably the world's first long distance multiple unit trainsets). In my view, the new controller design had many in common with CS-10, which were used for Series 80 in 1950 and became the JNR's successor to CS-5. Both MMC-H-10E and CS-10 had more steps than CS-5; and more importantly, both seemed to be equipped with "bridge switchover" circuit used to switch connections of traction motors from series to parallel, reducing shock that would originate from varying accelerating force.

The bogie for Series 7330, called "FS-10" by the manufacturer Sumitomo Metal Industries Ltd., uses simpler and less comfortable primary suspension mechanism - putting a single coil spring just above each axlebox - compared to its predecessors FS-106 or FS-7 used for Tobu's express trains, probably due to the fact that Series 7330 was intended for commuting trains, for which simplicity and cost-effectiveness were of first priority. This primary suspension configuration was of the same sort as TR-25 bogies that came with Series 63 / 73, which might also have affected Tobu's decision to adopt the idea. Constructed by cast steel (casting the whole bogie frame at a time - the famous Sumitomo's technique of the time), the bogie frame was simply strong. The combined use of laminated steel springs and bolster anchors for the secondary suspension was characteristic of Japanese bogie design during the times of change. I have seen a document stating that the designed maximum speed of this bogie was only 80km/h. It may have been a printing error, or even my false memory, but apparently the riding comfort at higher speeds was not satisfactory. And, not all Series 7330 cars came with FS-10s when new; two of the newly built cars, both without traction motors, were fitted with even older TR-25 which Tobu managed to find somewhere.

The brake was so-called automatic air brake system, with the brake cillinders to be equipped on the carbody, not the bogie frame.

In 1953, seven two-car trainsets, each of them having one car with traction motors and the other without, were built and put into service as Series 7330. Before the end of the year, all the cars were re-numbered to Series 7800, and in 1954 eleven more trainsets were built, making the total fleet size to 36 cars.