M-OV's first indirectly-heated HT rectifiers were the MU12 and MU14. These were in fact the same valve but only a minority of each production batch were good enough to withstand the full rating when tested. These few were labelled MU14 but the 'also-rans' (initially the large majority) were labelled MU12 and sold against a reduced specification (350 V).
In time, M-OV learned how to make better valves and introduced the MU12/14 as a single type able to meet either specification. However, customers who had learned to shun MU12 'also-rans' in favour of MU14s, even for applications requiring only the lower specification, distrusted the MU12/14 label. M-OV therefore reverted to labelling them MU14 and discontinued supplying MU12s. Even so, the MU14 was never quite the equal of its Mazda equivalents such as the UU5.
After WWII M-OV often bought in and supplied relabelled equivalents made by other manufacturers.
The M-OV ellipse containing the MU12/14 designation.
The twin rectifier sections.
The pressed anodes with ridges for strength. The heaters are run in parallel with one side strapped to the cathode tube.
The classic envelope is 45 mm in diameter and, excluding the B4 base pins, is 100 mm tall.
References: Data-sheet. Type MU12/14 was first introduced in 1939. See also1939 adverts.
