Real Brimar 13D5 tubes made in England.. Brimar codes 4D1/1571 and 4I1/1571. The codes indicate that these are 12au7 tubes that were specially selected for use in industrial equipment I am not sure what attributes caused them to be used as a 13D5 tube but they are a 12AU7 tube. The tubes were tested on a TV7/U tube tester with the following results Tube 1 - 85/88 tube 2- 88/87 The TV7/U produces mutual conductance numbers and there are conversion tables available. ( google tv7/u mutual conductance) The numbers are 2125/2200 and 2200/2175 which indicates that these tubes are testing at NOS levels.
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