Once the 6550 was introduced however and the E元4 became available here in America, everything changed. That's why almost anything with a UL output stage generally employed some sort of 6L6 family tube throughout the first half of the 50s decade. While the 6L6 family of tubes performs well in UL mode, it is not an optimum tube for that mode of operation due to the significant power loss displayed when employed - but it was the best tube available at the time and still produced a significant amount of power output in that mode for that day. At the time, the 6550 and KT88 did not exist, and the European E元4 was not to be had here in America when UL was introduced (Dec, 51). That resulted in the first ever UL OPT, the Acrosound TO-300. Remember that the original tube that UltraLinear was developed with was the 6L6GB/5881.
