[MrFixr55]

There were a series of rectifier tubes, starting with the 84 / 6Z4 7Y4 and 7Z4 that were designed for Auto radio use where the potential voltage differential between the cathode and heater, by the nature of being supplied by the Auto's 6 Volt electrical system, is essentially equal to the DC output of the rectifier. The max cathode vs heater potential difference is 450V, comfortably higher than the 180V B+ voltage used by Philco in most car and home radios where this tube is used. Newer versions of this tube include the 6X5 and 6X4 which have a somewhat lower max cathode / heater differential of 350V. (The voltage was 450V in the RC15 (1948) version but seems to be specified differently in subsequent versions such as theRC25 (1963?). It became popular to use these tubes in AC powered (home) sets as well as low to medium power HiFi amplifiers and preamplifiers, eliminating the need for a separate winding for the rectifier filamentary cathode which by nature, that winding is also above ground potential by the raw B+ output of the rectifier circuit. So, do we have the differential in the tube or in the transformer?