RETMA tube designation

• First figure group: indicates heater/filament voltage; 0 means a cold-cathode tube.
• Letter group: letter indicate the serial order of assignment of the designation; combinations like AB, AC, AD, AE... were used, avoiding same-letter repetitions, after the single letters were exhausted. Wherever possible, the 12V equivalent of a 6V tube had the same letters, just 12 instead of 6.
• Second figure group is the number of useful elements ; in metal tubes the shell counts as one element. Internal shielding also counts, but base shielding does not. Electrodes connected together internally are counted as one element. There is an inconsistency in the way tapped heaters or filaments are counted. For example, the 12BH7 counts the center tapped heater as one useful element, whereas the 12BY7 counts the center tapped heater as two useful elements. There are also inconsistencies in the way internal screens are treated. In the 6BX6, the screen around the assembly is not counted, whereas the 6BR7 has its similar screen included.
• Optional letters: