I t was my understanding (perhaps erroneous) that 2.5 volt heater tubes were introduced to respond to the need for tubes suitable for AC receivers, which were not too much later replaced by the 6.3 volt tubes which were also suitable for 3-cell battery/mobile applications. I was curious about the continued development of 2.5 volt tubes long after 6.3 had become accepted. What was the justification for the 1619 and 1624 and the RK-41 for example, 2.5 volt (filamentary in some cases) versions of the 6L6 and 807. Yes, I have seen equipment that uses them, but I wondered why 2.5 was chosen over other voltages. Inquiring minds want to know. Thanks in advance.