On Wednesday 18 June 2003 12:27, GGLL wrote: > Reading RCA manual I've seen that 4CX250B (6V heater) tube requires > no warm-up period, but 4CX250F (26 V heater) a 30 seconds period, why > the difference? There is no difference. The instructions for the 6V version are incorrect. Personally, I use at least 1min. warmup. Steve