The 3B28 has the same pin out and filament voltage (2.5 volt @ 5 amps) as the 866 but is filled with Xenon gas rather than mercury. Peak inverse anode voltage is 10000 volts at peak plate currents of 1 ampere or less and 5000 volt at peak plate currents of 2 amperes.
The 3B28 can operate over a wider ambient temperature range (-75 deg C to 90 deg C) than the 866 and can be operated in any position; the 866 must be operated vertical, base down. Contrary to popular belief 3B28’s do require warm up before high voltage is applied. The Altec 260A amplifier uses a pair of 3B28’s in its full wave power supply to provide B+ to a pair of 813B tubes operating push-pull.