6BD4

Early high voltage shunt regulator for television applications.
Plate dissipation power = 20 Watt; max. plate volts = 20,000; max. unregulated power supply voltage =40,000; max. DC plate current = 1.5 mA.

The 6BD4 was only used in the very early color sets like the RCA CT-100 chassis as well as the Westinghouse H840CK15. It was replaced by the 6BK4 and derivatives, which dominated the market until the advent of pulse regulators in the late 1960s. The leaded glass construction is needed because the tubes emit soft X-rays during normal operation, and are installed inside a shielded compartment on a TV chassis. It is common to see used tubes that have turned a dark brown color due to X-ray and electron bombardment of the glass over years of operation.
These tubes saw also few applications outside of color TV sets, e.g. in ion implanters for semiconductor manufacturing.
[Text from Bob Weiss N2IXK]