Today's valve component is METALLISING. Many valves have an external metallic coating applied to their envelope which is either copper or more commonly zinc, often finished in a coat of coloured laquer so we see silver - grey, red, gold and occasionally green colour coated envelopes. Typically, this coating is connected to a valve base pin, often the cathode by wire hoop and connecting wire placed near the junction of the envelope - base join.
Although small signal valves tend to be fully covered, many output valves have the metallic coating confined to the lower half of the envelope ala the Mazda 6P25. This is done to allow heat to be dissipated which could otherwise shorten valve life. The purpose of this metallising is not just to look pretty but to prevent external electro-magnetic fields from affecting a valve's correct operation.