Explanation 1:
Transcribed for your convenience from "RADIOTRONICS"Technical
Bulletin No. 39 - 24 February 1936
Radiotronics was published by the Amalgamated
Wireless Valve Co. of Australia using technologies derived from RCA USA,
Marconi, Telefunken and others.
BLACK COATING ON BULBS
"Many modern valves are given a black coating on the inside surface of the bulb, whose purpose has frequently been queried by engineers.
The real purpose of this coating is to provide a conducting surface so that electro-static charges may be spread uniformly over the inside surface of the bulb.
If the coating were not present, charges would tend to accumulate on small areas of the inside surface of the bulb, and these charges would lead to erratic performance of the valve, especially at high frequencies, and cause noisy operation.
The black coating used with Radiotron valves is effective in preventing this trouble."
(Examples include 6AQ5, 6L6GT, 6SN7GT and 6V6GT)