BLACK COATING of GLASS BULBS in Electron Tubes
 

Explanation 1:

Transcribed for your convenience from "RADIOTRONICS" Technical Bulletin No. 59 - 24 February 1936

Radiotronics magazine was published by the Amalgamated Wireless Valve Co. of Australia using technologies derived from RCA USA, Marconi, Telefunken and others.

Radiotronics was edited by Fritz Langford-Smith, the guru of Australian valve electronics
 
 

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)

Explanation 2:

Contributed by Jac van de Walle - EML

http://www.emissionlabs.com

The conductive layer inside prevents static discharge along the glass.  Some electrons always  land on the glass.   This always comes into a balance, where the amount of electrons  landing, and the amount which gets removed is the same.   If not the same, electron accumulate which would make the voltage locally on the glass rise (to very high level) until something happens. At some point a corona discharge takes place along the glass surface.    In the dark this is visible as blue glow on the glass itself.   Such blue spots are warmer as the rest of the bottle.  This blue corona glow becomes audible as noise,  because it adds up to the plate current.  

A conductive layer flattens out the hot spots, and the electrons find their way back to the anode, one way or another.   Just not via (sputtering) corona discharge any more, but more like DC current.  So the noise is gone.

Explanation 3:

Courtesy - “Google AI” - (The sources are not identified)

A conductive layer inside a device, like a vacuum tube, helps prevent static discharge by allowing accumulated electrons on the glass to flow back to the anode, rather than causing a disruptive corona discharge. This prevents

localized high voltage buildup on the glass surface, which can lead to a blue glow (corona discharge) and audible noise. The conductive layer effectively "grounds" the glass, creating a more stable and less noisy environment. 

Here's a more detailed explanation: 

Static Discharge:

• Static electricity occurs when electrons are transferred between materials, leading to a buildup of charge. In the case of a vacuum tube, electrons can accumulate on the glass surface.
•  
• Corona Discharge:
•  
• If the charge buildup is significant, a localized high voltage can develop on the glass. This can lead to a corona discharge, which is a visible blue glow (often around hot spots) and audible noise.
•  
• Conductive Layer:
•  
• A conductive layer inside the device acts like a "ground" for the glass. It provides a path for the electrons to flow back to the anode without creating the high voltage and disruptive discharge.
•  
• Reduced Noise:
•  
• By preventing corona discharge, the conductive layer also reduces the noise generated by the sputtering of electrons across the glass surface. The electrons flow smoothly back to the anode as a DC current, rather than through a disruptive corona discharge.
•  
•  
•  
•  

Back to Home Page