laurie54 wrote:
Does anyone here know if these tubes are really encased in leaded glass or sand glass.
...
Secondly, wouldn't the gas inherently make the amp noisier? Especially for HP use?
The second question is an easy one. Not to a level that you would notice. The secondary emission current in the tube is very uniform for equipotential points in the glow field. As such, the noise is purely white and relatively low. Only at very low bias currents (~ tens of µA) would the noise level rise appreciably.
Your first question is much more interesting however. So, I have reviewed both the 1940 and 1962 versions of the RCA Electron Tube Design manuals, as well as the seminal 1948 text by Karl Spangenberg "Vacuum Tubes" looking for answers. It is important to note that with the exception of some very special purpose tubes, most tube envelopes are either "lime bulb" glass or "lead bulb" glass. The short answer to your question is that it doesn't appear that the transmissibility of the glass was an issue of concern in glow tube design. J Gallup of RCA in the 1962 paper says the following in the section on
Light Transmission, Luminescence, and Index of Refraction.
Quote:
The light transmission of receiving-tube envelopes and stems is of value in that it allows nondestructive visual inspection of completed tubes. The transmission of clear glass is approximately 91 percent in the visible spectrum (wavelengths between 4000 and 7000 angstroms). The major portion of the loss in the visible spectrum results from a loss of about four per cent by reflection at each surface. However, the so-called clear glasses start to absorb energy in noticeable amounts in the ultraviolet region below 4000 angstroms. Light transmission through ordinary lead and lime glasses is down to nearly zero at 3000 angstroms. A similar absorption occurs in the far infrared region, and complete cutoff occurs between 45,000 and 50,000 angstroms for all normal glasses. As a consequence of these effects, special ultraviolet transmitting glasses must be used for ultraviolet lamps, and electron tubes and electric lamps run hot because of the absorption of infrared, or heat, rays in their envelopes.
The luminescence of commercial glasses is also of little importance with respect to electron tubes, save as a method of identification. Under short-wave ultraviolet illumination (2537 angstroms), all high lead glasses glow a bright blue. This effect permits ready discrimination between lead and lime bulbs because 0080 lime glass fluoresces very weakly in comparison with lead glass.
... [Emphasis added.]
So given this information, I would suggest that, whereas the general properties of lead glass did lead to it's use in some tubes, the limitation of ultraviolet radiation was not among those properties. However, when the leaded glass was used in gas tubes where short-wave ultraviolet was produced (i.e. glow discharge tubes) the bright blue color observed is not due to the ionized glow region within the tube but rather the luminescence of the lead glass under bombardment of the ultraviolet radiation from within the tube.
For these reasons I strongly suspect that the emission of such tubes does not present a significant risk to vision or retinal health.