[Amps] Fw: "Tubes 201" - How Vacuum Tubes Really Work

[Joe Subich, W4TV]

Karl-Arne writes: 

> It also appears that most modern power grid tubes are very 
> robust, as a "glitch resistor" usually will absorb sufficient 
> energy to prevent destruction of the grids and cathode. 

Large power grid tubes can be very rugged, particularly when 
some of the newer materials are used in their grid structure. 
However, there are still some applications,  Inductive Output 
Tubes (IOT) used for UHF television that have failure modes 
that cannot tolerate the arc.  In those cases the crowbar 
(generally a triggered gap) is universal with the bigger tubes.

> Preferences of individual design teams seem to govern if the 
> "glitch resistor" or "crow-bar" route is taken. 
> Collins used the "crow-bar" on the 204J-1, but not on the 
> 208U and HF-8022, Philips and Telefunken used ignitron
> "crow-bars" in their 30 and 100 kW systems. 

Use of a "glitch resistor" or crowbar is often based on the size 
of the power supply and nature of normal operation as they influence 
the power dissipation requirements for the glitch resistor.  Those 
Applications with large standing or peak currents seem to gravitate 
toward crowbars due to either the size (dissipation) required with 
high standing currents or effects on HV stability with high peak 
currents.  

73, 

   ... Joe, W4TV