I have posted a simulation for investigating energy dissipation when a high power transmitting vacuum tube arcs over from anode to grid (A-G arcing). I have simulated energy dissipation in the glitch resistor, in a simulated SCR based crowbar circuit and finally in the vacuum tube itself. In this case an Eimac YC-179 high power triode with a 6KV DC supply is investigated. The PSU storage capacitor is 20 uF.
According to an Eimac engineering note, the maximum energy the grid circuit of a typical Eimac YC-179 or 8877 can take is 4 Joules. With only a fuse and a glitch resistor it looks like it will be hard to prevent the grid taking less than 4 Joules of energy. A typical Elschukom HV fuse opens after 10ms at 10x rated current according Elschukom datasheets. This is too long time to prevent the tube becoming damaged, if Eimacs data is not too conservative. I suspect the opening time will be less if loaded by 100-200x the rated current. I have contacted Elschukom engineering department to get more data.
In this simulation I have taken the time integral of the current x voltage product to investigate energy dissipation in Joules. (A not very well documented trick in LTspice is to CTRL+left click on the trace you would like to investigate to get the numerical time itegral of the trace data. If the trace data is a compound statement, the integral engine will give you correct units. For example P in Watts x t in seconds will give you Joules).
The findings in this simulation may be incorrect. Please contact me if you have comments or would like to share experiences from practical field tests.
If you want to disable the crowbar to see dissipation with just a glitch resistor, just change R1 to 10 MEG.
If you want to change the PSU DC voltage, right click V1 and change to desired value.
In case you would like to increase the capacitance of the capacitor bank, right click C1 and enter your new value.
If you want to change timings, please change pulse sources (they control the voltage controlled switches to simulate the sequencing in a real HV PSU).
You can download the simulation files here:
Here is the LTspice plot settings control file (Rename the file so it has a .plt extension and load it via Plot Settings / Open Plot Settings file) :
These files are licensed to you free of charge for hobby use. If you change them you have to share the results back. If you would like to use these files for commercial design / R&D purposes, please contact the author for more information.