Tubes are just like any other electrical component i.e. they can fail in three different ways.
The first one is to short out i.e. act as a piece of wire rather than some type of component. This will typically cause an increase in current to flow, resulting in damage in support circuitry and components.
The second way is for the device to open up i.e. break the connection of the circuit. Since the circuit is not completed, current flow comes to a halt. While it is possible that other support components could be damaged when this happens, in most cases, the device will simply not function until the part is replaced and the circuit is closed, allowing current to flow.
The third way is for a device to alter its' electrical characteristics to a great degree. While all electronic components ( resistors, capacitors, transistors, tubes, etc... ) tend to "settle" due to use and thermal conditions, some parts can literally shift value way beyond circuit tolerance. These parts can shift up or down in value, so performance can be altered in several different ways. This can cause either erratic operation or no operation at all. In lesser cases, several parts may shift value, causing the product to perform in a less than optimum fashion, but still appear to be working normally. In many cases, this is what causes "production tolerance" from unit to unit and / or "lemons".
As such, there is no way to determine if one part failing will cause another part to fail. It is all up to the circuit design, how the part fails, the quality and ruggedness of the support components, etc...
As a side note, i would count your blessings and practice preventive maintenance. That is, if you have tubed gear, start checking your tubes every once in a while and keep a log. If you've got a circuit with four or eight identical tubes, obviously, you'll have to have some way to identify which tube goes where. This will help you to know when a tube is starting to "soften up" and allow you to keep the unit operating at peak performance without suffering any major catastrophes ( or at least reducing your chances ). There are always tubes that do fail for various reasons, but staying on top of the game can help reduce the casualties incurred. Sean
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