Figure of Merit (FOM)

Figure of Merit (FOM) is the product of the Resolution and the Signal-to-Noise Ratio (SNR). It's dimensionless.

It was introduced as an arbitrary reference and shorthand overall performance grade for classes/groups of tubes and individual specimens to facilitate comparison and what performance can be expected.

A FOM number gives an indication of a tubes quality and usefulness but is not to be taken as a tell-all-end-all as a full spec sheet gives far more detailed insight as to a tubes performance albeit it might seem slightly more esoteric to people who are unfamiliar with interpreting a spec-sheet.

In order to fairly judge a tubes FOM rating one should have some knowledge of the category to which a tube belongs as well as an average or minimum number for at least one of the factors involved in producing a FOM rating. If one knows that a particular category of tubes has a minimum SNR of 30 with average at 36 then going by FOM alone when considering an individual tube of that category is less arbitrary

Mathematic formula

The formula is:

FOM=Resolution*Signal-to-noise ratio

As Resolution and SNR are two important numbers it is fair to say that a higher Figure of Merit is a fair indication of a particular tubes performance in general, but it is not possible to deduce from the number alone the individual number of either factor.