I have an e-mail today from Peter in Australia who was asking why my results for a given valve were 1000 times smaller than some he had bought elsewhere - I was concerned as to the uninitiated, my results could suggest that I may be peddling devices which were low emission rubbish!

I smiled wryly as the decadic factor indicated that Peter had bought valves with their Mutual Conductance (gm) expressed in micro-mhos, so, how best to explain this........

OK, let's start with the basics, mutual conductance is the measure of the 'goodness' of a valve  with some people colloquially referring to it as 'the slope' like Jeremy Clarkson (!) and may be expressed using the Van der Bjil equation where μ = device gain and rp = device anode resistance: -

g_m = {\mu \over r_p}

Although I have given you the equation, I have not used any units and this is where it gets interesting for you see, in the USA, gm is expressed in micro-mhos whereas here in the uncivilised world we express it in mA/V.  As a gm of 1000 micro mhos = 1mA/V you can see where the confusion may arise for the unwary and this handy decadic conversion factor should help Peter and any other user interpolate their gm results accordingly.

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