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Posted by STEVE M on

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.

You may like our photo above, however, Mutual Inductance is QUITE another thing indeed!

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