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Blog posts of '2014' 'December'

MUTUAL CONDUCTANCE - WHATEVER IS THAT?

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.

 
 
JUST BACK FROM THE IMITATION GAME

And what an exceptional film as Benedict Cucumberpatch was...... Alan Turing!  There were a few slight boobs though, the first being the 1951 use of an Eddystone 770R - which wasn't actually on sale until 1953.    Then there was the Anglepoise lamp in Alan's Styal flat - correct colour, correct two stage base BUT with an incorrect non-rolled wired shade rim not first seen until 1958 post austerity and all that.  Sticking with the lighting theme, the Dugdills patent lamp situated to AT's left during the bear joke scene had a fetching but decidedly not period white plastic 1970's bayonet light fitting.   The last thing to be spotted was when Alan & Joan took a train trip to see Mr MI6, they were sitting in an example of outwardly LMS coaching stock but with the corridor compartment unfortunately fitted with 1970's British Rail blue moquette seating.

But other than the above four very minor points  this was a most enjoyable film.