My two pennoth :- I fully agree with David's point of view. But, to get back to basics, whilst avoiding the merits & seemingly complicated theory of ferrite cored wee transformers, AVO's range of VCM's are just that - Valve Characteristic(Gm) Meters. The "Gm" being regarded as "Gm(static)", but in the 163's they introduce an element of "Gm(dynamic)" measurement. I.e. - the 15KHz signal superimposed on the 50Hz sourced 10mS pulses. Following on from Barkhausen's Law - - Gm(d) = Gm(s) x ra/ ra+ Ra. Ra being the Anode load. As long as this anode loading remains in the region of just a handful of hundreds of ohms, then Gm (d) will be just a few % lower than Gm(s). Simple enough to compensate for in the calibration procedure, so that 163 users can refer to the AVO VDM's "mA/V" used by all other AVO VCM owners. In the real world - valves in radios have a wide variety of ra's & recommended Ra's, hence nearly all valve testers just test for Gm(s) & just have a test anode load of 1 or 2 hundred ohms.
In other words, "if in doubt - shout", & don't fanny about. Complicated VCM's, particularly 163's - send them off to Mike Barker, if he's still offering a professional service.
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Originally Posted by Radio Wrangler
I think that also a lot of VCMs are getting into the hands of people who don't know how to use them, but most importantly don't have anyone to learn from. No-one is born with fully formed knowledge. Not knowing things is natural.
Back in their day, these were very expensive pieces of equipment and in a workshop anyone who was allowed to use one had been trained to the workshop manager's satisfaction.
David
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Regards, David(Simpson)