What is Valve Mutual Conductance and why is it so important?
Also called Transconductance, mutual conductance is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. A mutual conductance checker will apply voltage to each element of the valve, supply bias and a signal to the control grid and subject the valve to a test much like it may be used in an actual circuit, measuring plate current and indicating such in micromhos. If you want to pair up valves in “matched pairs”, this is done by comparing the “Gm“**reading and finding a pair with equal (or within 10%) reading. You cannot do this with an emission checker.
This test checks valves for shorts & cathode (or filament) emission only. This reading gives a good-bad indication by virtually tying all the elements of a vacuum tube together (except the cathode & heater) and testing the tube for cathode emission by subjecting it to test as a diode (or rectifier). An emission check will tell you whether a valve is usable or not and give you an indication of the strength of the cathode emission.
No. They are variously known by any of these names depending on where you live. Somewhere sometime people started mixing and matching words. But it all means the same thing so on this website you will see we use the names interchangeably too. To add to the confusion many valves have multiple identifiers, again this depends on which part of the world they were made in and if the valve was made for the domestic or the military market. They are often (but not always) a near match in terms of their technical specifications.
If a valve is “microphonic” it will “howl,” or feedback, like a microphone. No valve is free of microphonics, and some people would cautiously say that a little is actually wanted for a livelier sound. Microphonics are caused when the insides of a valve start to loose their solid construction from vibration. There is no fixing this other than to replace the valve with one that has been previously checked for microphonics. If the valve rings like a telephone it suffers from “filament rattle.” Like a microphonic valve it should be replaced. Filament rattle isn’t technically the same since the valve isn’t acting like a microphone. Often you’ll hear people mistakenly identify filament rattle as microphonics. It’s well known that you can tap on a valve to hear if its microphonic or suffering from filament rattle, but be careful. You can easily damage the valve when it’s hot. I can test valves used in audio applications for microphonics.
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Other TERMS that are useful to know:
NOS: Stands for ” NEW OLD STOCK “. In other words it is a new tube but made a long time ago such as in the 50’s, 60’s and etc. While it’s impossible for us to know the exact history of “old stock” of such age this assessment is done using best endeavours to accurately assess the status of the tube.
UT: stands for ” USED and TESTED “.
NEW: Well. . . that pretty much says it all! Not too much choice here unless they are manufactured in Russia or China these days. This is why many of the NOS tubes are highly sought after, especially for audio applications.
So, how does a Vacuum Tube actually work?
I think this 1943 movie describes the theory and practice quite well (the 21 minute movie will open in a new window). Thanks go to YouTube and Westinghouse.
We support the Kapiti Coast Museum. The museum has a very good collection of vintage domestic radios, amateur radio and other transmitters from the early days of radio.