Understanding valve data.

[awkwardbydesign]

I've built a basic valve tester. http://www.valveheaven.com/2015/03/an-inexpensive-easy-to-build-diy-valvetube-tester/ and http://www.valveheaven.com/wp-content/uploads/2015/03/An-inexpensive-easy-to-build-diy-valve-tester.pdf
It will test for shorts and gas leaks. The anode voltage can be set at 100/125/150/200/250/300v (more or less). The grid voltage can be varied. The anode current can be read. Basic matching can be carried out (anode current at typical grid voltages).
But the designer says that it will " Provide Gm or mutual conductance readings, directly or indirectly". So how do I go about this? If needed I could add other meters. I admit my constructional abilities outstrip my technical knowledge, but is there a primer or idiot's guide that will walk me through this? I have Morgan Jones' Valve Amplifiers and Building Valve Amplifiers (although I haven't got very far into the first one yet), so is all the info I need in there? Valve data sheets are all very well, but I don't (yet, if ever!) understand them.
As I get older (67 now) my brain is slowing and my memory is going, so I need to start simply. Or just need help! Any of which would be appreciated.

[lurcher]

Yep, its all in MJ. See how far you get and then come back and ask what you are unsure of.

For example gm (or mutual transconductance) is a measure of how much the current through the valve varies for a voltage difference on the grid with a constant anode voltage. So for example if you set the grid to -1v, Anode at 200v, and the current is 2ma, then you change the grid to -1.5v and the current is then 1.4ma, then the gm is found by ( 2.0 - 1.4 ) / ( 1.5 - 1.0 ) or 1.2ma/V

[Firebottle]

A very good primer can be found here Richard:

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

:)

[lurcher]

Though some of the things in the first few pages are not entirely correct. Stating that "current is a flow of electrons" is incorrect seems a little odd given that its hard to explain the operation of valve without that. Yes, as it says, current is the flow of electric charge, in the case of a valve or conductor, the electrons carry that charge, so current is a flow of electrons. Sounds like he is setting himself up to for a definition of how semiconductors work.

[awkwardbydesign]

Yep, its all in MJ. See how far you get and then come back and ask what you are unsure of.

For example gm (or mutual transconductance) is a measure of how much the current through the valve varies for a voltage difference on the grid with a constant anode voltage. So for example if you set the grid to -1v, Anode at 200v, and the current is 2ma, then you change the grid to -1.5v and the current is then 1.4ma, then the gm is found by ( 2.0 - 1.4 ) / ( 1.5 - 1.0 ) or 1.2ma/V
Thanks Nick. Just what this idiot needed! MJ is a slow read, so it may take some time.


A very good primer can be found here Richard:

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

:)
Alan, I'll give that a go. Thanks.
I used to be a quick learner when I was younger, but those days are sadly gone.