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Measuring transconductance

2/28/2004 3:31 AM
Pat FMeasuring transconductance
I recently finished building a little Champ type amp to be used primarily as a test bed for tubes, and to try different tube combos (i.e. 12BZ7 & 6AQ5).I have an octal socket for 6V6's, 6L6's and 6CA7's, a 7 pin socket for 6AQ5's and a 9 pin  
socket for 6BQ5's.  
For the preamp tube I have a socket set up for a EF86 and one for the 12AX7 and compatible tubes. I have it on a chassis on my bench now, and just have to install test point terminals to make measurements from on top the the chassis. To me this is more practical for my purposes than getting and using a tube tester, since I can test the majority of tubes I'll come across working with tube amps.  
But, I need a formula for measuring transconductance. I've looked around a bit on the net but haven't found anything real concise. I plead ignorance here, and am not sure what is measured to determine transconductance and how the figure they use to match tubes is arrived at. Is this a fairly simple task?

Rob Mercure Pat,With all other voltages... -- 2/28/2004 12:38 PM