# Can someone educate me on transconductance?

elkym
October 24th, 2008, 12:13 AM
I understand something of the different tube types...

I'm now trying to understand measurements taken of tubes...

for example: just got a couple of 12at7's. each of these was 'measured' at 3000/3200, 3200/3200, and 3700/3900. I have no clue what this means... and I'm not sure what to Google to find the answer... can you help me?

elkym
October 24th, 2008, 10:09 AM
bump

mojo2001
October 24th, 2008, 02:48 PM
Transconductance is the ratio of a change in voltage at the input of the tube to the change in current at the output.

In basic terms, this indicates how "sensitive" the tube is to input voltage changes. A high transconductance tube swings a lot of current for a given change in input signal.

Remember that a tube creates output voltage swings by varying the current through a load resistor or a transformer.

You can visualize this like the voltage operates a water spigot (British term for tube= valve). Signal voltage changes are like wiggling the knob changing the water pressure. The load resistor is like a water powered generator, cranking out higher voltage as the water flow increases.

A "high transconductance water valve" would create a large change in water flow for a small twist on the knob.

In plain language you could say that a higher transconductance tube "amplifies" more in a given circuit. As the tube gets old and transconductance drops, it "amplifies" less.

backline
October 24th, 2008, 04:23 PM
This is kind of a big question. Are you trying to make an amp run cleaner, or maybe brighter?