Mutual Conductance

[mike shaw]

What exactly does mutual conductance measure in an output tube? Is it the gain of a tube?
Example, if one 6V6 measures 3000 mho, and another tube measures 4000 mho, would the 4000 tube be a 'stronger' tube with higher gain? This is assuming all other parameters are equal.
The tester in question is a B&K 500.
Thanks! Mike

 

[mike shaw]

So if there was greater change in one tube over another, that tube would be stronger or have more 'life' versus a lower amount that would be more worn down?

 

[Wakarusa]

So if there was greater change in one tube over another, that tube would be stronger or have more 'life' versus a lower amount that would be more worn down?

That's the theory anyway. As the tube ages it becomes less efficient (gas in the tube, weakening of internal elements, the emissive coating on the cathode gets used up). Some tube testers include a "life" test (like the Hickok 733). You throw a switch and watch to see if the gm reading drops off. All it really does is reduce the filament voltage in the tube, so what you're measuring with this test is if the cathode has enough coating left to operate efficiently at reduced temperatures. Before they put the switch in, we used to just flip the filament voltage selector one step down

Also, as an added bonus, discovering that two tubes (or two triodes inside one tube) have matched gm at a particular point in their lives doesn't necessarily predict how well they'll match in the future. It's like assuming that both of the front tires on your car will wear exactly the same.

 

[mike shaw]

Thanks for the info guys!!

 

[HCStraub]

Here is an article from a 1958 magazine describing different tube tests including Mutual Conductance. I used to have this on my Hickok page back when I ran it....it is a very good article.

TESTER ARTICLE

 

[mike shaw]

Thanks for the article Harry!!

 

[Fuchsaudio]

...mutual conductance (gm) is the "change" in plate current (/\Ip) versus "change" in control grid voltage (/\Vg)...ie: "output" divided by "input":

gm = (/\Ip)/(/\Vg)

...notice that both variable are AC-signals, not DC-signals, they're actually "rates-of-change" variables.

...it's also sometimes called the "transfer ratio", and thus it's other name: "mutual trans-conductance."

...in words: how much "change" in output plate current occurs for a given "change" in input control grid voltage.

Shucks, you only make posts like this because you can get those cool greek symbols in them.....showoff...Well put btw, just kidding.

 

[donnyjaguar]

I guess look better than ⌂ ⌂ ⌂ or ▲▲▲ ♪♪♪♫♫♫