Tube-Town Forum

Technik => Tube-Talk => Thema gestartet von: Pandjesbaas am 14.01.2021 23:15

Titel: How to interprete TT Gm?
Beitrag von: Pandjesbaas am 14.01.2021 23:15

Hello good people,

Sorry for me writing in english, my deutsch isn't any good.
Please forgive me...

OK then, some time ago I bought four matched JJ 6CA7 tubes.
The TT boxes said "Gm = 3,6"
Recently I've tested these tubes on a mutual conductance tube tester and the results were 11000 micromho's...

My question:
How does these two figures correlate?
Is there a relation between the TT Gm- measurement and the mutual conductance?

(BTW: the tubes work just fine!!)

Please enlighten me!

F.O. van Mourik

Titel: Re: How to interprete TT Gm?
Beitrag von: Pandjesbaas am 31.01.2021 23:17

>>BUMP<<
Nobody? Really?
I'm caught in the dark...

Titel: Re: How to interprete TT Gm?
Beitrag von: Dirk am 1.02.2021 10:06

Hallo,

a tube is a none-linear working tube so Ip and Gm depends upon the voltages used for testing so you can not compare the reading of your tester with our test results, however I am pretty sure the or test-setup reflects the "real-world" better then the voltages use by your tester.

Best regards, Dirk

Titel: Re: How to interprete TT Gm?
Beitrag von: dimashek am 1.02.2021 10:14

simple tester uses 1V bias voltage, low Uanode and small AC signal on the grid to "measure" the transconductance.

Take a look on the curve of the tube. The transconductance is the slope of this curve.
As you can see, it's very nonlinear, and the maximum is at low Ug.

This curves are also very dependant to applied Voltages (Ua, Uscreen)

So your tester measurement could be correct, and the TT measurement as well, using higher Voltages and different bias points -which is much more practice-relevant

Titel: Re: How to interprete TT Gm?
Beitrag von: carlitz am 1.02.2021 10:27

Would be good to get to know, HOW Tubetown has set the parameters for their measurement.

By this, transparency is given and it is easy to compare with own measurements.

Is that possible?

Titel: Re: How to interprete TT Gm?
Beitrag von: Dirk am 1.02.2021 15:08

Zitat von: carlitz am  1.02.2021 10:27

Would be good to get to know, HOW Tubetown has set the parameters for their measurement.

Steht doch oben auf der Verpackung bzw. den Etikett, schon seit Jahren. Alle Spannungen in DC.

Gruß, Dirk

Titel: Re: How to interprete TT Gm?
Beitrag von: carlitz am 1.02.2021 15:13

Echt???

Bin ich wohl ein Blindfisch.... :danke:

Titel: Re: How to interprete TT Gm?
Beitrag von: Dirk am 1.02.2021 15:14

Brille ?

Titel: Re: How to interprete TT Gm?
Beitrag von: Pandjesbaas am 14.02.2021 22:45

Hello good people,

Thank you for your replies!
I now understand, that Gm isn't an absolute value, but depends to the applied voltages.
Could have figured that out for myself. Call me stupid, lazy...

Anyway, thanx again!

Frits