Fundamental tube questions

[Earl]

Hey all, new kid here. I have a couple of questions about tube parameters.

For example, the typical number used to describe a 12AX7's internal Plate Resistance is 62.5K ohms, which seems to work perfectly in the standard gain equation(s). I'd like to know how that number was derived.

I use P-Spice, and have a number of pretty accurate tube models. I've simulated a set of Plate Characterist Curves, and the results are virtually identical to the published PCC's for the 12AX7.

I was told that this number comes from delta Eb over delta Ib, however, I've not been able to get that particular number. I was also told that this delta Eb/Ib needs to be done using a very small incremet of Eb change.

Thought?

[Enzo]

Get the RCA RC-30 tube manual or something similar and read up on the tube function. Change in voltage at the grid causing change in current through the tube is pretty much the definition of gain isn't it?

[Arthur B.]

Change in voltage at the grid causing change in current through the tube is pretty much the definition of gain isn't it?

No, that is the transcountance. You could multiply the transconductance by the load impedance to get the gain, though.

[Enzo]

Thanks.

[Earl]

Actually, transconductance is a change in Plate Current / a change in Plate Voltage with the grid held constant. It's Gm, and is Amp/Volts described in mhos.

I have RC19, I believe, and will have to get it open and see what I've managed to forget. I've also found a good web page that talks about this (ra) in Load Line terms.

[Enzo]

RC-19 is cool, but it is real old. If you want to get into it, I really suggest getting the RC-30 as well. The 19 has a lot of older tube stuff, but the 30 will include modern tubes we use a lot, as well as the tutorials and component value charts and stuff. The 30 will have all the data for tubes we currently use.

[Arthur B.]

Actually, transconductance is a change in Plate Current / a change in Plate Voltage with the grid held constant. It's Gm, and is Amp/Volts described in mhos.

That's the plate's conductance, not the mutual conductance.

[Earl]

Right you are, Author...thanks for the correction.