impossible readings on multisim probe

[Irakli]

Hi guys. I'm new to multisim and this forum.

I've got impossible readings on a multisim probe. Here's the image demonstrating it. Please tell me what should I do to rectify this error. FYI, RSHUNT checkbox dos not change anything.

[johnsold]

I do not have Multisim, so I cannot address the probe issue directly.

1. The circuit would have a nominal gain of 1000 (until limited by the tranconductance of the tube).

2. The circuit would have a gain of zero because the filament is not powered.

3. The bias is suspect. Replacing the tube with a short from anode to cathoode results in 300 uA flowing and a cathode voltage of 0.3 V.  Most tubes operate at much larger grid bias voltages and plate currents.

4. If the internal model of the tube includes sources, it could produce voltages higher than V2.

5. Many device models poorly represent what happens when the device is operated far from its nominal parameters.

What tube type are you using? Do you have a SPICE model for the tube?

Lynn

[Irakli]

Thanks for your reply.

The tube being simulated is 12AX7. 

2. You are right, but I did not power  the filaments because the readings on tube do not depend on them being powered.

3. Yeap.

5. I guess this is the case. I've done little research on the subject and seen similar SPICE model sources with comments saying the model is accurate for "Grid voltage   : 0..-3.5V" and they are not applicable for low plate voltages, in which I am interested in right now. My intention was to simply experiment with different tube conditions and see the results. It seems I'll have to get a scope and tackle with the real tube.

Here's the SPICE source of the model: 

* EWB Version 4 - Triode Model
* Vpk= 250 Vgk= -2 Ip= 0.00131 mu= 83.5
* Cgk= 1.7e-012 Cpk= 1.6e-012 Cgp= 4e-013
Rgk 1 4 1e+006
D0 4 3 Diode_12AX7
Cgk 1 3 1.7e-012
Cpk 2 3 1.6e-012
Cgp 2 1 4e-013
B0 5 0 v=83.5*v(1,3)+v(2,3)
BIp 2 3 i=1.73242e-006*sqrt(abs(v(5)+v(5))/2*v(5)^2)
.MODEL Diode_12AX7 D(N=0.001)

Btw, could you tell me what this "B" means? 

[johnsold]

The B is a non-linear dependent source.  The syntax is:

Bxxxxxx N+ N- [ I=expr  | V=expr ]

N+ is the positive node and N- is the negative node.  The V or I expression determines the voltage or current respectively.

It looks like the B0 line defines a voltage as a function of the grid and plate voltages (I think) and Blp defines a current in terms of the voltage produced by B0. Neither is constrained internally by the power supply voltage of the circuit.

The pin numbers on the image of your schematic diagram do not match the node numbers in the model.  Is this a subcircuit?

Lynn

[Irakli]

Yes, it's true, it's a sub-circuit. Now I'm assured that this is all caused by unrestrained voltages. Also, one thing that bothers me, as far as I know pin '0' is always ground and if I'm not wrong B or pin 5 is referenced to the ground, when in fact when the tube is cathode-biased, it has no direct connection to the ground. Isn't it a bit weird? 

[Irakli]

So, it turns out that I cannot have a tube with this kind of model in saturation or cutoff mode, right? 

[johnsold]

I think the fact that the B0 source is ground referenced does not matter because that source is only used to define the output of the BIp source which is connected to nodes 2 and 3. So, you can use this subcircuit in a configuration where no pin of the tube is grounded.  I have seen similar behavior in other models.

The two non-linearities in that model are the Rgk - D0 circuit which will draw some grid current at positive input voltages and the BIp source which sets the plate curretn by a power law.  But since your V1 source is connected directly to the grid and the maximum current will be in the microamp range, the effect of the diode is negligible in your circuit. The BIp source produces an output like the one shown below for a sinusoidal input. This ignores the 1.732e-6 scaling.

Lynn

[Irakli]

Lynn, I need your oppinion on this idea: would it not be better and more safe to calculate variable resistance between plate and cathode rather than creating a virtual source? I believe using a variable resistance the value of which would depend on grid and plate volages would have been better, don't you think? 

[johnsold]

I think the theoretical models used for vacuum tubes are power law current sources controlled by the grid-cathode voltage.  In that respect the model is reasonable. What would make the model better would be to limit the voltages and currents to the constraints imposed by the external circuit.

For example in your circuit the plate current could never exceed 300 uA and the plate voltage range would be approximately 0 to 300 V.

Lynn