Since there have been some discussions on the usefulness of SPICE models and circuit simulation on several recent threads, I thought it might be of interest to some of us – the non-believers can skip this – to see a comparison of the different SPICEmodels in action… The accuracy of any simulation depends on the models used, so model verification is an important step in the design process, alas tube models are notoriously in-accurate particularly in their grid modeling. While it is not so bad for hi-fi designs, where the designers try to avoid non-linearity of the tube at all cost, in guitar amp designs, quite the opposite happens, where the grid is often pushed way beyond its normal operating range, that’s when the trouble starts… So we are talking about the extreme cases here, actually most tube models work fine in their linear region.

Anyway, in the following comparison charts, I used Loudthud’s great scope trace as the reference. And three 12AX7 models were used to generate the plate voltage/current vs. grid voltage curves, the only change was to flip Loudthud’s scope shot around, so the coordinates match up.

First, the the plate voltage traces (with plate resistor of 100k) of the models showing the differences in saturation voltages:

12AX7 Ia_Gk

Next, the plate voltage vs. grid voltage traces of the models:

12AX7 Va_Gk

And, a comparison of Ayumi’s model vs. Loudthud’s:

12AX7 Real vs Ayumi

So it can be seen that not only do the models differ greatly from each other, none of them matches the real thing! Although Ayumi’s model does come pretty close. For a more in depth look at the various models, please refer to the following:

Spice Models For Vacuum Tubes
Norm Koren SPICE Models
Duncan Munro Triode Model
Ayumi’s Lab (use Google Translate for non-Japanese speakers)