There is 26.5 V across the 12.6 V heater, but the current is not too excessive, so, yes, the model is suspect. But there are other issues, some related to the heater perhaps. But it's a weird circuit anyway, including a valve/tube/ a FET and an IC.

I don't know if you all missed what I wrote yesterday.? Yes, there is 26.6 V across the heater, but wait.? The current flows BACKWARDS.? It flows OUT of the heater.? The heater in this simulation GENERATES power!? It should consume power.

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Without UIC:

Normal solver -> 26.6 V @ -166 mA (166 mA flowing BACKWARDS out of the heater).

Alternate solver -> 266 V @ -166 mA (166 mA flowing BACKWARDS out of the heater).

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Almost all of that backwards current flows through R4, which is why R4's current looks backwards.? The heater in this simulation is an energy source, acting like a battery, generating 4.4 Watts out of nowhere and powering the rest of the circuit.

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With UIC, and no other changes:

Normal solver -> 1.165 V @ 186 mA into the heater.? That's somewhat better.? But still not right.

Alternate solver -> 1.165 V @ 186 mA into the heater.

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It must be noted that the heater's model in your 12AU7 model has loooong time-constants, and a 10 ms simulation simply will not do.? Continuing the simulation for 60 seconds:

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Without UIC:

No change, with either solver.? Stays at 26.6 V and -166 mA indefinitely.? The heater still magically generates power.? No good.

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With UIC and no other changes:

The heater's voltage has a damped oscillation, which eventually settles.? After 60 seconds:

Normal or Alternate solver -> 5.9 V @ 149 mA into the heater.

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IIRC, a 12AU7's nominal heater voltage is supposed to be 12.6 volts (hence the "12" in the part number), so this is way below normal, and I doubt that the part works right.? Was your circuit supposed to provide the 12AU7's heater with the right voltage (12.6 V)?

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I think the heater model inside that 12AU7 model is questionable, and borderline bad.? It has multiple stable operating points, at least when coupled with the rest of your circuit.? Stable operating points means intersections between the 12AU7 heater's I/V curve and the I/V curve of rest of the circuit.? Even worse, at least one of those stable operating points is nonsensical, where the 12AU7's heater generates energy from (very) thin air.? A decent model "should never" stray into that region, not even for a millisecond.? But it does, which makes this model questionable.

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If you were to continue using this model, with UIC, to save time you ought to add some initial settings (IC= or .IC) to try to get the heater up to its normal operating point without needing to run very long simulations every time to get there.? It would be a pain to run simulations for 60 seconds (lasting minutes in real time) just to see a few milliseconds of signals.

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Andy

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