New Duncan Munro Vacuum Tube Models

New vacuum tube models created by Duncan Munro have recently been ported to the Micro-Cap 6 parts library. There are nineteen triodes and eight pentodes. Some have heaters and some don't. The breakdown looks like this: Triodes No Heater 2A3 3CX300 SV6AS7 6BM8 6DJ8 6N1P 6SN7GTB 12AT7 12AU7 12AX7 76 SV5723 SV57210 5751 Triodes With Heater 6SN7GTB_H 12AT7_H 12AU7_H 12AX7_H 300B Pentodes No Heater 6BQ5 6CA7 6KG6 6L6 6V6 SV83 6146 EF86 The models and several demonstration circuits have been ported to MC6 and will soon be a part of the MC6 package. As an example consider the circuit below.

This circuit was used to sweep the plate voltage to generate the plate characteristics of the 12AX7 tube. The figure below is a plot of the static plate characteristics of the 12AX7 tube as calculated by MC6.

Here are the actual plate characteristics from an old GE tube book (circa 1958). The match is very close and a tribute to Duncan's modeling work.

The figure below shows the tube configured as a linear amplifier.

Here is the transient response to a sinusoidal excitation.

Here is the AC gain plot of the same circuit.

Note that the gain at 1KHz matches the gain as measured in transient analysis. If you have an interest in the circuits, models, or other esoterica of vacuum tubes, you should see Duncan's site at http://www.duncanamps.simplenet.com. It's a tube-lovers's paradise. As a further example of tube schematics and simulation consider the circuit below. It is a schematic of an amplifier from the same GE tube handbook.

Here is the schematic rendered in MC6 using the 6L6 as a substitute for the 35C5 pentode.

Here is the transient analysis showing the input 20mV P-P, 1KHz signal and the resulting output voltage across the simulated speaker load.

Here is the AC analysis showing the dB value of the output voltage across the speaker load, RLOAD. Notice that it shows about 41 dB, the same amount measured from the transient analysis waveform.

The Duncan models should provide a consistent and useful basis for analyzing these old curmudgeons of technology. The basic 3/2 power model has been elaborated to include the change in anode resisitance and the reduction in mu at large negative grid voltages. It also takes into account the anode current limiting that occurs with high positive grid and low positive anode voltages.

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