A Javascript valve circuit design aid
Valve Designer started out as a Qt application and as a companion to the Valve Workbench with the aim of being able to simplify the design process for valve circuits commonly found in audio amplifiers (many of my builds have been guitar oriented but I have also dabbled with a bit of Hi Fi). The Valve Workbench allows you to measure (using an Arduino based analyser circuit after the Valve Wizard) and then model triodes and pentodes. For the modelling, I have explored a number of approaches and have implemented models based on the work of Koren, Cohen & Helie and Reefman, with some minor extensions of my own for pentodes. The value of having these models is that it then simplifies the process of creating automated design tools, and any of the models supported by the Valve Workbench can then be used directly in the Valve Designer.
At this point, I must also reference the work of Giuseppe Amato whose load line calculators on vtadiy.com demonstrate the principle of what can be achieved using this approach using Javascript rather than Qt (using ChartJs as the charting library - which behaves in a very similar manner to QChart in Qt). However, while these calculators are great tools, they only tell you what your load lines look like at your chosen bias point. What I find more helpful is being able to understand how the valves work in the context of practical circuits. The difficulty with this, however, is that it requires you to calculate the operating point of the circuit. In Qt, this is very easy to do by using ngSpice to do the hard work but you can’t run Spice in Javascript.
In my Qt experiments, I had already looked at plotting both cathode and anode load lines for triode amplifiers although I was still using ngSpice to calculate the operating point. Calculating the cathode load line for a given value of cathode resistor requires you to vary the grid voltage and find the anode voltage that will achieve the cathode current (i.e. the anode current in a triode) that should be flowing in the cathode resistor for the given value of grid voltage. The models allow you to determine the anode current given the grid and anode voltages but there is no simple way of finding the anode voltage given the grid voltage and anode current. Instead of attempting an algebraic solution, I implemented a gradient based search with half the maximum anode voltage as a starting point. Noting that triode curves are quite “well behaved”, the gradient search converges quickly and reliably (this also holds for pentode curves as well). All I needed now was to compute the intersection of the two load lines to find the operating point. Using a charting library is helpful here as your data needs to be supplied as a simple array of points and the algorithm to find the intersection of two line series with linear interpolation is pretty simple.
A quick check against the values obtained from ngSpice showed that the calculations were very accurate and so my solution for a simple triode amplifier was now free from a dependency on ngSpice. A bit more work was required to achieve a similar outcome for a pentode amplifier and most of the other circuits worth considering for audio are either derivatives of these or much simpler to solve. The Qt version currently provides support for the following circuits:
The following circuits are in the pipeline:
To return to this project, now that the Qt version is no longer dependent on ngSpice, the static HTML pages represent the porting of the Valve Designer to Javascript (using ChartJs as the charting engine). The currently implemented circuits are available from the index page.
In addition to the circuit designers, there is also a generic anode characteristics viewer and a pentode modeller.