After nearly 18 months of using the curve tracer, I guess that everyone would find ways of improving any testing equipment. I personally traced so many valves on this curve tracer that I already know all its limitations, advantages, what it likes, what it doesn’t like, and all. Just like a pet (or a partner) or whatever analogy you would like to make.
Tracing anode current has always been a real challenge. I opted for sensing the cathode current as it’s easier to implement. For low current valves (or high gm ones) I have a set of cathode resistors to avoid any significant impact on the bias point.
Obviously sensing anode is the right way to go, but that is a hard challenge if you are looking at just using sand. A transformer couple version is fine, but not as easy as the op amp in the cathode.
Generally you would like to sense screen current and also grid current if you are looking to create some SPICE models for the valves you are planning to use.
Grid current in A2 mode plots are scarce. They are only available for transmitting valves. If you want to use 2A3, 300B or 45 in A2 you won’t find those traces around. Hence, your SPICE model will lack of accurate response in A2, unless you are able to plot the A2 grid current of course.
Here is the new circuit based on the AD8479 which will allow 600V in common mode. This op amp will help us to easily sense not just anode, but also screen and even grid current if necessary.
The AD8470 stage will convert the current across the sense resistor. The gain stage is provided by the INA122. A set of preset can be arranged to provide x10 or x100 or whatever gain you want to implement that will fit your oscilloscope Y input.
I will need to breadboard this and report some results…
Ale
