The AM Forum

Just wanted to guive a report on learning how to use MicroCap 12, which was made free to the public some months ago. There have been several things to finish in LTspice first but I think most of what will do from now on with be in MicroCap 12. It's so much better. Gives a ton more information about many parts of the circuit at once.

Here is something fun. Part 15 "100mW input" AM transmitter - with tubes. I'd started in LTspice with a MOSFET, but this is much better. The 12AU7A runs on 30V and 100mW plate power. A modulated DC supply is being used here, same as would be in LTspice.

It's a start. Still working on the next step with a modulation transformer, and a separate DC supply and AC audio source for modulation. Its transformer model seem easy to use but I just started that today. It's not slow but it is using one processor core and maybe there's a setting for more.

disregard the many notes, if they don't make sense. I picked the 12AU7A for the class C amp because it's rated for a lot of negative grid voltage and looks like it's run nicely at the low voltages needed to stay within the 100mW DC input rule. In this schematic the pi network input impedance is wrong, and maybe the loading coil and presumed antenna reactance 57pF. Still, there is shown about 400V of RF at the base of the antenna and 54mA into it, so its not too far off. With 100mW, have to make the most of every mW. Anyway MicroCap 12 gets my vote for being easy nenough to learn to make this circuit in one evening, and other good stuff like showing the current at each point (and volts and power if wanted). The Transient window can accommodate many waveforms, not just one.

I have Multisim and it has a selection of vacuum tubes in its library  but nothing like Microcap12.  Microcap has a huge selection which I did not know. 

I'm just a neophyte when it comes to this design/analytical software. I used to have a program, I think it was called TUBES but that was on my old Xp machine and I never transferred it out. Could this MicroCap be used to design? I suspect so based on that you can also utilize it to analyze your circuit. I remember SPICE vaguely. But that was at my early days contracting for NASA LRC.

Mike,
Yes, Microcap can be used for design of a circuit and apparently a wide variety of tube circuit/simulations unlike my Multisim counter part.  Microcap cannot do circuit board design unfortunately but one can export files to other circuit board programs for circuit board creation. I don't recall which ones but 3 or 4 in all.  I think OrCAD was one of them.  Maybe someone else knows.

Bob said:Bob, I should have been more clearer. I meant doing the schematics. Years ago, (like about 2002) I purchased a Valiant. It was ok but no matter what I did, I could not rid it of 120Hz hum. So the previous owner 'JS'ed' components in the speech amp and other components. However, there was no trimming of the component leads and RTV everywhere! So I gutted the entire speech amp. Come along in about 2013, I start rebuilding what I removed. Fastforward to today, I realized that it was Hi-fi'd but not exactly which one. So since I'm starting clean, I'm going to incorporate the best of Tim, WA1HLR's mod's and Keith, WA1ZHK's mods. Hopefully along the way I will have eliminated the 120Hz hum, (and yes I did replace the filter caps!). Now talking with Tim last night on 160, he said something about connecting the secondary to the centertap of the primary and making it in phase with the audio. He was just musing but it is something to try and simulate. Didn't mean to steal the thread but I'm going to use this to see how I can use it to my advantage on said project.

MicroCap:
http://www.spectrum-soft.com/index.shtm
https://en.wikipedia.org/wiki/Micro-Cap