We’ve reached the point where our Hybrid Vacuum-tube mic preamp prototype works, and now we need to put everything together in an easy-to-assemble package, in other words a complete kit.
So it’s time to design the Printed Circuit Board.
Continuing our Mic Preamp Design Adventure… see past articles:
Part 4: Amplifying mic level input signal
Part 3: Distortion & Harmonics. Solid-State vs. Vacuum Tubes
Part 2: Impedances & Loading – Adventures in building a Tube based Mic Preamp
Part 1: Adventures in building a Tube based Mic Preamp
…. first of all, I apologize for this late follow-up on this blog. This Part 5 blog was drafted months earlier, but never got posted.
We’ve reached the point where our Hybrid Vacuum-tube mic preamp prototype works, and now we need to put everything together in an easy-to-assemble package, in other words a complete kit.
So it’s time to design the Printed Circuit Board.
First, we need some “ground” rules (no pun intended). In this case, we want all our components to be through-hole parts for easy soldering even by an electronics beginner. Second, the vacuum tube shall have it’s own mini-PCB adapter to mount the tube-socket. Why? By using a tube-socket, you can experiment with using different brands/model of compatible tubes, different year vintage or country of origin… or if the vacuum tube fails, I want to make replacement easy to get you back up going as soon as possible.
I’m using Eagle CAD to design the Printed Circuit Board because I’m very proficient in it, and I have already built several Eagle libraries and schematics that I can re-use and import into this new design. I know my parts libraries are proven and work in the real world, and I only have to worry about a few new things (like the vacuum tube PCB adapter).
The main board:
The Vacuum Tube adapter board.
Yes, there’s an LED in the vacuum tube adapter board. You can enable it or not, just by leaving out the resistor. I didn’t put a specific value for the RLED resistor, since that will depend on your supply voltage or the forward voltage of the LED you want to use, or how bright or dim you want the LED illumination to be. I say “less is more”… you don’t want it blindingly bright, but just enough to enhance the effect.
Sent the boards for manufacturing, wait a few days, and we get our finished prototype boards…. version 1.0!
You know how version 1.0 boards go… somehow, we’ll find a mistake or two AFTER we received the physical boards. Doesn’t matter how much we review the boards and check and double-check BEFORE sending it out for manufacturing, we always find that one or two stupid mistakes AFTER we get the real boards back.
