Q: Hi, I’m working on a schematic from your website. How do you usually test your circuit, as you go, or once everything is wired?
This is a great question. The short answer is that it depends. On a simple build with just one or two stages and passive loads and power supply filters, I will probably finish all my wiring and then power up and test. On a complex build with things like active loads, multiple bias voltages, or regulated power supplies, I will test as I build. In both cases, my general testing process is fairly similar.
- Connect the project to a variac or light bulb current limiter (if available).
- With only rectifiers installed (no other tubes), power on and measure B+ voltages. These will be higher than the voltage levels with the rest of the tubes installed, but should be in the ballpark.
- 2a If using any circuits on PCBs, I will test before installing in a chassis if my external power supply and loads allow it.
- Install preamp tubes and measure bias points to be sure they’re in the right ballpark. If fixed or directly biased output stages, measure bias levels. The B+ is still a little high at this point.
- Install output tubes and dummy loads, and measure current draw and bias point. The B+ should now be at roughly the calculated level. Adjust bias if needed.
- Connect to cheap speakers and debug hum/noise. Let the project run for extended periods of time and generally abuse it a bit.
- Hook-up to the main system and crank it!
At each step, any trim pot adjustment appropriate to the stage would be adjusted as needed. Typically I will have one digital multi-meter (DMM) on the B+ at all times and additional meters to measure individual tube bias. I use alligator clips and connect/disconnect meters with projects powered down. Don’t poke around live amps if you can help it!