The B-17A bass preamplifier prototype is complete! It works as intended and is now ready to ship to the South Lab for some real musician testing. There were some minor modifications to the circuit during testing and the resulting circuit diagram is shown below:
The modifications include removal of R20, and biasing of the heater circuit to approximately +50V DC. R20 was previously a 4.7 megohm resistor in series with the volume control, with a switch across it to choose higher or lower gain. In practice because of its high impedance it was really just a noise source, and is not needed to get the input range we are looking for. Biasing of the tube heaters prevents them from supplying electrons to the cathodes of the tubes which can inject 60Hz noise.
The measured frequency response of the preamp is very close to the target predicted by the LTspice simulation, proving that the simulation models are pretty accurate. The amp is nice and quiet once sufficient time is given for the tubes to warm up, and sounds good when playing recorded bass inputs. The heater elements are running slightly below their rated voltage with this power transformer, so they do take a while to reach operating temperature.
The circuit board was installed by wiring the tube sockets and the rest of the controls as shown below.
So as I said, next step is to get the prototype to the South Lab to test it with real instrument input.
After that goals for the next prototype include:
- Improved circuit layout and a better case
- Use of a commercially available power transformer rather than the scrounged unit used in this prototype
- Perhaps a power amp stage?