The start of a new project – the B-17 Bass Preamplifier

If you scroll back far enough in this blog you’ll find an entry for “Project Thor” where we had intended to build a copy of the Gibson Thor bass amplifier. After breadboarding the first stage of the preamp, however, it turned out that it was capable of handling about 200mV of input but no more. The Gibson Les Paul bass that this is intended to be used with is capable of much more output than that – around two volts at its maximum setting and driving a high impedance input. So obviously it was time to rethink this project.

The end result was the circuit below – with ideas taken from several excellent vintage amplifiers, a lot of LTspice simulation, and my own design ideas.

This circuit is for the preamplifier only. The plan is to build a separate preamplifier first, and then an amplifier (still to be designed).

This preamp features a switch to select “normal” or “high level” inputs – basically it changes the gain of the second stage so it can handle a larger input voltage range. There are also two outputs to allow driving of two amps simultaneously if needed.

Here is the predicted frequency response (and phase shift) with all controls set to the middle position and switch set to normal input.

Next step will be construction to see how closely the circuit matches this model.

Finally the name. Why B-17? Most of the names like B-1, B-2, etc have been used by other devices for the bass already. But so far as I can tell B-17 is still available! And of course, the original B-17 pictured below was certainly capable of producing some “explosive” bass sounds – although it is not our intention to cause any sort of damage!