B-17A bass preamp is complete!

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?

Stay tuned!

Progress on the B-17 preamp prototype

Good progress today on the B-17A bass preamp prototype. As you can see from the pictures below the chassis is complete and the power supply is built. I went ahead and put the knobs on and tubes in the sockets so you can see what it will look like.

The power supply is in the enclosure on top, to keep it isolated from the preamp circuitry. Here is what it looks like inside.

The inside of the chassis still needs a few things, as you can see below…

The standoffs are there for the circuit board, so hopefully its just a matter of adding that and getting everything wired up. But this is as far as I’m going today.

For the next version of this preamp we have already chosen a different case which we believe will add to the appeal. But for now the next step is to make sure this prototype works and make any adjustments needed.

That’s it for now!

B-17 Update

Once again work activities have not left me with much time to spend in the lab. I finally did get a chance yesterday to breadboard the first stage of the B-17 preamp and it works great!

I also ordered an aluminum chassis and the materials to make a turret board. I had considered just doing point-to-point wiring, but after drawing out the resulting layout it turned into more of a rat’s nest and could be difficult to modify if improvements are needed.

I also was getting tired of searching through the resistor bin to find the right values, so spent some time organizing my resistors by the first two numbers of their value as shown below.

Now it is much easier to find the right value quickly. That’s it for now – hopefully more to come soon.

Updated B-17 Circuit Diagram

Here is a refined version of the B-17 circuit, including power supply. The transformer used in the supply is one that I scavenged from an old vacuum tube pH meter. The supply is a voltage doubler type and includes a dropping resistor to get to the required 390 volts. Likewise the filament circuit requires a dropping resistor since there is one less tube than there was in the pH meter. Here is the circuit.

Next step is to build the bill of materials and order the parts. Stay tuned!

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!