Finally have some time to get back to the B-17B design after the holidays. The focus lately has been on the preamp circuit, getting the input stage right and making adjustments to the tone stack. The amplifier section has been reworked as well – simulation of the previous design showed way too much distortion without some type of global feedback. But that will be the feature of a future update. Right now here is the latest preamp circuit:
As mentioned, the amplifier has been moved to a separate diagram.
You may notice that there is a new potentiometer labeled ‘shift’. This idea came from a circuit in Designing Tube Preamps for Guitar and Bass by Merlin Blencowe. The shift control changes the center point for the bass and treble controls, and is a more flexible alternative to a midrange control. Otherwise the tone stack is a standard James design used in several guitar and bass preamps.
Here are some bode plots (phase is omitted) for the range of tone controls. Each plot shows all combinations of the bass and treble controls set to zero, 50%, and 100%, to give the total range of frequency response. Click on the individual plots to see them full size.
The first plot is with the shift control set to zero:
Here it is with shift set to 50%:
And with shift set to 100%:
The effect may appear subtle, but our ears are very sensitive to the midrange and this appears to be a good starting point to at least try in the first revision.
Next steps are to do some additional simulation of the preamp with some wav files to see how it sounds by itself. Then its time to choose exact components and get them on order. There’s no reason why the preamp can’t be built before the amp is finalized, they are really two individual circuits by design. This will allow substitution of a more powerful amplifier in the future, or a different preamp while keeping the same amplifier.
More to come…