The BASS AMP
The BASS AMP combines the original circuits of the Ampeg B-15NC (1964) as well as the later B-15NF (1965-67). Maintaining these circuits keeps the same gain structure and biasing found in the original B-15N amp designs, preserving their true heritage.
It has two channels each have volume with a ‘Pull Bright’ switch, treble and bass controls. The first channel, NORMAL, from the B-15NF, has the volume control section before the tone stack. The second channel, CASCADE, comes from the B-15NC which has the volume control section after the tone controls.
The power amp section has two switch selectable bias modes - fixed (NF) and cathode (15NC). While 1N4007 solid state rectifier diodes were used for the "NB" configuration, this was not incorporated into this design. This could be built into special builds or user modifications.
The BASS AMP is designed to output approximately 30 watts in cathode bias mode and 40 watts in fixed bias mode.
The tube set used in both the B-15NC and B-15NF is incorporated in the design:
Three 6SL7-GT dual triodes for preamp and phase inverter circuits, two 6L6GC power tubes, and a 5AR4 rectifier. A small change in the phase inverter to power amp coupling capacitors in original B-15Ns from .022uF to .047uF opens things up a bit.
The Output Transformer has a multiple impedance secondary which allows a choice of 4, 8 or 16 ohm speaker loads. The primary impedance is 6800 ohms. There is a 22K negative feedback resistor from the 8 ohm tap.
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