CtG FCBN 51B - Enhanced version of the original CtG FCN-51-B
The FCBN 51B integrates circuits of the Ampeg Portaflex B-15NB, B-15NC and B-15NF
FCBN 51B Description
The FCBN 51B is CtG's enhanced version of the CtG FCN-B-51.
The CtG FCBN 51B design like the FCN-51-B design was inspired by Mark Gandenberger's B-15NC restoration project, which can be found at the Vintage Blue blog B-15NC project.
Whats different ? - A solid state rectifier mode (B-15NB) has been added AND new packaging in a 7" wide chassis which allows it to be used with any B-15N type Portaflex cabinet.
With just a custom chassis tray the FCBN 51B can become a true flip top !
The FCBN 51B combines the original circuits of the B-15NB (1963), 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, helping to preserve their true heritage.
Its two channels each have volume, treble and bass controls. The first channel, from the B-15NF, has the volume control section before the tone stack. The second channel comes from the B-15NB and B-15NC which have the tone stack before the volume control.
The power amp section has two, switch selectable, bias modes - fixed (B-15NF) and cathode (B-15NB and B-15NC). The FCBN 51B is designed to output approximately 25 watts (RMS) in cathode bias mode and 30 watts (RMS) in fixed bias mode.
A front panel rectifier mode switch selects between a 5AR4 tube rectifier (B-15NC and B-15NF) or a solid state rectifier (B-15NB). This allows the power supply to provide tube sag or solid state tightness.
The tube set used in the B-15NB, B-15NC and B-15NF is incorporated in the FCBN 51B design:
Three 6SL7-GT dual triodes for preamp and phase inverter circuits, two 6L6GC power tubes, and a 5AR4 rectifier. 1N4007 solid state rectifier diodes are used for the "NB" configuration. (note: the phase inverter to power amp coupling capacitors in original B-15Ns are .022uF not .047uF as in the FCBN 51B)
The power transformer can be a Super Reverb type. The output transformer can be a WOT45HHR. After removal of their bell covers both can be potted with thermally conductive potting in metal cans from THD electronics.
The OT has a multiple impedance secondary which allow a choice of 4, 8 or 16 ohm speaker loads. The primary impedance is 6800 ohms.
The chassis is an 7" x 17" x 2" aluminum Bud Industries p/n AC-1416-ND. Brass plates in front and back are used to facilitate grounding.
 FBCN 51B rear view  FCBN 51B custom 7' x 17' chassis tray |
 FCBN 51B chassis layout  FCBN 51B inside chassis view |
FCBN 51B Design Documentation
| ctg_fcbn_51b_chassis_081810.pdf |
FCBN 51B chassis
 FCBN 51B schematic
 FCBN 51B custom chassis tray |
 FCBN 51B layout
 FCBN 51B tone stack vero board layout | ||||||||
| ctg_fcbn_51b_081810_bom_r0.1.pdf |
