The Markbass Classic 300 amplifier is the
first amplifier from Markbass featuring a
tube output section. The head cabinet is
covered in black tolex, and the face of the
cabinet is accented by neatly-applied
(surprise!) yellow piping. A yellow logo,
suspended to a screened vent arranged
within an oval port on the face of the
cabinet, provides a cool 3-D feature. The
chassis is also black, and prominently
features yellow silk-screened control
labeling and a display screen positioned
adjacent to the power switch.
Internal Construction
The tube complement of the Classic 300
consists of one ECC083 (12ax7) preamp
tube, one ECC083 phase inverter tube,
two ECC099 driver tubes and a sextet of
KT-88 output tubes, as shown in Fig. A.
The test amp was received with all new
production, JJ-brand tubes. I have to
admit, working on the Markbass Classic
300 felt more like working on a personal
computer than a bass amp. The Classic
300 chassis may be accessed by removing
nine screws from the back panel and four
hex head bolts from the bottom of the
cabinet. To avoid weight and stress on the
chassis, the transformers are bolted to the
head cab and the wiring from the
transformers connects to the circuitry
within the chassis using heavy duty molex
connectors. In all, there are seven molex
connectors that connect the power
transformer, output transformer and fan
to the amp circuitry. All of the molex
connectors are heavy duty with the
exception of the fan connector. When
removing the back panel, care must be
exercised as the fan, which is secured to
the back panel, plugs into the chassis
using a thin wires and a small,
plastic connector. There is very little
slack in the wires to position the
panel out of the way enough to
easily unplug the fan. As the fan is
an important part of this amp and is
computer controlled, it would be nice if
this wiring were more robust.
Fortunately, the front panel bearing the
Markbass logo is secured to the head cab
using Velcro, so a user can access the tubes
without the need to remove the back
panel.
With reference to Fig. B, the internals
of the amp consist of a main circuit board
that contains the tube circuitry as well as
2008 Markbass
Classic 300
Bass Head
AMP
LAB
the microprocessor circuitry (discussed
below). The amp also includes a circuit
board for the preamp/tone control
circuitry as well as a few smaller circuit
boards for the display, tuner and effects
loop jacks, etc. The tube sockets are
soldered directly to the main circuit
board, so use care to gently remove tubes
when performing tube swaps.
With reference to Fig. C, as noted
above, one of the circuit board contains
the circuitry for the tone controls. This
board is a neatly arranged collection of
surface-mount components, including
resistors, capacitors and operational
amplifiers (opamps). The potentiometers
are soldered directly to the pc board and
ECC083
Preamp Tube
Sextet of KT88
Output tubes
ECC083
Phase Inverter
ECC099 Post PI
Buffer/Driver
ECC099 Post PI
Buffer/Driver
Fig. A - Chassis showing tube location
Fig. B - Gut shot
Fig. C – A view of the surface mount
circuitry that comprises the
equalization section
Fig. D – The
microprocessor
controller
Tom Lees’
Summary of Contents for CL 810
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