is used in this amp is well-implemented,
and it appears that Markbass engineers were
thoughtful in implementing features that
add value to the overall amp from a
performer’s perspective.
With regard to the overall construction,
I understand why Markbass chose to
mount the toroidal power transformer
and the large output transformer to the
head cab and not the chassis. However, it
appears that the transformers were bolted
to the cabinet before the tolex was dry, as
there is some wrinkling and bubbling
around several transformer mounting bolt
holes. The tolex inside the cab was also
not trimmed and glued very well. The
tolex lifted in several placed making
inserting and removing the chassis more
difficult than it needed to be.
The printed circuit boards inside the
amp chassis are clearly labeled and are
securely mounted. However, I wish that
the tube sockets were chassis-mount
instead of soldered to the circuit boards.
Flexure of the circuit boards due to
inserting and removing tubes can cause
stresses in the circuit traces on the boards.
That said, no significant flexing of the
circuit boards was noted on the test amp
during this evaluation; just be gentle if
performing tube-swapping.
I am not biased either for or against
commingling tube and active analog (e.g.,
opamp-based) circuitry that is well-
implemented – as it appears to be in the
Classic 300. However, the labeling on the
amp chassis, touting the amp as an “all-
tube amplifier” is arguably misleading.
The Classic 300 does sport a preamp tube
in the first gain setting section of the amp.
The amp has a tube phase inverter/driver
section and an all-tube output section.
However, the equalizer, DI, effects loop,
limiter, etc. are all analog circuitry, and
from the flow diagram in Fig. M, it
appears that there is no way to pass a
signal from input to output that does not
pass through some form of active analog
circuitry.
Semantics aside, however, the Classic
300 is an impressive melding of “old
school” and modern technologies, and it
was a strong performer in our tests.
output to the effects loop. The illustrated
traces have each been normalized for sake
of comparison. The red trace corresponds
to the output measured across the speaker
load. The green trace corresponds to the
effects send at the same amp settings. The
effects send signal includes a “pre-
emphasis,” which slightly boosts the treble
of this signal. The blue trace corresponds
to the effects return at the same amp
settings. The effects return includes a
corresponding “de-emphasis” to cut the
treble of the received signal by an amount
that corresponds to the amount of boost
added to the treble by the pre-emphasis
network at the effects send. The overall
response is, theoretically, a flat frequency
response to a device in the effects loop.
However, the pre-emphasis/de-emphasis
has the advantage of potentially lowering
noise that may be caused by the device in
the loop.
Output Power
The Classic 300 performed well in power
testing. During the half-power/half-hour
test, the transformers remained cool as the
PIC adjusted the fan speed periodically.
During the output power tests, the test
amp exceeded the manufacturer spec of
175 watts at 8 ohms, down to 31 Hz. At
4 ohms, our power tests measured 295
watts at 1 kHz, which is extremely close to
the manufacturer rated power of 300
watts. The 2-ohm load testing yielded
power measurements that were a bit lower
than the 4-ohm tests, measuring 266
watts at 1 kHz. The Classic 300 has a
built-in limiter, so tests were not extended
up to the full 5% THD for every test
frequency, as doing so
did not have a material
affect on the measured
output power.
Conclusions
There is a great deal to
really like about this
amp. The engineers at
Markbass are clearly
not afraid to embrace
and pioneer new and
emerging technologies.
The control system that
b
g
:
Frequency Response
Referring to Fig. E, the amp was adjusted
to obtain its flattest response at the
speaker output, which is shown in the red
trace. Fig. F shows a photo of the tone
controls for that setting. The tone controls
were all adjusted to noon, and that trace is
shown in blue for sake of comparison.
Referring to Fig. G, the speaker output
frequency response for the optimally flat
tone control settings shown in Fig. F (red
trace) is compared to the output of the DI
(blue trace) and the effects send (green
trace). Each signal was normalized to 0 dB
for sake of comparison. The DI output
tracks the speaker output fairly well,
having a frequency response that extends
just a bit deeper and higher, compared to
the speaker output. The effects send is
discussed below.
From an inspection of the frequency
response sweeps shown in Figs. H-K, it
appears that the equalizer section comprises
four bandpass filters. These filters align
pretty closely to the manufacturer-specified
center frequencies of 80 Hz, 400 Hz, 800
Hz and 3 kHz, as published on page 11 of
the Classic 300 manual. One nice feature
of the Classic 300 implementation of active
analog filtering is that the adjustments to
each band are nicely distributed across the
rotation of the associated control knob,
providing a smooth transition as the
control is adjusted. In the frequency
response plots, the traces are as follows: red
= set to an approximate flat response; grey
= off; green = 9 o’clock; yellow = noon;
orange = 3 o’clock; and blue = full on.
Referring to Fig. L, frequency sweeps
were conducted to compare the speaker
Fig. N - Frequency set flat. Green trace-long life setting
Red trace-high fidelity setting
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