Rick Hogue, a good friend of mine who owns Garrett
Park Guitars in Annapolis,
Maryland, recently acquired an
older Carlsbro amp head. He
loved its sound and asked me to
take a look at it. It turns out he
has a black plexi-panel, 60-watt
Carlsbro CS60TC, in very
nice shape. In all the years I’ve
been servicing amps, this is the
first time I’ve run across one of
these, so I was curious. Carlsbro
amps were built in Nottingham,
England, and seem to be more
rare in the USA than any other
British amp.
The amp has two discrete
channels, Bass and Treble, each
with a low- and high-level
input. The amp’s only controls
are Volume, Treble, and Bass
for each channel, along with a
global Top Cut knob. A power
switch and pilot lamp round out
the front panel with the noticeable lack of a standby switch.
The rear panel is very minimal
as well. An AC socket, that
appears to have been changed
from a British Bulgin to a US
IEC jack, a voltage selector, a
couple of speaker output jacks
and an output impedance selector with 3. 75, 7. 5 and 15 Ω
settings. Definitely old-school
British! Rick offered to let me
take it back to the shop and see
what made it tick.
Before I disassembled this
rarity, I decided to try it out
first. Very nice—the CS60TC
sounded open and percussive
with just the right balance of
clarity, growl, chime, and grit.
Think plexi meets Hiwatt, with
a tilt towards the latter. What
it lacked in front-end gain, it
made up for in tone.
When I pulled the chassis,
I didn’t see anything out of the
ordinary. So what makes this
amp sound the way it does? The
cathodes of both V1 and V2 use
1k Ω resistors and 64 µf bypass
capacitors. Seeing that indicated
there should probably be more
front-end gain than the amp
was delivering. A quick look at
V1 revealed the first anomaly—
someone had installed a 12AT7
in this position. According
to this amp’s schematic, the
preamp tubes should be all
12AX7s. A quick change to
the proper tube resulted in ...
nothing more than a moderate
increase in gain.
A rare 60-watt Carlsbro CS60TC.
The rear panel with its old-school British design.
Nice vintage handwiring!
to it, but at lower volumes, the
amp sounded slightly better to
me with the additional 470 Ω
resistors in place. It had just a
bit more jangle. This reminded
me of what I always tell cus-
tomers regarding amp mods:
“Whenever you change some-
thing, there’s always a tradeoff.”
I also saw the bias supply
had been previously repaired.
The value of the stock dropping
resistor from the HT winding
to the bias diode is 180k Ω.
Someone, I’m sure doing the
best they could with the parts
on hand, elected to place three
47k resistors in series (yielding a
141k total value) to replace the
unusual value 180k bias resis-
tor. The result was that the bias
voltage rose from the schematic’s
stated voltage of -35V DC to a
much greater -50V DC, caus-
ing the amp to be substantially
over-biased. Lowering the bias
voltage to the schematic’s level
removed all the crossover dis-
tortion present in the output
signal, but set the idle current
of the output tubes at around
65 mA. This was a bit too high
in this amp, in my opinion.
Adjusting the bias resistor to
a value of approximately 160k
placed the tubes at a more
respectable 40 mA of idle cur-
rent and definitely improved the
amp’s overdriven characteristics
at higher volume levels. Chalk
up another validation for prop-
erly biasing output tubes!
JEFF BOBER, one of
the godfathers of the
low-wattage amp revolution, co-founded and was
the principal designer
for Budda Amplification.
Jeff launched EAST
Amplification in 2010, and he can be
reached at pgampman@gmail.com.
