In response to an email Joseph Centeno
sent me about his Carvin Belair 212, last
month we began exploring a series of mods
for this EL84-equipped combo [“Reducing
Output in a Carvin Belair 212,” September
2012]. Seeking to lower the level of this
non-master volume combo, Centeno had
come across a handful of mods for this amp
online and wanted my opinion on them.
(You can download these mods in PDF format
at hasserl.com, a cool amp and mod-kit
website run by Richard Hasse.)
Due to space considerations, I couldn’t
address every modification Centeno inquired
about, so we’ll finish the job now. If you
missed Centeno’s original email and the first
batch of mods, go to premierguitar.com and
check them out. So far, we’ve covered clipping
diodes, the tone stack, and cathode bypass
caps. Now let’s dive into two more mods.
Master Presence Control. In designing
this amp, Carvin chose to make the presence
control active only when the amp is in
its clean mode (channel 1) and not in “soak”
mode (channel 2). This strikes me as odd for
a couple of reasons. First, according to some
Belair users, the presence control doesn’t do
much to alter the clean channel’s sound,
so why not leave the control active in both
modes in the hope that it would be more beneficial
to channel 2? Second, the way the presence
control is disabled in channel 2 essentially
leaves it in what would be a “minimum”
setting. This offers no help for a channel that
was already a bit dark and muddy. For these
reasons, I’d highly recommend making the
presence control active for both channels.
Here’s how to do this: In its stock configuration,
the control’s wiper is connected
to a relay. The relay electrically connects
the wiper to the ground when channel 1 is
engaged. When channel 2 is engaged, the
wiper is electrically lifted from the ground,
causing the control to become inoperative.
The modification Centeno sent me removes
the connection to the relay and connects the
wiper directly to the ground, thereby leaving
the presence control always operational.
I think this is a great idea, but I’d
approach it differently. In the original mod,
you cut one of the wires near the connector
on the presence control circuit board,
attach a new wire to the connector, and link
the other end of that wire to the nearest
ground lug. This will indeed make the presence
control permanently active, but I’m
concerned with the choice of ground connection.
Seeing as how all grounds are not
the same—that’s another topic entirely—I’d
recommend against cutting the wire at the
connector, but instead following it back to
the circuit board, cutting it there, and then
finding a suitable ground point in that area.
This would put the ground connection
closer to where it was originally grounded
and may alleviate any unwanted issues, such
as parasitic oscillation.
Phase Inverter Master Volume. Master
volume circuits have been around for many
years. Their function, of course, is to set the
amp’s overall volume level after you’ve set
the gain to your liking, but there are different
ways to get the job done and they all
sound different.
The suggested master volume mod for
the Belair takes the post-phase inverter
signal and varies the signal level being sent
to the output stage. Because it is after the
phase inverter, there are two discrete signals
that need to be dealt with, each going to
one half of the push-pull output stage. For
that reason, this is the most complicated
and parts-heavy master volume circuit to
install. It is, however, a very “clean” master
volume circuit that should not color the
characteristics of the amp much, especially
with regards to the rhythm channel.
I think this is a good mod, but it’s
worth noting that the master is active in
both channels and not just in the overdrive
channel. That said, it actually might help
Centeno with his need to “reduce output to
allow for use in small rooms.” He may now
be able to reduce the overall volume of the
amp and increase the settings of the individual
channels to get a bit more crunch at
lower volume levels. This additional crunch
will be coming from the preamp sections
and not the output stage, as the signal levels
are being reduced before they can send
the output section into overdrive.
In my opinion, slamming the output
tubes yields the best type of overdrive.
If the goal is to push the output tubes
harder, yet reduce the amp’s overall volume,
there are a couple of other options
to explore. One is an external attenuator,
such as the THD Hot Plate, Tube Amp
Doctor Silencer, and Rivera RockCrusher.
The advantage here is that you can use an
attenuator with any amp by simply inserting
the device between the amp’s output
jack and speakers. An attenuator lets you
set the overall volume of the amp after the
output stage, and this allows you to drive
the output tubes into distortion.
Another quick, easy, and cost-effective
option to reduce the Belair’s output level
would be to replace its stock EL84 output
tubes with EL844s. I quickly auditioned a
pair of these in my EAST Club18 amp and
they reduced the output by approximately
30 percent and saturated more quickly than
the stock EL84s.
And speaking of tubes, Centeno had a
question about altitude possibly affecting
tube performance. Here’s what he wrote:
“Does altitude have an effect on tubes?
Most of the bars here are at about 5,000
feet, but we have some clients that have a
bar at almost 10,000 feet.”
I don’t believe altitude will have an effect
on tubes, since the bottle is pressurized and
also since tubes were used for many years
in aircraft navigation systems, which I’m
pretty sure fly higher than 10,000 feet.
All right—we’ve had fun with the Carvin
Belair. See you next month for more amp
madness!
Jeff Bober is one of
the godfathers of the
low-wattage amp revolution,
co-founded and was
the principal designer for
Budda Amplification. Jeff recently launched EAST
Amplification, and he can be reached at
pgampman@gmail.com.