I’m going to guess that the Crack is Bottlehead’s all time
most popular kit. A great little tube headphone amplifier at a price-point
nearly anyone can afford that sounds very good for the money, it’s really hard
to go wrong here. The Crack has gone up in price a little bit since its
introduction 3+ years ago, but it’s still quite a value. I also highly
recommend the Speedball upgrade, which replaces the plate
loads on the tubes with a constant current source and brings this amp into its
own with added detail and transparency. I
built a Crack shortly after its release and of course
replaced a number of the parts with racier versions and it sounded quite good.
Years later, I reminisced how much fun it was to put together that
point-to-point Crack and decided to build a new one from the ground up with
fancy parts and give it a layout that would accept that added girth.
While I normally would have turned to
Keith Kirby to shape
me up a new chassis, it was time for me to step up and build my own, despite
not being crazy about working with wood. The first task was to figure out what
the size of the top plate would be with the additional room for larger
capacitors (in both coupling and power supply positions) and a pair of
large-ish Hammond chokes for the power supply as well. This ended up being 12” x
12” (more than twice the stock 10x6 size!), which is perfect as sheets of
metal can be ordered in 12” x 12”. Rather than go with the typical aluminum
plate, I spent a few extra bucks and went with a 12” x 12” x 0.1” copper plate.
The copper provides even better grounding and conduction than aluminum and
comes with a nice natural color. Despite the fluctuating metal market, a copper plate this size isn't too bad at about $40.
The next chassis-related task was to create the wooden box
that the plate would sit on top of. This is not the most trivial of tasks for
those of us not blessed with nice power tools and a garage. So, I drew up the
dimensions of the plate, where the plate would sit on top of the routed edges
on the wood, noted how deep the route would be, and based on that drawing I
found the appropriate sizes for the wood to be cut and added about a 1/16” of
an inch for wiggle room. For the wood itself, I decided I wanted something a
little more intricate (“fussy”, as someone I know would call it) than flat
sides, so I went with a flat piece of pine and a piece of decorative molding
and glued the two together. I then used the router to make a decorative route
along the outer top edge and used a rabbeting bit to make the inner route that
the plate would sit on. Four sections of this piece could then be cut at 45
degree angles to make the box. Since I was cutting the wood by hand, I used a
plastic miter box and did my best to cut at a straight 45 degrees all the way
down to get the corners to line up properly. Obviously, this never works out
perfectly, so that’s what wood filler is for.
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| Wood frame for metal plate "gravity" mount |
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| Wood frame corner detail |
Once the box was assembled with wood glue and the wood
filler sanded down smoothly, four small wood blocks were glued to the bottom
corners to provide adequate surface area for the EAR damping feet to be placed.
For the finish, rather than simply staining the wood like I normally would, I
opted to use a copper leafing kit from Old World Art to bring the copper theme
into the base. The leafing kit is a multi-step process including a red base
coat, adhesive size layer, application and burnishing of the metal leaf, a
sealer coat, application of an antique top-coat, and then another sealer coat. The nice thing about copper leaf is it's incredibly cheap; a set of 25 sheets runs less than $10.
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| Old World Art Leafing Kit |
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| Red base coat |
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| Copper leafing process |
Once complete, it looks like the texture you would find on an old frame on a painting in an art gallery.
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| Copper leaf after burnishing |
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| Copper leaf over the whole frame |
Now the copper plate could be cut, punched, drilled, Dremeled,
etc. to hold all the parts in place. The layout used to plan the initial plate
size was used to determine where each hole would fall. Small holes were drilled
using the drill press, larger holes for tubes and the Neutrik locking plug were
punched using GreenLee punches, the extra-large holes for the capacitors were
cut using hole saws on the drill press, and square holes, like for the
transformer and IEC inlet were cut using a Dremel metal cutting disc. The
copper is a little denser than aluminum and requires a bit more effort to cut
through.
Once the chassis plate had all the holes necessary to
accommodate the parts for the build, it was sanded down, starting with 80 grit
sandpaper and ending with 600 grit. Next, a coat of clear satin finish was sprayed
on each side to protect the copper, otherwise it tends to pick up fingerprints
and oxidize. Gloves needed to be used after the sanding process so my fingers
wouldn’t leave any oil on the copper. Once the clear coat was dry, parts could
finally be fastened onto the plate.
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| Copper plate after sanding and satin finish |
Aside from the PT-3 transformer from Bottlehead, a little bit of the wiring, and the Speedball upgrade, all other parts
were swapped. For the wiring, silver plated copper in PTFE/Teflon was used for
power connections, including the heaters, Cardas 20.5g internally litzed copper
in Teflon for the signal, and Kimber 19 gauge copper in Teflon for capacitor
connections, etc. Shielded Cardas copper in Teflon was used for the input and
output wiring with the copper mesh shield grounded. A few of the nice gold
plated solder posts from VT4C were used in place of the tin plated ones, and
short turret boards were used to handle the power supply connections, which I
find a little more robust for multiple connections. The tube sockets were
Teflon with gold plated copper contacts and matching miniature PCBs. Coupling
caps were a combination of a 47uF Obbligato Gold Premiums and 70uF Obbligato
Film in Oils, both good values for the money. The final power supply capacitor
was replaced with a set of three 75uF Axon caps wired in parallel (simply
because the three together were a bit cheaper than a single 220uF film cap).
The volume control was a relatively low-cost Taiwanese ladder stepped
attenuator. Resistors were all of the Kiwame / Koa Speers carbon film 2 watt
variety. The knob was a solid aluminum one with set screw from Kilo. Finally,
the switch was a nice little brass plated 3A DPDT that was made for guitars.
Wiring up the unit went just like a stock unit aside for a few
things. The Hammond 159M chokes (measuring 254 ohms resistance) replaced the 270 ohm resistors, the Obbligato Gold
and Film Oil caps were wired in parallel and soldered in place of the coupling
electrolytics, the pilot light was wired up to the 6.3V heater , and a few of
the ground connections were moved about. Also is the 6SN7 driver tube. A big thanks to
Grainger who posted the alternate wiring information at Bottlehead which is
duplicated below:
12AU7 PIN Wired
to 6SN7 PIN
1
2
2
1
3
3
4
7
5
N/A
6
5
7
4
8
6
9
8
I assume it is correct as it was confirmed on the forums by Caucasian Blackplate, but I make no personal guarantees ;)
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| Initial parts installation on the copper plate |
Even though the stock Bottlehead PT-3 looks great as-is, I wanted to try something a little different for this build, so I opted to use a transformer cover from VT4C. The transformer cover base had to be trimmed to fit the PT-3, but it's a perfect fit length and width wise.
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| Bottlehead PT-3 sitting on top of VT4C transformer cover base |
In order to make a nice clean install for the three 75uF film capacitors in the power supply, I fabricated a piece of FRP (fiberglass reinforced plastic) with a few turrets that would sit on top of the caps and accept solder connections so they could be cleanly wired in parallel. The three caps wired in parallel give about 228uF (75x3 plus a little bit of tolerance wiggle room), very close to the 220uF electrolytic they are replacing.
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| Fabricated FRP board |
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| Fully wired up in stock configuration |
After firing up the build in stock configuration and making sure it held smoke. I powered it down, removed the tubes and installed the very worthwhile Speedball upgrade. As mentioned earlier, the Speedball is a constant current source upgrade that replaces the plate loads of the input and power tubes. I also took the time to wire up the big 70uF oil filled Obbligato film caps.
Here's a shot of the completed amplifier sitting on the copper leafed base. I really love the look of the copper color and black on this build.
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| Completed Coppermine Crack |
Some nice little embellishments were a custom logo I made for the Coppermine Crack which I had engraved on a 3" wide copper plate, as well as some copper plated earrings which I attached to the top of the oil capacitors.
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| Engraved plate and copper plated earrings on top of the oil caps |
The final product looks and sounds great. I hope you enjoyed this build.
Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.
