Cabinet and chassis

This is the first part of the Bottlehead Crack build. In the following parts I will show how I made my Bottlehead Crack amplifier and maybe even give you some inspiration for your own DIY projects.

Bottlehead Crack is an OTL tube amplifier for high impedance headphones. OTL means that it’s an Output Transformer less design. The output transformer in a tube amplifier causes a lot of distortion because it’s located directly in the signal path. High-end tube amplifiers makes up for most of it by using hand wound transformers, made with pure silver wire. This makes them incredible expensive. In a normal amplifier it’s would be very hard to get enough amplification to drive a set of speakers with low impedance (usually 4-8 ohm impedance) using OTL design. Using high impedance headphones, like the Sennheiser HD650 rated at 300 ohm impedance, in combination with output capacitors, makes it possible to use an OTL design. The output capacitors causes some distortion as well, but it’s only a fraction of what’s caused by even a great  output transformer.

An OTL tube amplifier doesn’t sound like a regular tube amplifier. What someone would describe as ‘tubey’ sound, is often the sound of the output transformer making distortion. But it doesn’t sound like a solid state amplifier either. It’s somewhere in between , with the smoothness of the tube amps and the detailed and precise sound of the solid state. Best of both worlds.

 

Bottlehead Crack overview:

 

Bottlehead Crack Parts

This is all the parts you get in the Bottlehead Crack kit (without the Speedball upgrade I plan to install later). This picture is from www.Bottlehead.com and it’s not one of my own shots. What I received from Bottlehead was close to identical though, with exception from the tube boxes.

 

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The first thing I did was paint the Chassis body. I used a dark grey/black hammered spray paint and it got about 8 layers with about 2.30 minutes wait between the layers.  It’s important to know that 2-3 minutes is the right amount of time. If you wait a few minutes more, the paint will get bubbles and get ugly.  You can always let it dry out for a few hours and apply more layers later. The more thin layers, the better depth of the colors and the hammered effect will have a finer grain.

The power transformer bell is also painted in the same color.

 

Bottlehead Crack cabinet raw

 

The alder wood cabinet is assembled in a very clever way. All the pieces are placed on a table in a long row with the outside facing up. Then masking tape is applied the between the pieces and then glue. Then you can fold the cabinet together and apply masking tape to the last closing gap. It hard to get the last gap closed properly though.  The chassis can be used to maintain a perfect box shape.

 

 

 

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Alder wood is a very nice hard wood. Classic electric guitars is often made from Alder wood. I don’t like the pale wood for this project though. I have other plans.

 

 

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These pins is just to get some clearance when applying stain and varnish later. This is just the bottom of the cabinet, so the small holes doesn’t really matter.

 

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I use a classic powder based chestnut colored wood stain. It’s mixed with boiling water and you can add some alcohol if you like that more than water based. You just apply it with a brush or even better, with a sponge. Let it dry for two hours and decide if you wan’t it darker. Then apply another layer for an even darker color. It’s important to let it dry completely in between, otherwise you won’t get a perfect uniform finish. I applied three layers to get a nice and deep color.

 

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It’s important to apply varnish, oil or vax, because wood stain i going to wear off fast otherwise. The stain will look very matte and dull until you do so.

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The first coat of varnish will probably look like crap. No worries, you’re just filling the wood like when you use a primer. Don’t worry too much just yet. It will also take longer to dry than the following coats.

 

 

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The second layer will be much better. The wood is already starting to look like it’s supposed to be dark. The stain job turned out so much better than I though so, the varnish really help with the depth of the color.

 

 

 


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Third layer is adding even more glossiness and depth.

 

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Four coating of varnish with 400 grit sanding between layers makes for a fantastic result. This is much better than I dared to hope for and this is definitely not the last time i’m using wood stain. When done properly it’s a fantastic product and a good way to make something look luxurious and expensive, without being so.

 

 

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I really love the Bottlehead bronze logo. I have placed it a little higher than others do. I think it look better this way.

 

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The logo from the front…

 

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Let’s have a look at the combo, chassis + cabinet. I really like it. Nice and dark, like the sound from a tube amplifier. There is something classy about dark wood, and this looks like dark chocolate too.

 

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Go to part II: Mounting the parts