The last amp build was so much fun I ordered a new one immediately. The last one (50W) had a little too much headroom for my 10’x10′ closet/darkroom/studio (what was I thinking?), so trying one of Ceriatone’s ‘lunchbox’ amp kits. This is a relatively simpler circuit without reverb, but with something like 3 or 4 gain stages.
I wanted a smaller and lighter cabinet, but the Ceriatone’s chassis all have the tubes and transformers on the same face, which complicates space saving. Even though the OTS Mini chassis is only 14″ wide, it’s also roughly 7″ deep and 7″ tall. I opted to mount the chassis with the console facing up at the back of the cabinet, but to clear the speaker magnet the cabinet needs to be almost 12″ deep, even with the speaker offset. I even used a Celestion G12-Neo, with the substantially smaller and lighter magnet. I thought about mounting the chassis vertically on the side of the amp like a Tone King, but instead I just decided to embrace the bulk and make it look like an old console tube TV from the 50s.
To de-emphasize the depth of the box, I kerfed the cabinet around the perimeter for two-tone Tolex upholstery. (Tolex is a brand name of vinyl upholstery that’s used for amps). I used a 3/32″ bit in a trim router, setting up a fence around the amp. For the radius areas at the corners, I used a riffler to get to the right depth, 3/32″ all the way around. The kerf with allow for continuous piping to separate the tolex colors. (The green dots on the face are Bondo, covering the T-nuts for the speaker baffle mount so the won’t show in the tolex.)
The black back half was done first. The roll of tolex wasn’t quite long enough for a continuous wrap, so I did the bottom piece first. Center lines and other layout control helps enormously when gluing up, much easier to keep everything square and perpendicular. I used a blue chalk snap line to mark the cut and layout lines on the black tolex. There are lots of you tube videos on cutting corners and wrapping inside curves so I won’t do into it here. The material is cut oversized, then trimmed to the kerf strip and inside edges after layup. The tape is to mask off the contact cement for the next layer of tolex.
The amp face was traced on the white tolex cover, then taped down. Contact cement makes this stuff curl up dramatically, so it helps to tape it down taut. I used some plastic spacers to position the amp over the layout lines, which made it easier to apply. The cement should still be a little tacky when sticking together- the tolex needs to be slightly reposition-able to do the corners and stretch the material around the radii. I practiced doing one front corner on some scrap, it was pretty easy to do and the seams look decent, so I decided to do the corner seams on the top and bottom instead of the sides.
Since this has become my favorite hobby, I made a chassis stand to make assembly easier. This is on my darkroom sink, with a few boards resting on the sink rim to support it. I made the chassis stand fixed on one end but length-adjustable, with loads of room for tools. The stand also adjust from horizontal to vertical.
Since Ceriatone provides a lot of color photos, and has printable layouts for their kits, I found it useful to set up a music stand for the layout print while viewing the photos on their Flickr page (er, when not checking email that is). I’m not savvy with electronics, so I really have to focus and concentrate to not make a hash of it all.
From mess to test. I used the same procedure as the last amp, wiring in the pots, heater leads and installing all chassis hardware and grounds before dropping in the board. Once everything is connected, I trace out the circuit on the layout drawing with pink highlighter to make sure I didn’t overlook anything.
Testing was straightforward, with little drama. All voltages checked out. However…no sound when plugging it in. After troubleshooting with Nik (Ceriatone’s owner, designer, and just an all around amazingly generous individual), we realized I’d done the signal lead pigtails wrong. I’d pre-made the pigtails to save time, but for some reason I stripped the inner insulation when joining the shield wire to ground on all of them, so all the signal was being dumped to ground. Once those were redone, it came to life easily enough. Great sounding amp, and incredibly quiet- no 60 cycle hum or other noise to speak of. Can really push it at low volumes and get some nice Tweed-syle overdrive without all the rectifier tube sag.
I found some great grill cloth at Mojotone (same place I got the Tolex). I stumbled on a better way to do the corners. Instead of mitering all the way to the outside edge of the corner, stop about 1/2 away, fold the excess together, then pull it back tightly from the corner and hit it with a staple. Then just trim the excess away.
All that was left was to install the piping around the perimeter, and put in the speaker baffle. For the piping, I drilled two 3/32″ holes side by side in the kerf on the bottom, right next to the inner side wall of the amp. I inserted an inch or so inside to start, stapled it to the wall, and then tucked in the end to the other hole and stapled that to the inside floor. Worked fairly well. It’s hard to get the tolex fabric inside the kerf consistent all the way around the amp, which makes it hard to have an even depth for the piping.
I’m pretty happy with it the look. It was inspired by the Swart Antares and my old Supro. I wish the white wasn’t quite that white, but there’s definitely a 50’s vibe to it. Just need to leave it in a smoke-filled bar for 70 years.