Posted by GarMan [ 216.13.136.38 ] on April 01, 2004 at 15:45:05:
My new preamp is now 98% complete. All that’s missing is a set of feet. Photos can be found at photos.yahoo.com/lowgc. Click at the Rozenblits Portfolio.
Since this is the Craftsman Forum, I’ll limit my discussions to the building process. Previous to this project, I’ve built tubes from a kit (point to point), and Chips from scratch. But this is my first attempt at tubes directly from a schematic. The actual soldering was the easiest part. The most difficult and time consuming part of the build was drafting the wiring diagram and chassis work. I took about two weeks and twenty drafts to translate the schematics into a component placement and wiring diagram, and an entire weekend to construct the chassis.
The chassis was built from a Hammond 6x10x2 1441 Series steel enclosure in black satin finish. I highly recommend this finish. Very nice to the touch, just the right amount of sheen, and very durable. The only down side is that this finish is a non-stock item at many places and has a lead time of approx 3 weeks.
For holes, I just used a hand drill. One trick I learned to get nice clean holes was to clamp a clean thick piece of MDF very tightly underneath and drill right through metal and MDF. The MDF will hold the drill bit steady as you’re passing through the sheet metal. Otherwise, you end up with triangular shaped holes :p
To keep your drill pattern accurate, draw it out on a sheet of graph paper and tape it on the chassis as a template. You can’t get simpler than that.
It was not my original intent to build this amp with wood panels. Unfortunately, I reversed the hole pattern for the back of the amp and had to find a creative solution. With the pattern reversed, I had the input/output running in under the transformer and the power cord running through the signal tubes. I couldn’t use the chassis as drilled and didn’t want to wait another 3 weeks for a new enclosure. Solution? Cut the entire back side out and wrap the enclosure with wood. It turned out to be a happy accident.
If I knew from the start that I’d end up with a wood paneled chassis, I wouldn’t have used a full enclosure. It would have been a lot easier to just use a flat plate and build a wood frame for it Bottlehead-style. My choice for plates would be Hammond’s chassis bottoms, in the black satin finish of course.
I’ve never been a big fan of chassis top connections (ie Bottlehead). However, since the rear wood panel is 0.5” thick, I had do a bit of drilling to thin it out for the connectors. The transformers were open style and they’re covered up by another Hammond enclosure, spray painted black. Didn’t think it would be a good idea to have 120V out in the open.
I ended up going for a vintage look. I thought the dark wood works very well with the toggle switch and black/silver knobs. All and all, I’m pretty happy with how it turned out. But I also know of at least dozen ways I can improve on its construction, using my superior powers of hindsight. Good thing I have a long list of projects I want to go through to apply my new found skills.
Gar.
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