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Old 10th May 2010, 05:05 AM   #1
toufu is offline toufu  United States
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Default Plexiglass case for small tubes?

I have a few tube projects I want to start. They are preamps, phonopreamps and I/V for DAC's. They involve small signal tubes (12ax7, etc) and larger rectifier tubes. I am thinking of using plexiglass. What do you think? Can I just cut 6 pieces and put them together with brackets from a hardware store? I don't mind the transparent look. But what about the temperature? If I put the tubes inside, would they get hot enough to affect the plexiglass? What about grounding? Thanks!
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Old 10th May 2010, 05:19 AM   #2
tomchr is offline tomchr  United States
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Plexiglass is a bit...eh..."lady dog" to work with. But if you do choose to go that route, you may consider making the brackets out of thicker plexiglass that you drill and tap holes in. That would allow you to screw everything together and create a fairly cool transparent look.

However, preamps, phono stages, and I/V converters are probably the most EMI sensitive circuits you'll find in your audio chain, hence, they will benefit the most from electrostatic screening. You could clad the inside of the plexiglass cage with some copper wire mesh screen and then ground the mesh. You can probably find mesh like that at an art/crafts store.

If you enclose the tubes in any form of case, you need to drill holes in the case for ventilation. You'll need some ventilation even if you choose to have the tubes "top side" as resistors and other components may dissipate a fair amount of power. I suggest looking at George's (tubelab) Lexan Amp.

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Old 10th May 2010, 04:13 PM   #3
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Quote:
If I put the tubes inside, would they get hot enough to affect the plexiglass?
YES! My first attempt for a Plexiglass amp looked like a potato chip after a few months. The heat from the tubes and the weight of the transformers added up to warp the plastic badly.

I have been using Lexan without a problem. You still want to allow for some air movement so that the heat from the tubes won't fry all of the other parts though. The lifetime of electronics components (expecially electrolytic caps) goes down as the temperature goes up.

Lexan is GE's trade name for polycarbonate plastic. Other vendors have their own names. Hyzod and Tuffak are other names for the same stuff. I find that buying remnants off of Ebay is the cheapest way to get this stuff.
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Old 10th May 2010, 10:10 PM   #4
toufu is offline toufu  United States
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Thanks guys, George, what did you use to screw the whole thing together?
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Old 10th May 2010, 10:46 PM   #5
johnr66 is offline johnr66  United States
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you can use acrylic (common trade names: Plexiglas (one S), lucite, acrylite...) if you keep the power tubes away. It looks best with polished edges and glued construction. It however takes proper tools with proper bits and blades to machine it well, not to mention experience. Cell cast acrylic is a bit easier to work with but many plastic distributors don't stock it and it is more costly.

Polycarbonate makes nice bullet proof windows, but it's greyish edge tint doesn't look as nice and it has a soft surface that scratches easier than acrylic. It can handle the heat better.
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Old 10th May 2010, 10:50 PM   #6
tomchr is offline tomchr  United States
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For other on-line sources of various metals and plastics, I recommend McMaster and Online Metals.

~Tom
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Old 10th May 2010, 11:28 PM   #7
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Thanks guys, George, what did you use to screw the whole thing together?
I drilled and tapped the Lexan (ordinary metal drills and taps) and screwed it together with brass 4-40 screws. The Lexan amp was one of my earliest "alternative" construction projects. I have had it for about 5 or 6 years. It has been dragged around, loaned out and taken apart several times. It has held up well, but it does show some scratches now.

I have been taking woodworking classes for a few years now and my construction skills have improved a bit. I may rebuild the Lexan amp some day, but it still works great so I haven't wanted to mess with it. I have built (or started but not finished) several amps that incorporate Lexan in their design.

The easiest is a typical wood box with a metal deck, except the metal deck is replaced with Lexan to reveal the Tubelab SE PC board and all the wiring underneath. That one was complete until I stole its power transformer for another project. I have a new one, but no time to finish it now.

The latest project is an ambitious (maybe a bit to ambitious) project incorporating wood, aluminum, diamond plate, and Lexan along with 15 to 20 tubes, two Simple P-P boards, a still unfinished regulator board, some sand state components and a few other unusual design features. It should be finished in a few weeks. It is safe to say that it will look very different.
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Old 11th May 2010, 01:07 AM   #8
kuroguy is offline kuroguy  United States
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Look up the local farco plastics. they usually have scraps of Acrylic for very cheap. As you have found, it can be drilled and tapped very effectively. it can also be glued very easily. Get a can of Weld-on #4. Place the two pieces to be welded together and use a bottle with needle dripper to place some weld-on #4 right at the joint. Capillary action will draw the glue into the crack and the glue sets in a matter of seconds. It takes a little practice to get the joints looking perfect, but it is way easier than drilling and tapping for permanent joints.
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Last edited by kuroguy; 11th May 2010 at 01:10 AM.
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Old 11th May 2010, 03:11 AM   #9
toufu is offline toufu  United States
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If I glue them together, how am I suppose to change the tubes?
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Old 11th May 2010, 03:28 AM   #10
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Quote:
Originally Posted by toufu View Post
I have a few tube projects I want to start.
I believe that aluminum plate and framing lumber is as good a place to start as any...

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