Material and thickness for top-plate

[RhythMick]

HI all. Approaching my first real build (phono stage), having only modded other amps previously. Design is all but complete including layout, positioning etc.

My brother is a woodworker and is going to find me a nice piece of wood and help me construct. I have in my head a copper sheet as a top plate, lacquered to prevent it from discolouring over time. On this copper top plate will be mounted step-up transformers and large caps. Valve bases will be mounted to a circuit board (plain epoxy sheet) which is itself mounted to the top plate, valves obviously sitting above the copper. Resistors, small caps and point-to-point wiring mounted beneath the circuit board. It will need a bottom sheet to prevent anyone accessing the gubbins.

- How thick does copper sheet need to be to support it's own weight, transformers and caps and not bend over time - I'm probably looking at 26 x 20 cm.

- Is copper a bad choice for the top plate ? I could use aluminium sheet but like the colour of copper, especially with the right wood.

- None of the components will be designed to run hot, but it's still a box with no airflow. How concerned should I be about that ? I'm over-specifying dissipation ratings by some margin - for example plate loads the dissipation is calculated assuming the valve is a short and then I multiply by AT LEAST 2. Normal running will see for example 1W run into 3 x 2W resistors in parallel.

 

[nigelwright7557]

I'm over-specifying dissipation ratings by some margin - for example plate loads the dissipation is calculated assuming the valve is a short and then I multiply by AT LEAST 2. Normal running will see for example 1W run into 3 x 2W resistors in parallel.

1 watt of heat whether in a 1 watt resistor or a 10 watt resistor still gives off the same amount of heat.

There will be some heat from heaters going through the copper plate and warming up the enclosure.

I would add vents in the side of the enclosure to keep things cool.

 

[chrisb]

Aluminum will be much cheaper for the required thickness, is generally easier to machine with hand and hand operated power tools, and can be powder coated to achieve some very nice verdigris effects that won't oxidize.


On my last large tube amp project, I used 1/4" 6061 Aluminum, water jet cut and powder coated - more than rigid enough for at least 30 lbs of iron on the top plate (400 x 260mm)

 

[JonSnell Electronic]

3mm or 1/8" aluminium is ideal and then when cut out, anodise it.

 

[rayma]

How thick does copper sheet need to be to support it's own weight, transformers and caps and not bend over time -
I'm probably looking at 26 x Is copper a bad choice for the top plate ? I could use aluminium sheet but like the colour
of copper, especially with the right wood.

I would use 1/16" or more stainless steel for the top plate, and 1/32 to 1/16" copper over that if you like it.

 

[chrisb]

Isn't stainless rather more of a bear to fabricate with the average DIY tools than aluminum?

 

[rayma]

Isn't stainless rather more of a bear to fabricate with the average DIY tools than aluminum?

It won't corrode, or react with the copper, and is not very hard to work with.

 

[jcx]

as usual, depends - some stainless grades/tempers bends, spins wonderfully - but that ductility causes it to move out of the way of cutting tools making precision hard

others work harden like crazy - difficult to machine without very specific cutting tools, speeds/feeds/lubrication


"Aluminum" has similar wide range of grades/tempers but with lower ultimate strength its not such a problem when hardened

and geometry wins for stiffness - thicker is much stiffer so for the same weight, Aluminum wins due to lower density, being used in thicker sections

 

[rayma]

some stainless grades/tempers bends, spins wonderfully - but that ductility causes it to move out of the way
of cutting tools making precision hard

I mostly just shear sheet metal, and punch the holes.