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-   -   Building power electronics into a wooden cabinet? (http://www.diyaudio.com/forums/instruments-amps/219263-building-power-electronics-into-wooden-cabinet.html)

ingenieus 7th September 2012 04:42 PM

Building power electronics into a wooden cabinet?
 
Hi all

It surprises me that guitar amps use wooden cabinets to house the electronics. Wood is great for a speaker cabinet, but when heat is applied to it the results can be disastrous if care is not taken. Then there is the issue of grounding.

This is however the way things are done in the guitar world. An approach used in subwoofers is to put all of the electronics on a piece of metal that also acts as heatsink and bolt it to the back of the enclosure. I haven't this in a guitar amp yet, but I may be wrong. Even separate head units are built out of wood. It is therefore obvious that the safety concerns has been figured out. A cursory examination of any guitar amp show that there is quite a bit of metal used inside the cabinet.

I have looked around for information on how metal and wood is combined in the construction of a guitar amp, but there seems to be very little of it. So my question is - how does one go about it?

Build a wooden box and line it with metal plate? Or is that oversimplifying things a bit? What about ventilation? Most guitar amps seem to have very little of that. The reason is probably that a guitar cab with ventilation slots will get liquid spilled into it by a careless musician sooner rather than later. Also, just how much heat does a transformer put out in the real world and how do you mount it inside the cab? It would make sense to keep it away from the input, but just how far away does it really need to be?

Any experience and knowledge that you could share would be helpful. I am keen to get into building guitar amps and am OK as far as the circuitry goes, but the thought of a self-combusting cab fills me with trepidation.

CharlieLaub 7th September 2012 05:23 PM

I run in to some issues related to this with my own projects - I build chip amps into the back of DIY speaker cabinets. These are mounted to a heat sink, but there is no metal "enclosure" around the electronics. I find that this can sometimes result in quite a bit of hum and noise injection into the line level wiring. I have found that using shielded cable internally, from the input to the board, pretty much solves the problem. Although it's not so much of a problem for a guitar amp, I thought maybe someone will be interested in hearing about this anyway.

-Charlie

doozerdave 7th September 2012 06:03 PM

Maybe I'm missing your point, but every single guitar amp that I've seen has a metal chassis for the electronics and this chassis is boxed up with wood (usually wood comprised mostly of glue these days). Any grounding and heatsinking is taken care of in the metal chassis. Some amps have fans that blow out the back and draw air in from somewhere else (hopefully somewhere safe). Some amps (I know Fender does this) have a metal screen stapled inside the wooden cabinet where the open side of the metal chassis would be. I believe this is meant as electrical shielding.

If you're building an amp you would start with a metal chassis, wire it all up, and when you're done you box it up. If you have high heat components you would design your chassis to keep them away from the wood. It would take some seriously hot components to burn the wood though, especially if there's an air gap. I'd guess the components would fail before the wood caught fire.

Keriwena 7th September 2012 11:10 PM

Guitar amp cabinets, both combos and heads, are subjected to a lot of abuse on the road. The also get vibrated a lot when in use. So while the basic construction dates back to console radios, it hasn't changed much because wood offers a very good strength to weight ratio, damping to weight ratio, and construction to cost ratio. It's only real drawback, as you noted, is that it's a good insulator.

Nearly any modern material that might be used would also need a layer of damping, and there goes any heat transfer advantage. For the new generation of small amps like the Vox Night Train and the Orange Tiny Terror, metal cases have come into fashion, but even those are a nod to '50s PA gear.

Take a good look at guitar amps and notice how big the cases are, and how the components are spread out. They're not designed with the same philosophy as a cell phone. ;)

Printer2 7th September 2012 11:47 PM

Have you actually looked at a guitar amp in person or in pictures? Heat having disastrous effects on the cabinet? Very few amps have heat related damage in normal use. Also Google 'did I hurt my amp I accidentally put its vinyl cover on and left it for a week', more than a few with the amp still working and not burning up.

As said, the cabinet's purpose is to protect the chassis (the metal surrounding the electronics) and the tubes.

Amp chassis.

amp chassis guitar - Google Search

Combo Cabinets, Twin Reverb, Super Reverb, 18 Watt, Bluesbreaker, Vibrolux, Vibroverb

Nigel Goodwin 8th September 2012 10:04 AM

Quote:

Originally Posted by ingenieus (Post 3155844)
Any experience and knowledge that you could share would be helpful. I am keen to get into building guitar amps and am OK as far as the circuitry goes, but the thought of a self-combusting cab fills me with trepidation.

As you know so little about it (absolutely nothing!) perhaps you shouldn't even consider such a venture - all your worries are simply your imagination.

Why would a wooden cabinet be 'self-combusting'? :rolleyes:

mickeymoose 8th September 2012 02:44 PM

If the OP would go to a radio museum he/she would find out that 100 years ago wood was a common material in this industry, including RF transmitters, which got a lot hotter than a couple of EL84s. E

shanx 10th September 2012 03:29 AM

From a safety standpoint this is not entirely invalid point to ask about wood flammabilty. Yes all of the electrical components are normally housed within an enclosed metal chassis. But this was not always done and a museum visit, you may find some examples of stuff that would not be acceptable to today's safety standards. An electrical inspector would check that the metal enclosure is not open in any way or exposes the internal components to the wood surface. Some amps hadchassis with no covers that are bolted to the wood cabinet. That would not pass basic safety electrical inspection (at least in Canada). But if all components are encased in metal chassis , then flammability of wood is not a concern as any potential electrical fire in a chassis is contained.The wood is not considered part of the basic assembly, merely further mechanical protection or embellishment. Tubes are 'external' only above the socket but they are enveloped with glass so not a fire risk from an inspection viewpoint. Just need to keep them away from wires, speakers and towards an open back end of the cab or provide vent slots to allow ventilation.

ingenieus 11th September 2012 11:14 AM

Thanks for all of the replies. It helped to set my mind at ease and it seems that things are not complicated after all. My concern was that there might be hidden traps for the uninitiated.

I have had a look at guitar cabs in shops and also at the practice amp that I do have around. I then started wondering if there are any published safety guidelines for building a guitar amp.

The correct way seems to be to build the electronics into a metal enclosure (as one would do normally) and enclose it in wood, leaving the back open for ventilation.

Now that I think of it - some drawings (from a repair manual) on how the big boys do this would help a lot. I will go and look for some.

ingenieus 11th September 2012 03:06 PM

2 Attachment(s)
These two drawings show how it is done in a commercial amp. It is only a 25W unit using a chip amp, though. The entire chassis acts as heatsink. The top part is open to the wood. It is practises like these that lead to the posting of this thread. Just how much heat is OK and how far away from the wood the heat generating components are mounted seems to be a matter that is left to the designer/builder's knowledge and experience.


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