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.
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.
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
-Charlie
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.
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.
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.
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.
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
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
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'?
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.
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.
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.
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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That is the issue, here. Maybe different countries would have agencies that accept the open frame metal chassis like that. But that would be further subjected to extensive agency tests and there are plenty of companies willing to design commercial stuff that meets minimum electrical safety requirements (barely). Maybe adding $1 worth of sheet metal to enclose it properly is too much out of the profit margin. But if you are designing a DIY build, better to make sure you surpass the minimum safety standards by a wide margin..IMHO
Except for tubes, as a rule of thumb, no electrical component should get too hot to touch. So I think you'll generally be far below the kindling point of the wood. For tubes, based on 50 years of observation, I'd a say a couple of inches will suffice (assuming adequate airflow), and to be really safe you could add a small metal foil reflector on the wood over the tubes.
Good point, Kindhornman, But when does it start smoking? I will have to look it up.
After all the input, I think the way to go is to build the electronics into a fully enclosed metal box that does not get too hot to touch. That is easy to test beforehand and wrapping it in wood should then be OK.
Here are two more drawings I have found. This one is from screen 17 of a Google search for "guitar amplifier service manual filetypeDF" BTW. These things are not easy to come by.
It is a 60W amp with sophisticated digital processing, therefore it has a lot of circuit boards. There is a heatsink, but once again the top is left open.
Now to figure out how to build such a metal enclosure without a sheet metal bender. This might work:
Making a simple sheet metal Bender - By Nev Sweeney
After all the input, I think the way to go is to build the electronics into a fully enclosed metal box that does not get too hot to touch. That is easy to test beforehand and wrapping it in wood should then be OK.
Here are two more drawings I have found. This one is from screen 17 of a Google search for "guitar amplifier service manual filetype
DF" BTW. These things are not easy to come by. It is a 60W amp with sophisticated digital processing, therefore it has a lot of circuit boards. There is a heatsink, but once again the top is left open.
Now to figure out how to build such a metal enclosure without a sheet metal bender. This might work:
An externally hosted image should be here but it was not working when we last tested it.
Making a simple sheet metal Bender - By Nev Sweeney
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You're worrying far too much about absolutely nothing - there's no problems with wooden cabinets at all.
Hi Guys
The CSA match test is clearly written and requires holding a flame to the material for a set amount of time. Particle board does not even warm up in the allotted time. Neither does most tolex or rubber coatings for amps. However, this did not stop the local CSA inspectorfrom stating "That test does not apply to wood, only to plastics" even though the test is written with no qualification as to its applicability.
The amp's electronics must be housed in a metal chassis for electrostatic shielding if nothing else. A metal box provides heat shielding and a path for fault currents to the safety ground. The chassis should NOT be used for random grounding of the audio circuit.
CSA tries to make it sound as if it is illegal to sell product without their label, but this is not the case. If you want third-party approval, the local hydro inspector can do this for a lot less money. A lot of equipment arrives here with just a Los Angeles County approval sticker. Electricity is the same everywhere so one agency approval is really as good as another.
Have fun
Kevin O'Connor
The CSA match test is clearly written and requires holding a flame to the material for a set amount of time. Particle board does not even warm up in the allotted time. Neither does most tolex or rubber coatings for amps. However, this did not stop the local CSA inspectorfrom stating "That test does not apply to wood, only to plastics" even though the test is written with no qualification as to its applicability.
The amp's electronics must be housed in a metal chassis for electrostatic shielding if nothing else. A metal box provides heat shielding and a path for fault currents to the safety ground. The chassis should NOT be used for random grounding of the audio circuit.
CSA tries to make it sound as if it is illegal to sell product without their label, but this is not the case. If you want third-party approval, the local hydro inspector can do this for a lot less money. A lot of equipment arrives here with just a Los Angeles County approval sticker. Electricity is the same everywhere so one agency approval is really as good as another.
Have fun
Kevin O'Connor
It is nice to see Mr. O'Connor on DIY Audio, thank you for your clear and concise explanations.I deal quite regularly with CSA special inspection service, and found the interpretations of electrical safety code could sometimes vary from person to person. I do believe they do a good job of it, generally thorough in looking at basic safety. As you say there are other agency's that are equally good and competitive price wise. For example Intertek, UL can do inspections and their service is legally equal as far as acceptance by government bodies.
Thanks to all the input received, I have now come to the conclusion that the wood is in very little danger of burning. The semiconductors will melt down long before that happens. Another variation of the old story of the fuse in the speaker line being protected by the output transistors.
If the metal enclosure and heatsink is sufficient the keep the electronics at a safe temperature, the wooden cabinet will therefore be in no danger. Doing that brings its own challenges, though. I was thinking of doing a 150W amp but I have now scaled that down to 70W. The 150W project can wait for next time.
If the metal enclosure and heatsink is sufficient the keep the electronics at a safe temperature, the wooden cabinet will therefore be in no danger. Doing that brings its own challenges, though. I was thinking of doing a 150W amp but I have now scaled that down to 70W. The 150W project can wait for next time.
Hi Guys
I'm glad your dealings with CSA have been smooth, Shanx. My experience left me feeling that they are basically making whatever cash-grab they can interpreting "guidelines" in whatever way invokes further expense. In the 1990s their web site was fairly clear about what they do; today you cannot tell what business they are in.
The altruism of a self-regulating industry usually gets overtaken by a body that realizes money can be made. Fortunately that invokes competition by other parties. The fact is that it is insurance companies - again with their hand out - who want to reduce their own liability. Your insurance company wants to get out of paying claims, so if they can cast the blame onto another party they will. This is predominantly a USA problem where the country is run by litigators and corporations and everyone thinks that sueing someone is the answer to everything.
As far as 70W vs 150W for a solid-state output, the cost for both is about the same. What will matter is the entire package and how most players will use it. Where you might intend for the PA to be operated cleanly by fitting the preamp with a master volume, players will use it how they wish. What you might deem to be abuse is normal in the guitar world. Fortunately, robustness and functionality are the main criteria for solid-state PA design for pro use of any kind. The extra watts may require a heavier speaker, which is a nonissue if you build a head or rackmount rather than a combo amp.
Have fun
Kevin O'Connor
londonpower.com
I'm glad your dealings with CSA have been smooth, Shanx. My experience left me feeling that they are basically making whatever cash-grab they can interpreting "guidelines" in whatever way invokes further expense. In the 1990s their web site was fairly clear about what they do; today you cannot tell what business they are in.
The altruism of a self-regulating industry usually gets overtaken by a body that realizes money can be made. Fortunately that invokes competition by other parties. The fact is that it is insurance companies - again with their hand out - who want to reduce their own liability. Your insurance company wants to get out of paying claims, so if they can cast the blame onto another party they will. This is predominantly a USA problem where the country is run by litigators and corporations and everyone thinks that sueing someone is the answer to everything.
As far as 70W vs 150W for a solid-state output, the cost for both is about the same. What will matter is the entire package and how most players will use it. Where you might intend for the PA to be operated cleanly by fitting the preamp with a master volume, players will use it how they wish. What you might deem to be abuse is normal in the guitar world. Fortunately, robustness and functionality are the main criteria for solid-state PA design for pro use of any kind. The extra watts may require a heavier speaker, which is a nonissue if you build a head or rackmount rather than a combo amp.
Have fun
Kevin O'Connor
londonpower.com
Struth,
I think that the CSA situation is a bit more complex than how it seems to be presented. It seems that if you are to produce a product these days you will have to deal with more than one standards organization. If I get a UL label on a product here in the USA that is fine for here but if I plan on selling that product in Europe I don't think that label alone would fly. I am going to need a CSA or TUV or some other stamp of approval also for that market and then the Japanese or Asian markets will demand something else. Just meeting the connector standards in Europe are going to be very different than in the U.S. It would be nice if we had a worldwide standards body that we could all use and everybody could agree to, but I don't see that happening any time soon.
Steven
I think that the CSA situation is a bit more complex than how it seems to be presented. It seems that if you are to produce a product these days you will have to deal with more than one standards organization. If I get a UL label on a product here in the USA that is fine for here but if I plan on selling that product in Europe I don't think that label alone would fly. I am going to need a CSA or TUV or some other stamp of approval also for that market and then the Japanese or Asian markets will demand something else. Just meeting the connector standards in Europe are going to be very different than in the U.S. It would be nice if we had a worldwide standards body that we could all use and everybody could agree to, but I don't see that happening any time soon.
Steven
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