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I have a 1x12 tube combo and i'd like to close its back to get a beefier sound. Would this cause the power tubes to overheat (altough I have a fan inside)?
Also, what is the best material to cover the back? I know most companies use plywood to build their cabs, but i've also seen some poplar ones as well. My dad is building proffesional home speakers cabs and he builds them with MDF, is that any good for guitar cabs?

13-ply Baltic birch is the standard for instrument cabs amp; high end monitors. You want to get void-free furniture grade material for it's stiffness, durability and lack of resonance...it's pretty expensive but IMO it's worth it. If you can't swing the baltic birch MDF is a good second choice, tonally it'll work but it's not as durable over the long haul and will chip amp; dent easier. If the cab takes a really solid hit like a 5' drop off the back of a truck chances are the baltic birch cab will survive with zero or minimal damage...MDF is basically sawdust amp; glue pressed together, I'm sure you're imagination can do the work on that one!
Still, a LOT of cabs are made from MDF but it's mostly the lower-end to mid-grade stuff like the cheap Marshall cabs amp; Crate. Rivera, Bogner, etc. all use baltic birch. You could also use oak ply, maple...anything really as long as it's void free.

Though, if you're after a beefier tone I gotta suggest adding a cabinet rather then closing off a 1x12 combo. Open back cabs sound fuller amp; bigger because the sound fills the space, a closed back will project forward and 'beam' the sound. Sure, you might get a little more thump but that depends on the speaker, amp amp; quality of the enclosure. I'd experiment a bit before commiting to anything, and yeah...you'll have to leave it at least partially vented for the tubes. Drilling a few 2-3quot; holes would work but then you're getting closer to the open back thing.

Best of luck with it,

I would never used MDF in any speaker cab and surely not my guitar cab but even more important than that I would never close an open back cab...if you do your just asking for short tube life, component failure as well as the possibility of fire...

I'd have some issues with completely closing up a tube amp. Heat build up would be pretty bad.

The other thing to consider, closed back cabs are often engineered to get just the right amount of space enclosed. I'm not an expert in speaker cabs, but I've seen the algorithms for calculating just the right size. With open backs, just about anything can sound good. The deeper the better IMO. Stuff like what Rivera does with their Chubster and what Matchless and Bad Cat and I (and probably several other builders) do with the extra deep chassis' make a nice addition to the typical narrow combo amps. Bigger, fuller low end.

Three things:

I would be concerned with heat build up if you closed that cab. Even with a fan, if the box is sealed, all you are doing is blowing hot air around. You need some sort of vent.

MDF is an acceptable cabinet for building enclosures. MDF is great because as a material it#8217;s a little more lifeless than Baltic Birch. In other words, a speaker made of MDF relies more heavily on the voice of the speaker than a cabinet of Baltic Birch. Birch cabs do sometimes sound more #8220;alive#8221; because the birch simply adds more coloration to the tone. Both are fine in terms of strength. MDF is actually fairly dense and survives facial impacts well. It#8217;s the end grain that you have to be cautious of. Because of its durability, stiffness, and rather sterile (by itself) sound, MDF is the industry standard in car audio enclosure construction. It#8217;s simply great for playback. But for guitar cabinets, I#8217;d build with Baltic Birch before I would build with MDF. I would never consider low grade plywoods, OSB, or particle board of any type.

Closing an open back guitar cab isn#8217;t as big of a deal as people may make it out to be. In open back (free air/infinite baffle) applications, cabinets do not resonate as well as sealed enclosures. The sound is a little more open but does not have the fullness of a sealed enclosure. In sealed enclosures, as the volume of air within the cabinet decreases, the speaker#8217;s power handling increases, the bass response drops, but the sound emitted has more #8220;punch#8221; especially in the lower frequency ranges. As the interior volume increases, the speaker#8217;s power handling decreases, the bass response increases, but the speaker begins to lose the punchiness. BUT, given the design of guitar loudspeakers (they don#8217;t move much air), their materials (which aren#8217;t as stiff as woofers), and the volume of air we are talking about inside a guitar cabinet, much of what I just talked about doesn#8217;t apply much to guitar cabinets. Simply stated, your average cabinet sealed guitar cabinet has so much volume per driver that the box#8217;s properties approach a free air situation. The tonal variations you hear are more resonance inside the cab and a more directional sound.

Scott is right#8230; there are programs that exist for the design of enclosures for audio playback, but they are typically more geared towards subwoofer enclosures, where the physical dimensions of an enclosure have a far greater impact on what you hear than guitar loudspeaker enclosures.