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12J8 Headphone Amplifier
https://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6291
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Author:  ILoveHiFi [ 18 Feb 2019, 19:38 ]
Post subject:  Re: 12J8 Headphone Amplifier

Colour should have negtlible small effects on heat disipation.
Tube disipation is not to be worried about if you have few holes and vents.

Author:  Suncalc [ 18 Feb 2019, 22:45 ]
Post subject:  Re: 12J8 Headphone Amplifier

BowToEd wrote:
Yes, color would have something to do with cooling. If I painted the box black inside and out, the infrared radiated from the tube plates would not bounce off the inside of the aluminum box and find its way back to the tube. It would be absorbed because black objects absorb more light (which is what IR is). Then the aluminum box would warm slightly. It would be able to conduct the heat away but would also transfer the heat to the air around and also radiate it a bit.
This is correct.

When dealing with heat transfer through an object (i.e. as in tis case, the chassis wall) there are two important parameters. One is absorption, the second is emissivity. The inside surface should have the highest absorption possible in the infrared bands to effectively transmit the radiated heat to the chassis material, the outside should have the highest emissivity possible to effectively reradiate the energy to the environment. Typically, black coatings have both high specific IR absorption and high emissivity. This is why they are used on heatsinks.

Ideally one might investigate the best IR absorber possible for the inside, which may or may not be "black". (Note: I have actually seen some "black" coatings which, while they had high absorption in the visible bands, the IR absorption was poor. Research in this area is a good idea.) Bare Aluminum offers >86% reflectance (i.e. <14% absorption) from visible down to mid IR. Most black coatings will increase the IR absorption to greater than 80% (and some over 90%). This is about a 6x improvement over bare aluminum.

Second choose a very high emissivity black coating for the outside such that the heat can effectively escape the material. Bare aluminum has an emissivity of around 0.1. A simple coating of a good matte black paint will get you over 0.80 emissivity. Black anodize is about 0.88 emissivity.

So yes, color and coatings matter a great deal. And while air circulation is good to keep the cavity from heating up, it is important to remember that the IR emissions from something hot have almost no effect on the air. All this energy gets absorbed by solid items in the radiation's path.

Author:  BowToEd [ 18 Feb 2019, 22:50 ]
Post subject:  Re: 12J8 Headphone Amplifier

Thank you. That's precisely what I was talking about.

Ed

Author:  ILoveHiFi [ 19 Feb 2019, 01:21 ]
Post subject:  Re: 12J8 Headphone Amplifier

Diffrent dudes have diffrent oppnions but I'm still strongly with myself on paint being negltible in terms of cooling effect.

Furthermore I always find valves to sound better when running hot and closer to maximum powerdisipations.

No one can save a overheating audio build by only using paint for extra light absotion.

Author:  BowToEd [ 19 Feb 2019, 01:28 ]
Post subject:  Re: 12J8 Headphone Amplifier

Have it your way. Either way, I'm working on the project as it stands. I'm also with you that vent holes will work to keep the enclosure cool. If they were power tubes, it would be helpful to keep the plates from overheating.

If you really want to know more about this, I'd suggest reading up on Black Body Radiation.

Ed

Author:  ILoveHiFi [ 19 Feb 2019, 01:34 ]
Post subject:  Re: 12J8 Headphone Amplifier

Not sure if you had planned to put legs and holes in the bottom so cold air flows and allow hot air to rise and circuitlate better.
Highly suggested.

Author:  BowToEd [ 19 Feb 2019, 01:40 ]
Post subject:  Re: 12J8 Headphone Amplifier

I've been doing this for a little while, and have done that with a project or two. Vent holes don't tend to do much when blocked. Keeping the temperature down will help ensure the electronic components don't bake and suffer shortened life spans.

The 12J8 tube is a 12v car radio tube. The plate dissipation will be very little, and almost all the heat in the tube will come from its filament. In this case it won't really help much, but I might want my project to look sleeker than plain mill finish aluminum.

Ed

Author:  ILoveHiFi [ 19 Feb 2019, 02:37 ]
Post subject:  Re: 12J8 Headphone Amplifier

If you have legs on the bottom can be done easily by screws even better is propper legs with dampening.
Then the holes on the bottom will not be blocked and the case will be nice and cool.

The heat from the tubes is also in the form of heat as like air, so air should help it cool down.
Most of the heat should be in the form of air, not infared/light radiation.
Not sure where your getting the idea of air cooling only helps elecotnics cool but not tubes.
Air blowing on the glass bottle will also help to some extent.

Author:  BowToEd [ 19 Feb 2019, 02:39 ]
Post subject:  Re: 12J8 Headphone Amplifier

I'm not worried that heat will kill my power tubes, although since 12J8 isn't being made anymore...

Tubes sounding more linear at higher plate dissipation has more to do with being in higher parts of its curves chart and little to do with the plate temperature. The heat is a by product and not a benefit. That's also not necessarily true with any power tube. It might be because certain tubes are designed for linearity and higher plate current is required for that.

Lastly, the plate dissipation in my tubes will be less than 500 mW each. We aren't exactly discussing a bank of EL34s here.

Ed

Author:  BowToEd [ 19 Feb 2019, 02:41 ]
Post subject:  Re: 12J8 Headphone Amplifier

ILoveHiFi wrote:
If you have legs on the bottom can be done easily by screws even better is propper legs with dampening.
Then the holes on the bottom will not be blocked and the case will be nice and cool.


That's likely what I'm going to do. I can turn some nice wood feet in my wood shop and bolt them on. Or, I can get some sliding door bumpers which are rubber and also work well.

Ed

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