Hi,
when looking at 300B builds, some use holes around the socket of the tube and some do not.
The argument made is that these holes will produce air flow around the tube in some "chimney effect". I am sceptical that this has any significant effect, especially if the case is closed at the bottom, but even if not.
Are there any real measurements available that prove the chimney assumption?
Also, the tube has a heater / filament to, well, heat it. So, given a design with some room around it and a grill/mesh above / around it at max, is it even desired to cool it?
Thanks
when looking at 300B builds, some use holes around the socket of the tube and some do not.
The argument made is that these holes will produce air flow around the tube in some "chimney effect". I am sceptical that this has any significant effect, especially if the case is closed at the bottom, but even if not.
Are there any real measurements available that prove the chimney assumption?
Also, the tube has a heater / filament to, well, heat it. So, given a design with some room around it and a grill/mesh above / around it at max, is it even desired to cool it?
Thanks
Good question. I have built 2 enginner amps, and after a few hours I feel they are too hot. If I add an external fan 30cm away on my cupboard blowing at the amp - even with super slow rpm, the temperature stays much lower. I dont have numbers but with the fan it's really better. My bottom plates have holes, the top plate none besides those for screws and tubes, with only 1mm radius supplement.
The question is, would more convection helps? In any case my next amp top plate will have larger/more holes.
The question is, would more convection helps? In any case my next amp top plate will have larger/more holes.
The problem comes down to anode dissipation, which is very variable. In a Class AB amplifier with no signal, Pa might be very low, but with full signal much larger. Some valves have "optimistic" maximum anode dissipations and need forced air cooling to keep envelope temperature within bounds. But a few slots or holes drilled around the valve base aren't going to do much. Mounting the valve socket in the middle of a fan guard gives a nice open area for air flow. But convection doesn't move much air, so even that won't give much cooling. But a slow-running fan underneath will make all the difference.
IMV the holes have nothing to do with cooling the valve, which is designed to work at a high temperature, and everything to do with creating convection currents to circulate hot air out of the chassis and draw cooler air in through holes in the baseplate. If there is air flow through the holes then they are working.
Yes, there should be holes, in the bottom of the chassis and in the top as well. Every component in the circuit has a temperature rating, most of them generate heat, that hot air needs a place to go. Placing the top vents around the bases of the hot power tubes will cause more movement of air, more heat will be removed from the tubes, components in the chassis will be cooler, last longer, everybody wins.
E I use Testor enamel to paint the edges of the little holes. Apply with a q tip.
E I use Testor enamel to paint the edges of the little holes. Apply with a q tip.
A lot depends on the configuration of components and vents. In my view, the cooler a tube runs, within reason, the less trouble you're likely to have. Overheating a tube is never good.
Another great point is that under chassis components need cooling. They are all derated for temperature when you properly design something. This is one area I see overlooked in new equipment. Some older equipment wasn't well thought out either. If you look at 1950's and 1960's tube PA equipment, you will see generous cooling and spacing between power tubes. Same for under chassis components. This stuff was designed to run as problem-free for years - decades with tube replacement.
So I would say, every little bit helps. One thing it will not do is hurt you in any way.
-Chris
Another great point is that under chassis components need cooling. They are all derated for temperature when you properly design something. This is one area I see overlooked in new equipment. Some older equipment wasn't well thought out either. If you look at 1950's and 1960's tube PA equipment, you will see generous cooling and spacing between power tubes. Same for under chassis components. This stuff was designed to run as problem-free for years - decades with tube replacement.
So I would say, every little bit helps. One thing it will not do is hurt you in any way.
-Chris
I am not asking about components inside the case. Of course they need to be able to stand the heat inside.
Also, I am specifically asking about a 300B (in SE class A). In the one I am currently building the 300B is running at about 27W idle.
It is more work and results in dust entering the case.
I live outside of a city so don't have heat retaining concrete/asphalt etc and, generally, it cools down at night. Our house has a second floor and is relatively open in design so if we open several windows both upstairs and down, it cools off nicely. If it's one of those muggy nights we can put a fan in an upstairs window to draw air out, and it's a big help. So, does convection work? Yes. Does a fan increase the affect? Yes, but it can be noisy and irritating. My opinion then is that holes about the power tubes can't hurt, and probably help while a fan can be more effective, but with drawbacks. Your choice.
If you have that much dust, you have a problem.
The tube type does not matter. High dissipation such as you have does matter. As long as you measure the inside chassis temperature and check the derating for your components, you're okay. However I have seen many pieces of equipment where the designer actually boasted about derating. The components were cooked after a few years.
No fan? Why not? You run them at reduced power and they are not noisy at all. You use a filter of course. It depends, if you need a fan and don't use one, you have created your own issues. It doesn't matter how you cool it, just cool it.
Anyway, you asked. You did get solid suggestions. Do as you will.
The tube type does not matter. High dissipation such as you have does matter. As long as you measure the inside chassis temperature and check the derating for your components, you're okay. However I have seen many pieces of equipment where the designer actually boasted about derating. The components were cooked after a few years.
No fan? Why not? You run them at reduced power and they are not noisy at all. You use a filter of course. It depends, if you need a fan and don't use one, you have created your own issues. It doesn't matter how you cool it, just cool it.
Anyway, you asked. You did get solid suggestions. Do as you will.
The tube envelope is extremely hot. Touch a tube like this and you may leave skin behind. The difference between ambient and under chassis shouldn't be that great to matter.
Any air flow aids in cooling the tube. It also cools under chassis components. Two birdies with one rock. Add some light air pressure (fan) and the effect is greatly enhanced.
Any air flow aids in cooling the tube. It also cools under chassis components. Two birdies with one rock. Add some light air pressure (fan) and the effect is greatly enhanced.
I hope it's a given that hot air rises and the tube is hot. Given these constants, if you open the chassis around the tube to provide a pathway for the air inside to chassis to displace the hot air around the tube that is moving upward, It will move some air through the chassis. This is no different than the thermosiphon cooling some automobiles used. Equally obvious I would hope, if there are no holes in the bottom of the chassis, there will be no air movement. This is much less about cooling the tubes, but using this natural convection process that is happening to move some air through the chassis. Also you should strategically place the holes in the bottom cover to direct the incoming air over heat sensitive/heat generating parts.
Have you ever looked into an old tube TV after a longer time?
I asked if these holes are useful. So far, the result seems to be "only for the parts" in the case. I was more after if it is better for the tube, but seem not to have made that explicit enough in my OP.
What I am using inside the chassis I always ensure can stand the temperature that is there.
Thanks Stephe, yes, I am familiar with hot air flowing upwards. I see you do not have cooling holes around the EL34 in your builds.
So I take that if not necessary for the case innards, no need for them.
Thinking like that imperfections can be copied and design specifics of another device are valid for your device that has different tubes dissipating 27W idling. The amplifier itself is also not needed 
Post #16 sums it up pretty well. There is no penalty having passive cooling the best way possible. On the contrary, there are even a few reasons to do so (and none against it). Of course there must be holes/slots at the bottom as well for air flow.
Post #16 sums it up pretty well. There is no penalty having passive cooling the best way possible. On the contrary, there are even a few reasons to do so (and none against it). Of course there must be holes/slots at the bottom as well for air flow.
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Every piece of equipment collects dust over time. I refused to work on TVs for a number of reasons, dust collection being one. You should see bar power amplifiers and older public address equipment. You have a very hot surface, you will (hopefully) have moving air. This will drop dust no matter whether you like it or not. I have to clean dust from teh tops of tube amps that don't have ventilation holes. Worrying about some dust seems silly.
Even solid state equipment accumulates dust. It is simply maintenance to clean these things out occasionally.
Anyway, all points have been made. You choose.
Even solid state equipment accumulates dust. It is simply maintenance to clean these things out occasionally.
Anyway, all points have been made. You choose.