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Old 7th April 2013, 04:33 PM   #1
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Default Chassis design question / cooling of small signal valves

Hi there,

a short question regarding chassis design:

For power valves and rectifiers, I am planning to have cutouts in the chassis plate around the valve base to allow for a cooling air flow (see chassis drawing below).

Click the image to open in full size.

Does this approach make sense for small signal valves too? Cutouts are the most expensive part of the chassis, and I would only make them if it is of any use...

What do you think?

Rundmaus
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Old 7th April 2013, 04:48 PM   #2
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Far from being an expert (and I am sure some will weigh in with some firther information), but from what I have read, large tubes like the EIMACs and large signal tubes that they use for broadcast need to be force cooled (they have fins built in and are supplemented with fans, and water at times) because they run very hot.

Other tubes don't require it because the envelope is glass (poor conductor) and the parts inside are in a vacuum (extremely poor conductor). What you need the cooling is probably more for the other components in the chassis -- but that is only if you have the tubes inside and under the plate drawing you have posted.

From what I recall in reading older texts, is that you kind of need the heat so that the electrons will be in a more excited state and can be ripped away. Smaller tubes octal tubes like the KT88s and EL34s (and probably even up to KT120s) don't really require the cooling because they will be convection cooled.

If you look at the McIntosh, Dynacos, and Conrad Johnson amplifiers, they don't have the cooling and I only hope that the engineers and designers took cooling cutouts into consideration.
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Old 7th April 2013, 05:18 PM   #3
12B4A is offline 12B4A  United States
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I think it would be a good idea to have some ventilation around those 3 small signal tubes that are grouped together. I assume there will be a few passive parts close to those sockets that will be more reliable if kept cool plus the heater wiring won't be helping with the heat either. Maybe just a line straight down between them.

Last edited by 12B4A; 7th April 2013 at 05:26 PM. Reason: added reasoning
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Old 7th April 2013, 05:43 PM   #4
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I think that all such cut-outs where tubes are fully above the open chassis are not necessary. Then the air flow and cooling is already efficient.
But if the plane where tubes stay is inside the cabinet, then the case is different.
Certainly in such case there must be sufficient openings at the bottom of the chassis.
But even then the cooling of voltage amplifying (small) tubes is useless.

My opinion is that mostly these cut-outs are used for outlook desing purposes.
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Old 7th April 2013, 06:11 PM   #5
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I just tried to follow the considerations M. Jones gave in his books, stating that the convection cooling of tubes is more efficient if airflow is allowed from below.

If I remember right, he even mounted power valve sockets onto the metal wire grills available for fans, allowing unrestricted vertical airflow.

As already said, I'll do this for the EL34 and the rectifiers, and maybe add just some holes for the small-signal valves...

Rundmaus
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Old 7th April 2013, 07:35 PM   #6
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Quote:
Originally Posted by Rundmaus View Post

As already said, I'll do this for the EL34 and the rectifiers, and maybe add just some holes for the small-signal valves...
Even if it doesn't provide any practical extension of tube-life for the small-signal tubes, it can't hurt!

You will be providing extra ventilation for the electronics inside the chassis and the heat from even the small tubes will set up a convection airflow to help pull hot air from inside the chassis.

If you're willing to put the holes in, go for it!

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Old 7th April 2013, 08:38 PM   #7
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Quote:
Originally Posted by rfengineer2013 View Post
If you're willing to put the holes in, go for it!
That's what I have in mind, though the extra costs will hurt. But you're surely right about the airflow from the chassis and prolonged component life.

Rundmaus

PS. Chassis drawing below.

Click the image to open in full size.

Last edited by Rundmaus; 7th April 2013 at 08:49 PM.
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Old 8th April 2013, 03:01 PM   #8
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Rundmaus, those cutouts do look expensive even if done by a CNC machine. The pricing is probably more because of tool wear, having a computer plot it out (which we know is essentially free), and cutting fluid used. The price also include having the operator stand by and watch the progress (or goes for a cup of coffee).

If you do not really require the cutouts, then why not try a set of holes instead of a partial radius for the cutout. Like having them pop 6 holes for the larger of the cutous in your drawing. I realize that this will somewhat impede the flow of air around the tubes versus the cutouts, but it could be cheaper for you.

The heat from the tubes will draw a sufficient amount air from across the surface of your plate. To draw it from inside the chassis and depending what might be heating the inside of the chassis, you'll have to consider also. Things like, will the bottom have a vent screen/louvers/holes? Is my transformer/resistors/choke/wiring going to create heat from inside the chassis?

That is a lot of tubes also. I am only guessing at maybe four power tubes, a couple of rectifier tubes, a mystery tube (a magic eye of some sorts possibly), and a bunch of signal tubes? Has the amp you are building been built before? How have other builders handled their chassis top plates? How have others dealt with placement of tubes, maybe you could space yours out a little more (more separation = less localized heat build up) and do without the cutouts?

Whatever the amp you have planned, it already looks impressive just from this.

And the biggest is how much are you willing to spend? If you do finally determine that the cost is less than the life of the tubes you have on top, the determination of not having the Duelunds cast capacitors and going with regular Auricaps, or maybe skipping getting a months of those doggie treats for your dog, then you make the call.
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Old 8th April 2013, 04:26 PM   #9
M Gregg is offline M Gregg  United Kingdom
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There is more than one way to do this..

It depends on the type of valve base you are using...If the valve base is all in one ie not using a retaining ring, then you can make the holes for the base larger and screw the support onto the chassis this will give a gap around the base and stop heat transfer. The air flow is what is important so work out how the air is going to flow around the chassis. You can create a chimney effect. If the bases are all in one you can mount them from under the chassis with silicone washers for damping...

I think you can do this a lot smarter..look at air flow from the base and where it will flow to leave the chassis. Make the hot parts drive the air around the chassis with convection. If it can be done with wood then it can be done with aluminium easier.

If its just for looks then go with what you are doing..You can put a barrier or shield between the power tubes and the pre-section..the heat will rise to the top of the inside of the chassis..by putting a barrier inside below the top plate you can ensure the heat only vents through the power section and the heat from the pre section will remain cooler. So you dont want the pre and power section heat to mix keep the pre section and power section separate..its over kill...depends what you are protecting. Then you want to stop conduction of heat across the top plate from power to pre section.

Regards
M. Gregg
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Last edited by M Gregg; 8th April 2013 at 04:41 PM.
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Old 8th April 2013, 05:02 PM   #10
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Your extra cutouts are not really necessary! To leave a gap for air circulation, you simply need to make a larger hole into your chassis, (about 2mm larger than your power tubes diameter) and use 4 or 5 mm long spacers to fix the tube socket underneath your chassis, at a slightly lower level. I use this method successfully in all my 6S33S tube amplifiers. Circular hole are very easy to drill, and in the other hand, a natural airflow can surround the tube envelope. The easy way guys!
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Last edited by Blaireau; 8th April 2013 at 05:05 PM.
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