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Chassis design question / cooling of small signal valves

Rundmaus

Member
2005-08-21 10:46 pm
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).

chassis.png


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
 
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.
 
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.
 
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artosalo

Member
2010-02-16 9:00 am
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.
 

Rundmaus

Member
2005-08-21 10:46 pm
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
 
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! :cool:

~ Sam
 
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.
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
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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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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tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
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!

That's the same principle Morgan Jones uses. He uses a ring of holes close to the tube socket to allow the natural convection to drive air up along the tube envelope. It works... I think the main purpose of this is to suck the hot air out of the inside of the chassis, though...

Whether it's necessary for the tubes is another story. As others pointed out, vacuum is a very poor conductor of heat. Glass is slightly better. Recall, it's the anode and the tube structures inside the tube that you're trying to protect. In the Thermography on 6C33C... thread (post #1), there's a link to a rather cool video showing the temperature of a 6C33C under natural as well as forced air cooling. You'll notice that even with a 5-inch fan blowing air on the tube envelopes, the anode temperature doesn't change significantly (maybe 5 % as I recall).

Now, the big Eimac 4CX.... power tubes, that's another story. They have cooling fins that connect directly to the anode. They're intended for forced air cooling. That's a completely different animal.

Rundmaus - for your amp, I suggest going with what looks good. You'll probably need some ventilation in the chassis to keep the innards cool. But don't worry about the tubes as long as they're mounted outside the chassis.

~Tom
 
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Rundmaus

Member
2005-08-21 10:46 pm
Hey there, lots of answers! :eek:

Sorry, have been busy writing my PhD thesis the whole day and didn't look at the forum. Now it's half past nine... *yawn*

If you do not really require the cutouts, then why not try a set of holes instead of a partial radius for the cutout.

You already gave the answer - holes have significantly lower cross section available to the air flow. I'd need many holes or large holes, both being nearly as expensive as the cutouts...

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?
Yes and yes. The bottom will be perforated sheet, and yes there are components within the chassis creating heat.

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?
Yes, it's 2x EL34 power tubes and 2x ECC88 drivers plus 1x EF184 CCS per side/channel. The others are 2x GZ34 rectifiers for B+ and 1x EZ81 for B-.

Has the amp you are building been built before?
Yes, the amp is the well-known PP-1C by Allen Wright. Just changed to tube rectification and pentode CCS to see if it is doable purely with vacuum devices...

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.
Unfortunately, the bases I am using need to sit in holes, being held down by a retaining ring. Therefore cutouts seem the only possibility to create a gap for air flow around them.

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.
That is what I hoped to achieve with the cutouts. The valves sit on top of the chassis plate shown, and the chimney effect of the hot valves should be able to draw cooling air from below the chassis, which would also assist in keeping the components below the chassis cooler.

If its just for looks then go with what you are doing..

It's definitely not for the looks! In fact, nobody will see these cutouts later, as the chassis will be covered with a second (black painted) plate, spaced approx. 10-15mm. The second plate will only have large round holes (sufficiently large for air flow) for the valves.

To leave a gap for air circulation, you simply need to make a larger hole into your chassis [...] and use 4 or 5 mm long spacers to fix the tube socket underneath your chassis

Sounds like a very good idea, but this will require one-piece valve bases with fixing holes. The bases I have are held down by a metal ring and are not suitable for that method, I think...


I think the main purpose of this is to suck the hot air out of the inside of the chassis, though...
Yes...

You'll probably need some ventilation in the chassis to keep the innards cool. But don't worry about the tubes as long as they're mounted outside the chassis.
Ok then. Surely it won't do harm to keep the innards of the chassis cool, as there are quite a number of electrolytics and resistors around...

I'll finish drawing the whole chassis and then check if the cutouts are within reach, in a monetary sense...

Rundmaus
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
If you are covering the plate with another plate,

Then go for holes around the tube bases, you might even be better using a mesh plate instead of the milled idea..with the top plate over it.

The chimney effect would come from the top plate..seems a bit complicated..is the top plate to hide the bolts and fixings?

If I was doing this with two plates I would have the pre/driver on one plate and the O/P section on another with holes drilled around the tube bases. Cover with the top plate.

Regards
M. Gregg
 
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Rundmaus

Member
2005-08-21 10:46 pm
If you are covering the plate with another plate,
Then go for holes around the tube bases, you might even be better using a mesh plate instead of the milled idea..with the top plate over it.

Thought about mesh plate, yes, but ruled it out:

a) The mechanical stability would not be sufficient to carry two power transformers, four chokes and a lot of other stuff... The chassis plate will be 4mm or even 5mm aluminium. I doubt that mesh plates are available in that thickness.

b) It is really difficult to fix other stuff in a mesh plate. The classical 'threaded holes' approach will not work.

The chimney effect would come from the top plate..seems a bit complicated..is the top plate to hide the bolts and fixings?

Yes, that is the idea behind the two plates. Lower one mechanically stable enough to support the weight of all the components and with lots of holes, bolts and fixings, top plate with nice finish and holes only for tubes, switches and pots.

Rundmaus
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
Why,

are you just using one type of plate...why can't you have the transformers on a different plate..so it would look like one plate with a few mill gap between..something like three wide strips..Tx's on the one, power section on the other, and pre-driver on the other? You can use different thickness plates for each section..leave a gap between so there is no heat transfer..
I see what your saying with the holes not being in line with mesh..it can be a pain to work with..they are always just out of line.
I would still go with holes around the tube bases..the slots are not necessary. Maybe even some 20-25mm holes in the center of the plate or in a convenient
place. I might even bend the edge of the preamp/driver plate up at 90 Deg to create a barrier up to the top plate with a few mill gap so it doesn't touch.
How hot are the O/P tubes your using..something like 6c33c?...If they run very hot I would make sure there was no heat transfer. You might save on weight as well/cost..


Regards
M. Gregg
 
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M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
I tend to use aluminium angle,

So this is an idea....make the chassis in aluminium angle and bolt the plates to it..I would put a frame of angle around/under the plate for the Tx's for extra strength...bolt the three sections onto the aluminium section support the top, bottom and plate section with all thread..use nylock nuts to set the height or slide tube over it. use the side plates/front back for strength and to make it solid.. I would think with the plates bolted to angle it would be quite strong..and the Tx plate should not sag.

Its just an idea..:)

Regards
M. Gregg
 
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I have no good tools to make good holes, beginning at one point and ended 0.5 mm farther.

I hope technology advances and sell pre made holes... :D

The last amp I did for a client, two class A 20 W monoblock, the MOSFETs on the PSU works quite hot, about 15 W (each one!), and the 2mm thick aluminium chassis serves as heatsink.
My client made things even more difficult, wanted wood cheeks... :rolleyes:
The air vents are in the bottom perforated sheet and a very small gap between chassis and valve sockets.

Zero problems, well, the chassis is a little hot, but valves work very happy, even the rectifier...

Many holes, I would fear a reduction on the resonance frequency of the chassis, and then microphonics.IMHO

BTW. Good luck with your PhD thesis.
 
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M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
Regards using the aluminium angle,

You don't need to weld anything....Use the side plates to mount two or three strips of angle.

Put angle along the Tx plate and cut it short so that the plate sits on the angle on the sides then fit angle along the Tx plate between the side plates, the plate will support between the side and the angle along its edge. So there are no overlaps of angle to worry about and you get a smooth fit.

If you put all thread through the angle on the side plates with tube to space the top/middle and bottom sections it will add strength. and you can reduce the fixings on the side plates

Vent from the bottom through the mid section to the top plate..the top plate WILL cause more transfer of heat from the power tubes to the pre and TX section.. so you only need supports don't support it all along its edges unless you can isolate the heat from the amp plates..

Just remember to bond the plates togeather with earth straps and take back to main earth.

I guess you are using a separate ground plane etc..

It will be interesting to hear what you decide to do..one large plate or a constructed design with different sections..I don't like using tapped holes with aluminium they can rip out easily with weight..so I use pin and nut/nylock or other..Is there any wood in this design or just metal?

Just more ideas..:)
Regards
M. Gregg
 
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From the looks of your answer to me in the previous postings, and the layout, might I could the water some more for you and have you consider one more option?

With respect to transformers (power and output) being on the top plate, the number of tubes you have, mention of L-angle brackets, the tubes you are using, the concern for structural integrity, and relative size of the amplifier when it is done...

Have you considered monoblocks?

It would be less strain on the top plate and your are already dealing with, as you said, 2x EL34 power tubes, 2x ECC88 drivers plus 1x EF184 CCS per side/channel, 2x GZ34 rectifiers for the B+, and 1x EZ81 for B-. You can always make add one more EX81 to the other side's B-. Other than more side panels and a redesign, maybe this will solve the weight on the top plate issue and you can still have cut outs.

Just saying...