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Old 23rd October 2012, 06:22 AM   #1
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Default How Many Tubes to a Rackspace?

Ok so here's an odd question, and I'm not entirely sure how to explain it...

Let's just saw (for argument's sake) that I'm making a circuit with 100+ tubes in it. Don't worry, I'm not, I'm just trying to figure some stuff out here...

Basically I'm wanting to know how many tubes I could comfortably fit in say, a 3U rack space. These will mostly be 12AX7's and other similar valves (so no power amp tubes).

I could cram them all in upright, but obviously heat is the big issue. I know I can add cooling fans etc, but let's just say that they'll be without forced air cooling for now.

As an example, the Drawmer S3 has ten valves, and fits them all into a 3U space. They suggest leaving a 1U rack space top and bottom (which is fairly standard for most units that get warm) or a fan to take the heat away.

Thanks in advance for your help people, I know it's an odd one!
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Old 23rd October 2012, 11:29 AM   #2
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It would depend on what else you need the space for. Fitting in audio transformers and a power supply if that is not external will take up space, and even circuitry will need room. The Drawmer has an on board PS and gets round this by well designed pcbs, but even they go all up the walls of the unit!

Theoretically you could fit quite a few quite a few 12A... type valves in even a 2U rack, the most we have done was 12 for an EQ... but as mentioned it depends totally one what else you need the room for.

Cheers

Charlie
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Old 23rd October 2012, 12:29 PM   #3
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it's not only a question of air temperature and forced air cooling. You should also consider radiated heat, which is the major part of the generated heat.

Please take a look at Morgan Jone's book "Building valve amplifiers", he made some suggestions about distance between tubes, and - if anode is not cylindrical - the preferred mounting direction of the tubes.

Regards,
Guthorst
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Old 23rd October 2012, 01:39 PM   #4
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My Euro card three tube mic pre fits into 3U. You can get 6 in a 19 inch sub-rack so that's a total of 18 tubes. Each pre has a 12AX7 and a pair of 6922 tubes. I usually fit the standard vented top and bottom screens to the sub-rack to ensure adequate ventilation. Dissipation per pre is 10W so that compete sub-rack dissipates 60W. I would suggest this is the maximum for a 3U case. Try placing a 60W bulb I side one and see how hot t gets.

Cheers

Ian
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Old 23rd October 2012, 01:50 PM   #5
TheGimp is offline TheGimp  United States
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Do a search on vacuum tube computer photographs and you can find photos of how the plug in units were constructed. That should give you a pretty good idea since they were probably cramming in as much as possible considering the size of a computer back then.

Google Image Result for http://infolab.stanford.edu/pub/voy/museum/pictures/display/2-2-Tubes.jpg
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Old 23rd October 2012, 03:43 PM   #6
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I succesfully stuffed a 15 WPC stereo guitar amp into a 3U rack mounted computer chassis. It was mounted in a road case with a matching chassis housing the computer. There were 4 X EL84, 2 X 12AT7, and 4 X 12AX7. All of the 12A_7's were mounted vertically on PC boards. The EL84's were mounted horizontally on an aluminum frame that placed them in the area where the PC power supply would have been. It also walled them off from the rest of the amp. The top and bottom plates in the output tube area has cooling slots. There are two small fans mounted in the front of the case that usually run from 12 volts. They are wired to rectified heater power which is about 8.5 volts, so they make very little noise. Some of the air exits through slots in the bracket that holds the EL84's and some exits through all the unfilled holes in the rear panel.

It worked out well, so I am building a bigger one. I am aiming for 50 WPC using sweep tubes. I will use a real 3 RU chassis this time so I don't have to stuff all the controls into a 5 1/4 DVD drive bay!
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Old 24th October 2012, 05:47 AM   #7
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All very helpful replies, thanks very much!

As for the power supply, this will be separate. For now I'm just considering the theory of a virtually naked case/PCB, but of course when I come down to the final designs there will be other componentry in there too.

Once I have some more finalized designs I'll be playing around with board designs etc. And Guthorst, thanks for the book suggestion - I'll be sure to check it out! Also TheGimp - great idea with the old computers, that hadn't crossed my mind!

Thanks for your tips all - of course I'm open to other people's input too! I know this isn't exactly a question with a definitive answer!
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Old 26th October 2012, 09:38 AM   #8
adalin is offline adalin  Sweden
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You could build like this perhaps, this is an old frequency counter in a 19" chassis 4U high, there are more tubes below the image.

Click the image to open in full size.
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Old 27th October 2012, 11:21 PM   #9
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a 250W amp I have uses 10 el34 tubes. a 350w use 8 6550 tubes. got a few 240 watt amps that use 8 6l6 tubes. and a few m0-200 that use 8 8417 tubes. You have to take into account the trannies need to power a lot of tubes and associated power supply. A pair of 1200 want amps do have 48 tubes each, but it would be inpractical to rack mout.
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Old 27th October 2012, 11:34 PM   #10
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Neat Stuff !!!!

I love tubes !!!!

jer
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