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Old 31st January 2012, 02:26 PM   #1
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Default General Tubelab PCB question...

To all:

As I read a few of the various posts regarding things like resistor selection, CCS and Diode placement, heat dissipation, etc. it leads me to a question.

Is there any rule of thumb regarding physical separation between components and a metal chassis / other grounding point?

Here's why: I seem to remember reading somewhere on TubeLab.com that there should be a "minimum" distance from some components and the chassis, which justifies mounting the tube sockets on the silkscreened side of the board, but most of the high heat release components (cathode resistors, CCS TO-220 units and their heat sinks) on the "other" side.

In normal circumstances, this is not the ideal for heat transfer and may lightly toast the PCB over time.

I'd like to mount some of the higher heat dissipation items (wirewound resistors and the CCS TO-220 packages) on the TOP side, but clearances are tight due to the limited clearance of the tube sockets. I believe there could be between 10-15 mm (about 3/8 to 7/16 inch) to play with from the board top side to the underside of a chassis plate (say 2.5 to 4mm thickness)

Any guidance from the forum?

Thanks.
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Old 31st January 2012, 09:48 PM   #2
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I've built my TubelabSE the way you are describing, with the components on the bottom and the tube sockets on the top, and the PCB is "hanging" from the top deck on standoffs.......at approx the distance that you reference. It works fine, although you need to triple check the semiconductors pin outs when installing them on the bottom, same with cap polarity.

I machined a couple of vent slots in the top deck above the heatsinks. The is a TubelabSE not a Simple Single Ended board so everything's a bit different.

I would personally be a little uncomfortable with HV resistors, etc being on the topside, close to the deck but YMMV.
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File Type: jpg Tubelab SE inside.jpg (193.6 KB, 359 views)
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Old 2nd February 2012, 01:28 AM   #3
rknize is offline rknize  United States
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I mounted mine up top because the ventilation is better. That is because in my case, there is a gap around the inset tube sockets to promote convection. The hot air above the PCB is pulled through the holes, whereas any heat under the PCB has no active air current to pull it away.

Click the image to open in full size.
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Old 2nd February 2012, 01:53 AM   #4
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Russ: What do you have as a "sub-floor"?....it appears that you have something around the tube sockets below the deck.
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Old 2nd February 2012, 04:22 AM   #5
rknize is offline rknize  United States
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Here is a picture of all of the pieces. It was made out of mostly hardware store aluminum stock (u-channel and angle). The extra plate is just a thin sheet of aluminum to act as a guard for the over-sized holes. You may notice that it has a few extra holes to ventilate hot spots on the PCB.

Click the image to open in full size.

More about the construction:

Tubelab Simple SE – Construction – metaruss
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Old 3rd February 2012, 08:04 PM   #6
wicked1 is offline wicked1  United States
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I did similar.. Most resistors are top side, and close to the aluminum chassis. Vent holes drilled

Click the image to open in full size.

Last edited by wicked1; 3rd February 2012 at 08:10 PM.
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Old 4th February 2012, 07:12 PM   #7
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Beautiful Work!

Arne K
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Old 19th February 2014, 12:08 PM   #8
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Quote:
Originally Posted by boywonder View Post
I've built my TubelabSE the way you are describing, with the components on the bottom and the tube sockets on the top, and the PCB is "hanging" from the top deck on standoffs.......at approx the distance that you reference. It works fine, although you need to triple check the semiconductors pin outs when installing them on the bottom, same with cap polarity.

I machined a couple of vent slots in the top deck above the heatsinks. The is a TubelabSE not a Simple Single Ended board so everything's a bit different.

I would personally be a little uncomfortable with HV resistors, etc being on the topside, close to the deck but YMMV.
I know this is old but I have a question that directly relates to it:

Is there anything to stop me putting everything on the top of the PCB EXCEPT the tube sockets and then mounting it upside down?
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Old 19th February 2014, 01:04 PM   #9
wicked1 is offline wicked1  United States
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Tube sockets have to be on the top of the PCB, else the pins would be reversed.
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Old 19th February 2014, 03:49 PM   #10
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I KNEW there had to be a reason no one else had done it

Oh well!

Thanks wicked1

BTW: you mention most resistors on the topside on your build, does that mean you put the caps and larger stuff underneath?
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