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Old 20th December 2004, 09:52 AM   #11
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You can get a surprising amount of airflow just by using the power of convection. The Apple G4 cube is a good example, where all the heatsinks are arranged around a central 'chimney' and air is drawn up through the unit. Judging by the amount of dust that collects underneath, a fair amount of air is getting moved.
It's quite a neat solution which avoids having to use a fan and has allowed them to use a very small case without overheating.

And no, I don't have shares in Apple, I just like the thing
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Old 20th December 2004, 11:06 AM   #12
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Default Re: The problem children

Quote:
Originally posted by gingertube
I've done as above as much as possible - the tubes are a liitle close together for my complete satisfaction BUT this was dictated by the PCB I used (Menno van der Venne BBB70100).

40mm holes around the EL34s which plug thru into a PCB.

No bases on the boxes as yet hence them sitting on the timber to allow air flow from underneath.

EL34s in triode mode giving about 40 watts Output Power.

Thanks,
Ian
Hello Ian, would those be the Plitron Boards? How do they sound?

Regards

Anthony
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Old 20th December 2004, 11:18 AM   #13
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Old 20th December 2004, 09:32 PM   #14
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Quote:
Originally posted by SHiFTY

If you do use a fan, I would mount it actually in the base of the amp, blowing upwards (ie sucking cold air from underneath the amp.) Use a 12V "stealth" fan running at 6VDC for maximum quietness. You can buy quiet PC fans (for example the Vantec stealth) and even in low profile. [/B]
Yes....snap....exactly my technique. If you use a brushless type at 1/3 voltage (to cut the drumming down) it may need a high value shunt cap across a series resistor to give it that little extra inertia on switch-on., then it will settle down.

The biggest drawback in my main amp is reflected radiation coming off the tubes and getting the chassis hot even with ample ventilation. Drilling large holes around the tube skirts is obvious. A large chassis with tubes mounted at 4 inch centers to one another using double quads 6550/88's etc in push pull, the chassis after an hour reaches 40-50C, even though there aren't any wirewounds underneath. It becomes obvious to mount the power supply electrolytics well away from any heat source. For every 10C temp rise, these components drastically reduce their working life compared to the life of tubes operated within their margins.
However, even the fan woring on reduced 1/3 speed the noise of is way above the S/N ratio of the amp.......unless (like me) one likes ones sound really loud then it doesn't matter.

rich
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Old 20th December 2004, 09:32 PM   #15
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Default Response to Coulomb

Yep - they are the Plitron Boards.

How does it sound??

In Ultralinear Mode Output Impedance is 13 Ohms so it sounds highly coloured as power out tracks the speaker impedance vs frequency characteristic.

In Triode Mode Output Impedance is 5 Ohms and it actually sounds quite lovely with my nominally 6 Ohm speakers (DF = 1.2)
You can hear those crystal clear, tingly highs that are so hard to get from an amp.

If you are serious about building an amp on a PCB and want to use Plitron trannies, I would suggest that you try to get the VDV100 PCBs (maybe from Amplimo?) in preference to the BBB70100 boards. Its a better circuit with higher sensitivity.

Cheers,
Ian
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Old 21st December 2004, 12:04 AM   #16
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Default The original issue

I dragged out my source material - in this case "Electronics Cooling" Journal and looked thru' a couple of relevant articles.

The fan type required IS an axial fan which is what everyone above has been talking about. Axial Fans are most suited to low pressure/high flow rate application. ONLY if you are trying to work agains a significant back pressure should you use a radial fan (sometimes called a blower) instead.

For low acoustic noise you want to keep airflow impedances low - by that we mean large air inlet, large air outlet (around each valve). There is some evidence to suggest that more fan blades are quieter than less blades at the same flow rate. Ie choose the 7 blade fan over the 5 blade fan.

Other evidence suggests a larger fan at slower speed is much quieter than a smaller fan at higher speed.

This also stacks up with the fact that we are only trying to provide a small positive pressure inside the box to bias or reinforce convective airflow.

In addition there are some axial fans designed specifically for low acoustic noise. A feather on the traling edge of the fan blade appears to be one way to reduce noise. Also sleeve bearings appear to be quieter than ball bearings.

So blow into the box from beneath - this allows the box to act as an acoustic baffle/damper - this action can be enhanced with a bit of acoustic damping material inside the box. Make sure you have no mechanical resonances - to this end use dampers on the fan mounts.

There - more than you probably ever wanted to know about fans.

Cheers,
Ginger
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Old 21st December 2004, 12:18 AM   #17
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Judging from the photo, I don't see that you have any problems. Is there some reason that you're convinced that you need fans?

Grey
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Old 21st December 2004, 12:19 AM   #18
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UK catalogues specify the noise (dB(A) @ 1m) and airflow of fans. Something you need to watch out for is that DC fans (12V, 24V etc) use semiconductor commutators and regulators. Lowering the applied voltage to reduce noise only works on traditional fans - doesn't make a blind bit of difference to a modern fan. However, you can get thermostatically controlled fans that bring the control element outside to a thermocouple - possibly allowing substitution of a fixed resistor to slow the fan. Unfortunately, my experience is that motor commutation/vibration is subjectively more of a problem than blade noise.
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Old 21st December 2004, 12:35 AM   #19
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Default Grey and EC8010

Ec8010 is correct about comutator noise etc.

BUT

Grey,
The ONLY electrolytics I could get to fit the PCB (4 off 47uF 500V axial for each board) were 85 degrees C jobs. In addition our forcast for tomorrow is 38 degrees Celsius - Sorry EC8010 to harp about the heat when your freezing your buns at the moment. I run my house aircon at about 26 to 27 degrees to keep the power bills sort of under control.

The boxes adjacent to the power valves get quite hot to touch - probably around 50 degrees C.
Thats why I want to put in afan.


Cheers,
Ian
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Old 21st December 2004, 06:36 AM   #20
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Quote:
Originally posted by gingertube
In addition our forcast for tomorrow is 38 degrees Celsius - Sorry EC8010 to harp about the heat when your freezing your buns at the moment. I run my house aircon at about 26 to 27 degrees to keep the power bills sort of under control.

The boxes adjacent to the power valves get quite hot to touch - probably around 50 degrees C.
Thats why I want to put in afan.


Cheers,
Ian
It was 42 for a few days (outside) at my place last summer, hence the fan, to move any stagnant air and to keep other elements like top plates cooler.
I have no a/c except opening the doors and hoping the breeze is onshore.
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