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Aerodynamics of a vent?

Probably equivalent to coalfire chimney design. Recently put in a cab top vent for a valve amp in desperate need of one. The convection air coming out was 40 degree C higher than ambient so metal of grill got fairly warm, better out than in. I was thinking next time instead of 2 x 3 inches would be double that and reduce the grill temperature. But would it, and would the convective effect be better, ie the output valves running cooler with a larger vent.

Restricting the aperature in a fireplace increases the draught, to make a lack lustre fire come to life, me was thinking. No actual chimney , here, just a vent directly over the output bottles, introduced milled holes in the pcb around the sockets and existing anular gap around the bottles, through the steel chassis.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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N_Cook
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Generally speaking, the ratio of outlet area to intake area should be at least unity. Greater than unity is better. To reduce the temperature of an air mass, increase its volume; either with a blower or by enlarging the outlet area.

The relationship between temperatures, power and air volume can be shown by

Qa = Pd/((To/Ti)-1)

where Qa = Airflow in CFM To = outlet temperature in degrees Kelvin Ti = inlet temperature in degrees Kelvin Pd = Power dissipated by active device(s)in watts

You can get an idea of the effectiveness of your cooling design by placing thermometers at the inlet and outlet of your cabinet, and using the calculated Pd of your amp, calculate the airflow volume with the formula above. Then, restrict some of the vent holes, remeasure the temperatures and recalculate. Those measurements should show that the more air flow, the lower the outlet temperature.

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster 
it goes.
Reply to
DaveM

(snip)

Not addressing the question on vents; I'll just add that my tube amplifiers are NOT in cabinets or cases (actually, one is, a Stromberg- Carlson, but I don't use it - just a "collector"!) My reasons are: 1) glass vacuum tubes (valves) look really good without cabinets - particularly in a dark room (you can always tart-up the chassis with polished wood sides, black-painted transformers, gold trim, etc.), and

2) they run cooler. Cheers, Roger Thornhill, Ontario, Canada.
Reply to
Engineer

(snip)

Not addressing the question on vents; I'll just add that my tube amplifiers are NOT in cabinets or cases (actually, one is, a Stromberg- Carlson, but I don't use it - just a "collector"!) My reasons are: 1) glass vacuum tubes (valves) look really good without cabinets - particularly in a dark room (you can always tart-up the chassis with polished wood sides, black-painted transformers, gold trim, etc.), and

2) they run cooler. Cheers, Roger Thornhill, Ontario, Canada.

You don't get windmilling guitarists in a domestic environment, let alone clumsy grunts throwing kit around.

Otherwise amp controls at the top and 12 inch speakers mean that the valves tend to be usually mounted upside down otherwise it means an extra six inches of cab height.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

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