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Re: [Amps] more - oil bath GS-35b

Subject: Re: [Amps] more - oil bath GS-35b
From: Manfred Mornhinweg <>
Date: Wed, 01 Apr 2015 20:49:02 +0000
List-post: <">>
You have tempted me to jump into the battle, guys, despite the more peaceful part of me saying "don't"!

Some facts:

- Oil, water, and most other liquid coolants, have very poor thermal conductivity, compared to metals. For this reason their ability to transfer heat to or from a metal surface depends critically on having a lot of turbulent motion, to quickly move away and replace any molecule of liquid that has equalized its temperature to that of the metal surface.

- The thermal contact between a liquid coolant and a metal surface depends dramatically, and in a nonlinear way, on the flow speed. Faster flow rate is better, always, within reasonable limits. Not only because it renews the liquid faster, but mainly because it creates more turbulence. It shouldn't be fast enough to cause cavitation, though.

- Some of you are mixing up different things. Yes, a liquid coolant will come out cooler from the radiator, if it flows slower through it, and enters at a fixed temperature. But that's mainly because the lower flow rate made it bring far less heat into the radiator! Our purpose is not cooling the liquid as far down as possible, but moving all the heat from the amplifier to the air, with the lowest temperature difference possible. That needs the lowest possible thermal resistance from the tube/transistors to the coolant, from the coolant to the radiator, and from the radiator to the air. And to achieve this, a high flow rate and speed is helpful. How much speed and flow rate is needed, depends on many things, like the amount of heat, the permissible temperature difference, the dimensions of all things, surface roughness, characteristics of the coolant, air flow through the radiator, altitude above sea level, humidity, and on and on.

- In a combustion engine different flow rates are used in different areas, to achieve optimal cooling efficiency. Very simply stated: If you allow a lot of cooling fluid to flow through an area that produces little heat, that fluid will NOT flow through those areas where more heat is produced, so those areas might overheat while the overcooled areas will stay too cool, thickening the oil there and causing more loss due to oil viscosity. So, carefully controlling how much coolant flows in each place allows obtaining the optimal temperature at each place of the engine, while needing the least amount of coolant pump power. This might find application in an amplifier, too. Like directing most of the flow around the plate of a tube, and a smaller amount to the base seals.

- In a well optimized system, coolant flow is so fast that there is very little temperature difference between the fluid moving from the amplifier to the radiator, and the fluid returning. Thanks to this, the whole system (all of the tube, all of the radiator, etc) are almost at the same temperature. This allows using the smallest possible radiator, among other advantages. In case of having many transistors, a high flow rate makes all of them work nearly at the same temperature, which is essential for even current sharing.

I followed this thread only superficially, but I think I saw questions about immersing the tube and tank circuit and everything in oil. I fear that doing so might cause trouble due to large stray capacitances. They are about 3 to 4 times higher than in air, depending on the kind of oil used.

And it's a mess, of course!


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