• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

How Do "Heat Fins" Work In a Vacuum?

DaveMcLain

Member
2007-10-03 1:07 am
I was just reading about some tubes that have "heat fins" attached at the plate. I was just sort of wondering how this does a whole lot of good in the evacuated environment inside of the tubes' glass bottle. I can see how it might add a VERY small amount of area to the plate which could help radiate some heat but does it really help? Or are these fins really there for some other reason? Making the plate more rigid or something?
 
One of the things that you learn by abusing tubes at dissipations far in excess of the ratings is that most of the heat dissipated by the plate comes from a small area of the plate structure. This is the area that glows when you crank up the juice. The fins that are welded to the plate are usually added to the area that glows and can double that area. What I don't understand it why there are sometines fins welded to the INSIDE of the plate structure.

Since there is a vacuum inside the tube there is very little heat conducted out of the tube. Most of the heat is radiated out of the tube. This why metal objects, especially black metal objects like OPT's that are near the hottest tubes (the output tubes) get hot. The fins allow more surface area to radiate more heat. I have tested tubes with and without the fins that are otherwize identical and can state that the fins can allow for a much higher plate dissipation.
 
re: Tubelab
"What I don't understand it why there are sometines fins welded to the INSIDE of the plate structure."

I believe the fins on the inside of the plate are called Barkhausen plates. They help short out UHF space charge oscillations near saturation. (space charge cloud oscillating back and forth thru the screen grid when the plate voltage drops suddenly) Usually found on TV Horizontal output tubes which get abused with pulses that stimulate these Barkhausen oscillations.

Don
 
Heat, Light & Sound

janneman said:
It's always a combination: conduction, convection and radiation.

Since there is a vacuum, convection is nil. There is of couse radiation, as mentioned. But there is also conduction through the structure to the pins and the socket.

Jan Didden

"Heat, Light & Sound" was our Physics text nearly 50 years ago.

Heat was divided into Convection, Conduction and Radiation.

Sub-chapters covered the experiments of all the greats: Newton, Kelvin, Boyle, etc etc

Do they still do all that at school?

Sorry! I am an old fart and reminiscing!
 
smoking-amp said:
re: Tubelab
"What I don't understand it why there are sometines fins welded to the INSIDE of the plate structure."

I believe the fins on the inside of the plate are called Barkhausen plates. They help short out UHF space charge oscillations near saturation. (space charge cloud oscillating back and forth thru the screen grid when the plate voltage drops suddenly) Usually found on TV Horizontal output tubes which get abused with pulses that stimulate these Barkhausen oscillations.

Don

I wonder if they could also be there to enhance heat transfer from the grids and cathode?
The thing with radiation is that it really works both ways, depending on the temperature of the objects in question. It is often forgotten that the plate actually radiates most of the heat produced by all the structures shrouded by it, because heat from those is mostly radiated onto the inner surface of the plate, then conducted to the outer surface and then radiated outwards (some is of course picked up by the bulb and then radiated on the outside). The rest would be conduction (for example through the grid wire support rods which frequently have fins attached on top and sometimes on the bottom).

Regarding fins on the outside, because there is no convection, these have to be strategically placed, unlike fins on a heatsink - to prevent them radiating too much onto other parts of the plate. The effective area of parallel fins such as found on heatsinks, would be much smaller than expected if this was done on a tube plate, because they 'shadow' each other as far as radiation is concerned. However, similar structures can still be used as heat spreaders to prevent hot spots.