Some serious heatsinks for GU-50: https://www.diyaudio.com/community/threads/gu-50-with-heatsink.379486/post-6849793
By my personal experience, for a 300B in reasonable Class A operation (plate supply voltage < or = to 400VDC), you do not necessarily need venting holes at the base, since that large bulb tube doesn't heat too much...
On my U-300B below :
You can see that there's a chimney provided around the socket of the 300B :
But finally, I drilled no hole nor installed any fan cooling under the chassis, so it's just a vent without chimney effect, so in fact no air circulation !
Conversely, on my U-OTL, where 3x 6080 operates in Class AB1, there's a chimney and fan cooling system :
You can see the chimneys around the 6080 sockets :
And the little fan at the back of the amp, with 3-speed settings : Hi/Mid/Low, the Mid speed being already enough and is inaudible :
I measured the temperature of a 6080 top bulb, cooling off, after 5 miniutes of operation : 190°C, that's much more than for a 300B...
So yes, in this case, a chimney plus light fan cooling is indeed welcome ! At LO speed, the chassis at the base of the tube stays lukewarm, and at Mid speed, it remains cold.
T
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So this is trying to say it can cause harm? Or that air can't move through a grid of holes? Trying to understand what you are claiming here.
I'm preparing to waste some time, electricity as well as some aluminum tomorrow drilling some holes. Mostly for thermally enhanced ventilation as well as a bit of decorative effect. Got two of these to do. 300B towards tge back, #41 up front and 5R4 to the right.
Man, that layout is what I lack in experience. That photo is revealing in how something should be approached. Damned human eye picks up too much in cases such as this and yet ignores stuff like motorcycles and pedestrians.
I mount my tubes on turret boards under the top plate. I make the tube holes around a quarter inch larger in diameter that the tube base at a minimum. This gives plenty of convection air flow from inside the chassis and up the tube and away. It has been doing a great job for years!
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One hassle with recessed tube bases can be difficulty in pulling the tube out if access to the base is restricted. Pulling up a tube using just the glass is a risk, especially for older tubes, and especially when the glass is loose in the base.
Yah, I bet we have all had one of those tubes that is no longer physically trustworthy. If not a base breaking free it is a rattle inside the tube and then told it's fine. Uh huh. You can have it.
?????
In much wisdom is much grief: and he that increaseth knowledge increaseth sorrow.😉
Why would the air flow through the holes or the slot underneath the tube if no pressure difference is present?
Air is not heated under the tube's envelope by the tube or does it? Funnel or chimney is yet another story...
Hotter air will rise, even from a warm chassis top (due to internal heat dissipation transferring to the chassis). Any aperture on the top of the chassis (holes or valve base openings), along with apertures for cooler air entry (eg. underneath) will achieve some airflow induced by hotter internal temp within the chassis.
I'd suggest that just the act of ventilating the chassis internals is important to minimise temp rise of all parts entombed within. It's pretty easy to poke a thermometer of K-type into a chassis and measure the internal temp both with and without apertures. The inherent reliability of any internal part will benefit long-term.
The hottest portion of an output stage valve's glass is broadside to the plate, and that is where highest level of convection to air will occur. There is typically a lot of distance between chassis top and the hottest portion of valve glass, which would allow air to come in from local ambient, especially for valves located along an edge of the chassis, or where valves are separated by substantial distances. Some datasheets indicate minimum separation distance advice, as well as orientation of plate structure so that radiation is improved by not pointing directly at the anode structure of an adjacent valve. Similar to inherent part reliability within the chassis, there are long-term benefits to a valve from operating in a 'cooler' environment, especially related to lower glass temperature and consequential lower outgassing.
I'd suggest that just the act of ventilating the chassis internals is important to minimise temp rise of all parts entombed within. It's pretty easy to poke a thermometer of K-type into a chassis and measure the internal temp both with and without apertures. The inherent reliability of any internal part will benefit long-term.
The hottest portion of an output stage valve's glass is broadside to the plate, and that is where highest level of convection to air will occur. There is typically a lot of distance between chassis top and the hottest portion of valve glass, which would allow air to come in from local ambient, especially for valves located along an edge of the chassis, or where valves are separated by substantial distances. Some datasheets indicate minimum separation distance advice, as well as orientation of plate structure so that radiation is improved by not pointing directly at the anode structure of an adjacent valve. Similar to inherent part reliability within the chassis, there are long-term benefits to a valve from operating in a 'cooler' environment, especially related to lower glass temperature and consequential lower outgassing.
Not sure what that quote has to do with physics.
There is gonna be heat inside the chassis, some of the tube's heat goes through the pins into the socket, there are resistors inside which by design create heat. The air outside the amp is cooler than the inside of the chassis, hot air is lighter so will rise if there is a way for cooler air to displace this hotter air from below. Seems pretty obvious to me that air is gonna move and if you simply seal the chassis, the heat will be trapped inside. It also seems to me that if this air is moving past the much hotter tube, that's probably not a bad thing. And that this rising hot air off the tube might help pull air through the amp chassis.
Another example of this, my computer router has no fan to create a pressure differential, just slots in the bottom and the top. I can hold my hand a few inches over the top of it and feel the heat rising from it, that's some sort of magic huh?
There is gonna be heat inside the chassis, some of the tube's heat goes through the pins into the socket, there are resistors inside which by design create heat. The air outside the amp is cooler than the inside of the chassis, hot air is lighter so will rise if there is a way for cooler air to displace this hotter air from below. Seems pretty obvious to me that air is gonna move and if you simply seal the chassis, the heat will be trapped inside. It also seems to me that if this air is moving past the much hotter tube, that's probably not a bad thing. And that this rising hot air off the tube might help pull air through the amp chassis.
Another example of this, my computer router has no fan to create a pressure differential, just slots in the bottom and the top. I can hold my hand a few inches over the top of it and feel the heat rising from it, that's some sort of magic huh?
Sure thing. If you put blower underneath the holes it will do.
Why expect air to flow through a crack in a window instead of nearby door wide open.
As trobbins has put it more elegantly few messages above.