Let me share my findings on tube temperatures, cooling and longevity:
About a decade ago, when I started using 807, I started experimenting with cooling the power tube. Standard tube coolers were off the menu due to the ST shape. I had a "thermal leak detector" IR temperature meter, and took measurements from a number of spots on the tube - top, shoulder, etc. I think I have the results written somewhere... as I recall there was a huge temperature gradient (about 100C) from coolest point to hottest (at the shoulder where it narrows). Folded fins from aluminum tape (sticky, thick aluminum foil) and stuck it on the tube, running along the hottest spot into the middle of the cool zone.
The readings were a few dozen C lower compared to no-fins.
Then, added force cooling: two 12V computer fans connected in series (running half-speed & more importantly quietly). With the fins the hottest spot measured on the tube surface dropped a whopping 70C!!! Also, the gradient from hottest to coolest dropped to 50C or less (can't recall exactly).
While maximum glass temperature is important, I also suspect that the temperature gradient across the glass is also important: with a big gradient you are much more prone to fatigue the glass, and make it release contaminants / loose vacuum.
However, measurements are not everything, who knows what I measured really? Glass surface might be cooler, but the aluminum might be reflecting the heat back to the plate, creating a worse problem.
I've got two more observations about that -
1. Did a reality-check on bulb temperature: Touch. Without fins, touching the shoulder my finger sizzled (=burned, loosing water content) instantly. With fins the sizzle got worse: I did not need to touch the bulb hard, when my finger got within .5mm of the fin, it already started sizzling. (This was expected, as aluminum is much better heat condutor than glass). When the fins were cooled by the fans, I could HOLD THE FINS without discomfort, or even touch the glass for a couple seconds without problem. Yeah, it was hot, and about 70-80C hot (I'm a microbiologist, can tell temperature by touch
. Meaning, the 70C surface temperature was real.
On the point of whether the heat was reflected or dissipated: when the heat is reflected back to the anode, and it's building up in the tube, you can hear that in the sound - it gets stressed. Also, upon prolonged exposure, it decreases tube life significantly. With the cooled fins the amp sounded significantly more relaxed, hinting that it was under less stress. Also, this pair of tubes lasted a crazy long time, even after 10 years they measure 90%. The amp runs in class A, was used a lot, and countless times I left it on for a week or longer.
Another interesting point:
with the exact same plate dissipation the 807 seems to be much cooler than the 6550. Right now I have a Quicksliver V4 monoblock by my 807 PSE (both sporting 4 tubes). The 6550s in the Quickie dissipate 16W each (+10W filament =26W total), and the 807 dissipates 18W each (+5W filament=23W total). The 807 feel cool, and the air sizzles around the 6550s. Weird. If I bias the 6550s higher, the perceived heat wave intensifies greatly, they warm up the room like crazy. When I crank up the 807 current, they feel just as cool... now, that's a murder-mystery. Why is the heat perception of the two amps dissipating the same watts so drastically different? : ) I can leave the 807 on overnight, and it does not change the ambient temperature in my living room. Turn on the V4, and within hours I feel the temperature rising. Also, if I count the driver tubes dissipation and the regulators dissipation, plus tube rectification, the 807 amp dissipates more total heat than the V4 does.
;
Janos
About a decade ago, when I started using 807, I started experimenting with cooling the power tube. Standard tube coolers were off the menu due to the ST shape. I had a "thermal leak detector" IR temperature meter, and took measurements from a number of spots on the tube - top, shoulder, etc. I think I have the results written somewhere... as I recall there was a huge temperature gradient (about 100C) from coolest point to hottest (at the shoulder where it narrows). Folded fins from aluminum tape (sticky, thick aluminum foil) and stuck it on the tube, running along the hottest spot into the middle of the cool zone.
The readings were a few dozen C lower compared to no-fins.
Then, added force cooling: two 12V computer fans connected in series (running half-speed & more importantly quietly). With the fins the hottest spot measured on the tube surface dropped a whopping 70C!!! Also, the gradient from hottest to coolest dropped to 50C or less (can't recall exactly).
While maximum glass temperature is important, I also suspect that the temperature gradient across the glass is also important: with a big gradient you are much more prone to fatigue the glass, and make it release contaminants / loose vacuum.
However, measurements are not everything, who knows what I measured really? Glass surface might be cooler, but the aluminum might be reflecting the heat back to the plate, creating a worse problem.
I've got two more observations about that -
1. Did a reality-check on bulb temperature: Touch. Without fins, touching the shoulder my finger sizzled (=burned, loosing water content) instantly. With fins the sizzle got worse: I did not need to touch the bulb hard, when my finger got within .5mm of the fin, it already started sizzling. (This was expected, as aluminum is much better heat condutor than glass). When the fins were cooled by the fans, I could HOLD THE FINS without discomfort, or even touch the glass for a couple seconds without problem. Yeah, it was hot, and about 70-80C hot (I'm a microbiologist, can tell temperature by touch
. Meaning, the 70C surface temperature was real.On the point of whether the heat was reflected or dissipated: when the heat is reflected back to the anode, and it's building up in the tube, you can hear that in the sound - it gets stressed. Also, upon prolonged exposure, it decreases tube life significantly. With the cooled fins the amp sounded significantly more relaxed, hinting that it was under less stress. Also, this pair of tubes lasted a crazy long time, even after 10 years they measure 90%. The amp runs in class A, was used a lot, and countless times I left it on for a week or longer.
Another interesting point:
with the exact same plate dissipation the 807 seems to be much cooler than the 6550. Right now I have a Quicksliver V4 monoblock by my 807 PSE (both sporting 4 tubes). The 6550s in the Quickie dissipate 16W each (+10W filament =26W total), and the 807 dissipates 18W each (+5W filament=23W total). The 807 feel cool, and the air sizzles around the 6550s. Weird. If I bias the 6550s higher, the perceived heat wave intensifies greatly, they warm up the room like crazy. When I crank up the 807 current, they feel just as cool... now, that's a murder-mystery. Why is the heat perception of the two amps dissipating the same watts so drastically different? : ) I can leave the 807 on overnight, and it does not change the ambient temperature in my living room. Turn on the V4, and within hours I feel the temperature rising. Also, if I count the driver tubes dissipation and the regulators dissipation, plus tube rectification, the 807 amp dissipates more total heat than the V4 does
. ;
Janos