Hello, all!
I am not a very good searcher - I couldn't find any info on the subject of my request, which is - thermal isolation of electrolytic capacitors in tube amplifiers.
The problem is obvious - the higher the operating temperature, the lower the life of electrolytic capacitors, and many tube amplifiers have el. capacitors very near the tubes. The whole amplifier runs quite hot, and the caps much more so, being close to the tubes.
Currently, I found a new favorite amplifier which I very much enjoy, and its two main V+ caps are exposed on the top plate few cm from the tubes.
They are too big to substitute for a film cap (330uF/450V each). In future, I may try substituting them for AN Kaisei for better sonics, but it will not alleviate the overheating problem.
I think of something like aluminum or steel cylindrical covers for the caps, lined on the inside with some heat-isolating material, and maybe even placing a small radiator on top of each cap.
What do you think, is it idiocy that may even worsen the things (caps slowly cooking inside the thermal isolation), or is it more or less sound? Also, maybe such devices already exist, and I just don't know?
Please, share your wisdom
Best regards,
Vlad
I am not a very good searcher - I couldn't find any info on the subject of my request, which is - thermal isolation of electrolytic capacitors in tube amplifiers.
The problem is obvious - the higher the operating temperature, the lower the life of electrolytic capacitors, and many tube amplifiers have el. capacitors very near the tubes. The whole amplifier runs quite hot, and the caps much more so, being close to the tubes.
Currently, I found a new favorite amplifier which I very much enjoy, and its two main V+ caps are exposed on the top plate few cm from the tubes.
They are too big to substitute for a film cap (330uF/450V each). In future, I may try substituting them for AN Kaisei for better sonics, but it will not alleviate the overheating problem.
I think of something like aluminum or steel cylindrical covers for the caps, lined on the inside with some heat-isolating material, and maybe even placing a small radiator on top of each cap.
What do you think, is it idiocy that may even worsen the things (caps slowly cooking inside the thermal isolation), or is it more or less sound? Also, maybe such devices already exist, and I just don't know?
Please, share your wisdom
Best regards,
Vlad
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You could use heat reflective tape: DEI Cool-Tape – Heat Reflective Tape | Kimpex Canada or Funk Motorsport: Reflective Gold Heat Tape for induction kits & bulkheads
Personally, I just buy 105°C long life caps. Here are some 105°C caps rated for 20000 hours: Nichicon LGZ2W331MELC50
Personally, I just buy 105°C long life caps. Here are some 105°C caps rated for 20000 hours: Nichicon LGZ2W331MELC50
You are right to be concerned, but the person who located the capacitors next to the tubes was not.
May not look good, but a simple sheet of aluminum mid-way between the capacitor and tube should help a lot. You need an air gap on both sides of the aluminum shield. Aluminum angle flashing for roofs from the hardware store is cheap and would work fine. Make the shield large enough so it will both block the radiating heat from the tube and have enough area to dissipate the resulting heating of the shield.
I suggest a non-contact laser thermometer to test the temperature of the capacitor(s) with and without the shield. These thermometers are available inexpensively on Amazon or at Harbor Freight, etc. Good to add to your tool kit - useful in many circumstances.
I suggest a non-contact laser thermometer to test the temperature of the capacitor(s) with and without the shield. These thermometers are available inexpensively on Amazon or at Harbor Freight, etc. Good to add to your tool kit - useful in many circumstances.
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If your buying new some caps are rated for longer life at higher temperatures than others. Go for these.
I think radiation is the main source of heat transfer. Convection takes the heat away upwards from the tube. I don't think conduction via the chassis is very large.
So what you want is something between the tube and the cap. It keeps the radiation from reaching and heating the cap. The more reflective it is, the less the radiation heats it up.
Heat isolation around the cap just slows the heating down, it does not alter the final temperature (a bit simplified...). I would not put any material around the cap as it would allow any heat reaching it (via conduction) to be taken away by convection.
Basically the same as what tgreese is saying.
So what you want is something between the tube and the cap. It keeps the radiation from reaching and heating the cap. The more reflective it is, the less the radiation heats it up.
Heat isolation around the cap just slows the heating down, it does not alter the final temperature (a bit simplified...). I would not put any material around the cap as it would allow any heat reaching it (via conduction) to be taken away by convection.
Basically the same as what tgreese is saying.
If the chassis is not enclosed I don't think the cap temperature in free circulating air will be high enough to be a real problem even if it is sitting close to tubes .
Did you mesured cap temperature? Else, why you suppose they are overheated and not only slightly heated?
I use 105 Degrees C electrolytic capacitors when possible, but sometimes use 85 Degree C rating.
I usually use power resistors that are rated for 5 times the actual power they dissipate.
That means they are bigger, and have more surface area to the air, to keep them cool.
I use holes in the chassis bottom cover, and holes in the top of the chassis (natural air cooling).
If my amplifier parts, (other than the tubes) reach 85 Degrees C (100 Degrees C is the temperature of boiling water) . . . then I turn the amplifier off, and redesign it (haven't had to do that yet).
I use an infrared temp meter to read how hot parts are.
I double fuse my tube amplifiers, for example, one uses a primary winding 1.25A fast blow fuse, and a 0.5A slow blow fuse in series.
I do not use power transformers at their rated currents, I use less current.
I use B+ with choke input filters whenever possible, to keep the power transformer cool.
Nuff said?
I usually use power resistors that are rated for 5 times the actual power they dissipate.
That means they are bigger, and have more surface area to the air, to keep them cool.
I use holes in the chassis bottom cover, and holes in the top of the chassis (natural air cooling).
If my amplifier parts, (other than the tubes) reach 85 Degrees C (100 Degrees C is the temperature of boiling water) . . . then I turn the amplifier off, and redesign it (haven't had to do that yet).
I use an infrared temp meter to read how hot parts are.
I double fuse my tube amplifiers, for example, one uses a primary winding 1.25A fast blow fuse, and a 0.5A slow blow fuse in series.
I do not use power transformers at their rated currents, I use less current.
I use B+ with choke input filters whenever possible, to keep the power transformer cool.
Nuff said?
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If they are large e-caps then measuring the cap surface temperature (eg. with a spot IR gun) on the 'hot' side does not really mean much as to what the internal temp gradient is, and hence how much heat is going in to the cap and how much is just being convectively transferred to local ambient air on that hot side of the cap.
I'd be suggesting the use of a thin reflective foil (applied to the hot facing surface of the e-cap) that is meant for elevated temperature (ie. the glue), and preferably has some semblance of a specification for reflection versus thermal frequency (where the thermal frequency is down below 100-150C). Of course that may unacceptably ruin the aesthetic appeal of your amp. And of course making sure there is a good ability for e-cap local ambient air to mix with room ambient temperature.
I'd be suggesting the use of a thin reflective foil (applied to the hot facing surface of the e-cap) that is meant for elevated temperature (ie. the glue), and preferably has some semblance of a specification for reflection versus thermal frequency (where the thermal frequency is down below 100-150C). Of course that may unacceptably ruin the aesthetic appeal of your amp. And of course making sure there is a good ability for e-cap local ambient air to mix with room ambient temperature.
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Regardless of the internal temperature, my electrolytic caps have run for 2 or more decades.
Be sure to use a capacitor with a high enough voltage rating.
B+ can often rise to 1.4 times the rms voltage of the secondary voltage to the rectifier.
That is because the output tubes may warm up after the rectifier.
And, a capacitor that is used as the first cap in a cap input filter will always get warmer, versus the same cap used in a choke input filter (the first cap in a choke input filter is After the choke).
The difference is UGH! High Transient current, versus smooth moderate current, respectively in that order.
I am sorry for the use of the word UGH!, it only has meaning in some languages.
Just think of a weight lifter in the Olympics lifting 500 pounds over his head, you will get the idea, I think.
Be sure to use a capacitor with a high enough voltage rating.
B+ can often rise to 1.4 times the rms voltage of the secondary voltage to the rectifier.
That is because the output tubes may warm up after the rectifier.
And, a capacitor that is used as the first cap in a cap input filter will always get warmer, versus the same cap used in a choke input filter (the first cap in a choke input filter is After the choke).
The difference is UGH! High Transient current, versus smooth moderate current, respectively in that order.
I am sorry for the use of the word UGH!, it only has meaning in some languages.
Just think of a weight lifter in the Olympics lifting 500 pounds over his head, you will get the idea, I think.
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One alternative, to minimise heat transfer, is to rotate the tube so that the smallest cross section of plate is facing the component. Not something you can really retro-fit without drilling etc.
Placement - point source radiated energy follows 1/R^2 so it drops off at distance squared. Jamming caps against tubes or hit resistors isn't a good design choice - especially for noise and life.
You could drill holes around the base allows convection cooling of the caps. If you have caps in the chassis itself, then cooling holes placed in the base allows cool air to flow across them.
Perhaps big copper plate with high temperature black paint would be best if you're retrofitting. Then ensure you have enough holes to allow the convection cooling.
I've been having fun with placement at the moment - banks of four tubes close together, but each rotated to minimise the heating effect on the more sensitive ones. The caps are under chassis but they will have cooling holes drawing air directly over them from underneath.
Placement - point source radiated energy follows 1/R^2 so it drops off at distance squared. Jamming caps against tubes or hit resistors isn't a good design choice - especially for noise and life.
You could drill holes around the base allows convection cooling of the caps. If you have caps in the chassis itself, then cooling holes placed in the base allows cool air to flow across them.
Perhaps big copper plate with high temperature black paint would be best if you're retrofitting. Then ensure you have enough holes to allow the convection cooling.
I've been having fun with placement at the moment - banks of four tubes close together, but each rotated to minimise the heating effect on the more sensitive ones. The caps are under chassis but they will have cooling holes drawing air directly over them from underneath.
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Ripple currents in the capacitors themselves generate heat, so don't insulate the capacitors; they need to be able to let heat out!
A radiation barrier between the heat emitting valves and the heat absorbing capacitor cases would stop radiant heat transfer. A shiny, mirror-like panel that 'shades' the capacitors from the tubes will work best, with a air gap around the capacitors to allow convention cooling of their own internal heat.
As others have said, if replacing the capacitors make sure you choose 105º capacitors, but also choosing a higher rated operating voltage for the capacitors has a huge effect on rated life, especially at elevated temperature. See if you can find a 500V or 550V rated version.
A radiation barrier between the heat emitting valves and the heat absorbing capacitor cases would stop radiant heat transfer. A shiny, mirror-like panel that 'shades' the capacitors from the tubes will work best, with a air gap around the capacitors to allow convention cooling of their own internal heat.
As others have said, if replacing the capacitors make sure you choose 105º capacitors, but also choosing a higher rated operating voltage for the capacitors has a huge effect on rated life, especially at elevated temperature. See if you can find a 500V or 550V rated version.
When in doubt, use a bigger chassis, or build 2 large monoblock amplifiers.
Or push everything beyond the absolute limits, and watch things go up in smoke.
Oh, did I mention that it is so hard to put the smoke back in!
So far, my amplifiers are doing well on these record breaking heat days (yes, I am using air conditioners, but the inside of the house has been the hottest ever).
Or push everything beyond the absolute limits, and watch things go up in smoke.
Oh, did I mention that it is so hard to put the smoke back in!
So far, my amplifiers are doing well on these record breaking heat days (yes, I am using air conditioners, but the inside of the house has been the hottest ever).
Large bottle beam tetrodes typically have a 'hot' direction and an orthogonal cooler direction. For a pair of such valves located side-by-side the cooler directions face each other (if the manufacturer new what they were doing), but that also means that other components may be in the 'hot' direction. Point being that increasing the service life of an e-cap may cause reduced life of the output stage valves.
Yep, in the end it's down to space and design. If the caps are being toasted by design.. you have to question the sanity of the designer.
Putting a shiny surface that reflects radiated heat back onto the plate is just as bad. Angle the surface to reflect in a different direction. Or alternatively paint it black and let the underlying copper conduct the heat away. It will re-radiate some but it's also possible to make it a heatsink with convection.
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If the capacitor is a round can then any direct reflection back to the tube will be diluted by the can curvature, and as the tube is also curved then any direct re-reflection from it back to the cap would also be diluted. Only some of the plate radiation passes directly through the glass envelope. The plate won't itself be heated up from any reflection, but its heat loss would decrease.
https://www.amazon.com/Design-Engin...hvlocphy=&hvtargid=pla-4583657821980831&psc=1
Your caps will stay cooler and make more horsepower!
Your caps will stay cooler and make more horsepower!