PT on nuts or washers for heat dissipation ?

[mroboto]

I sometimes see power transformers mounted on nuts or washers, and my power transformer laminations are mounted directly to the chassis, and he gets slightly hot at 128°F , so the chassis too.

Also have two 120 ohms thermistors in series on AC input prong that create heat in the chassis.

Do you think that if I install nuts to create a gap for air circulation between the power transformer laminations and chassis, most of the air will dissipate in air instead of dissipating in the chassis, and everything will not heat as much ?

Probably not be a very easy work to do, but if we think that can improove heat dissipation and won't cause any secondary effect, that could be a good project ?

 

[Retrovert]

The issue comes down to how much heat may be radiated to the air vs. how much heat may may be conducted into the chassis, a large surface area, and then radiated into the air.

That's an easy question to answer.

Consider the conductivity of air vs. steel and aluminum, all measured in Watts per (Meters x Degrees Kelvin):

Thermal Conductivity of Air: k = 0.024 W /(mK)
Thermal Conductivity of Steel: k = 50.2 W /(mK)
Thermal Conductivity of Aluminum: k = 205.0 W /(mK)​

The standard value here is k = 1, which specifies that a 1 mm cube transfers heat as 1 Watt for every degree Kelvin difference between two surfaces on the opposite sides of the cube. The lower the k value, the worse the heat transfer will be. Air is a fairly good insulator, which is why we use it trapped in fiber insulation, in both homes and clothes, as insulator to keep things warm or cool. Aluminum is a very good conductor of heat, which is why only thin aluminum may be cut with a laser while steel, relatively speaking to aluminum, is not a very good conductor, so it is relatively easy to cut, even in thicker layers.

Standoffs might be useful in forced-air cooling, since the air may flow over/under the object to be cooled, but this strikes me as implausible given the tiny elevations involved. I call internet nonsense on this.

The only time standoffs are generally thermally desirable is to insulate two devices to prevent heat conduction between the two surfaces. For example, if one had a thermal crystal oven in a shielded box it would be best to insulate that from the case. Or if one had a very hot surface with its own radiator that would be thermally decoupled from the chassis which was used as a heat sink for lower-power devices. Situations like those.

Washers are useful to spread the force of a nut across a larger area, and thus prevent tearout of a hole. So this would apply on the underside of the chassis and on top of the transformer mounting ears, but not between the chassis and the transformer where it would only reduce conduction into the chassis.

So I think that debunks the notion that standoffs could possibly be superior to conducting heat into the chassis which would then radiate it. In fact, one might want to use thermal compound to ensure the best possible heat conduction between the transformer body and the chassis.

The related issue is that black-body radiation is the most efficient radiator, hence black transformer paint. (Yeah, yeah, don't ask me if that's true then why are NYC radiators are usually painted silver or sometimes white, particularly when white shows the dirt, and also breaks down the lead paint to form toxic powder. Also, don't ask me why the subways occasionally run on time and don't bypass one's desired stop. All of these are great mysteries. New rules tomorrow.)

 

[mroboto]

I don't know how much mm3 of air can vent from the gap with M4 nuts, approx 3mm thick, but the air will not accumulate as the chassis that have good vents bottom, but bad vents on the top., that could be a good thing and not so easy to calculate for me.

If the transformer becomes slightly higher in temperature but chassis and capacitors inside (close to the PT) are much vented, receives less heat and obtain better cooling, this would be ok for me.

I am still tempted to test the temperatures with that, I'm probably fine if the PT doesn't reach 140°F or more.

 

[gadget73]

Yeah, yeah, don't ask me if that's true then why are NYC radiators are usually painted silver or sometimes white,

There is actually a reason for that. There is a chart I've seen (which I can't be bothered to locate just now) that shows the radiator efficiency and what color it is. Silver or white reduces output when the heat system or radiator is over-sized for the application. In the 20s there was a thing called the "fresh air movement", which basically was to keep windows open in the winter, and of course you'd need more heat output to make up for this. Once the Depression hit, there wasn't any money for that so the windows got shut and now you've got way too much heat system. Paint is cheaper than a new radiator, so there you go. There is a video on youtube of a guy discussing steam heating specifically in NYC and he talks about that. Its more interesting than it sounds.

Back to transformers, I'm not sure how much heat conduction actually happens through the mounting tabs though. Generally the heat will be concentrated in the core, and the mounting ears are generally attached off the side of the core on a vertical core transformer. The core doesn't sit directly in contact with the chassis. I expect you'd shed more heat with air space underneath to allow for air circulation than you would from whatever conducts through the mounting. Horizontal core transformers typically have nuts when the screw holding the core stack together is also used as the mounting screw. Not a good idea to remove that nut.

and all that said, 120F for a transformer is not considered to be hot.

 

[savatage1973]

It would have to guess that the transformer was intentionally mounted directly to the chassis for heat dissipation purposes, since the chassis acts as a heat sink. Separating the transformer from the chassis may actually cause it to run hotter than it does already, since the chassis would not longer be transferring and dissipating heat--refer to the thermodynamics discussion above.

I have seen transformers mounted off the chassis with thick teflon washers/spacers, to prevent transformer "buzz" (common in tube amps) from transferring to the chassis, but I am guessing that they are not running as hot as yours.

The other question--is your amp running "in-spec", and supposed to run that hot? That seems kind of high (to me, at least), and could reduce longevity.

 

[mroboto]

The tubes doesn't run too hot, this is a 250 watts transformer for a stereo SE KT88 with 6SN7 and 5U4G.

This is not the transformer temp that bugs me the more, but the temperature of the chassis around, all thermal sources except the small signal and power tubes are on the left of the chassis, thermistors, power transformer, rectifier, choke, and all this stuff close to the 1st cap and B+ cap.

The power transformer can survive with a temperature that do not exceed maybe 160°F , but the caps under the chassis will dry faster if I don't change anything, even if they are rated for 105°C

 

[mroboto]

You can see the vents, there is not a lot around the transformer, and the KT88 vents are half obstructed by the tubes bases, so the vents that works best (corectly) are the 6SN7 vents in the middle.

https://www.cjoint.com/c/HIbmKq5pmps

 

[savatage1973]

If you are that concerned, there are heat sink shields that slide right over the tubes to dissipate heat. They're not pretty, but functional. Besides, heat rises, so measure the top of the chassis and then the underside where the caps are, and see what the difference is--if nothing else (and you have room), just separate the caps a little further away from the chassis--just like "lifting" hot-running resistors up off the PCB board to prevent scorch marks on the PCB.

 

[mroboto]

There are plastic things (don't know the name) Edit: Cable Tie Mounting Pad under the capacitors to hold them with tie wraps, these plastic things are glued to the chassis, so the caps are not direct contact on the chassis, but the air around must be slightly hot.

I cannot measure the chassis directly with my IR thermometer because of stainless steel IR emission, except if I apply black tape but...

The PT heats the chassis more than the tubes, the temperature just beside of it is close to is core temp.

I think that separating the core from chassis would be the best option for vent except drilling holes, and what I don't wish to do, the vents are perfect on bottom but the top vents of the chassis is very faint.

I think I will try lifting it for about 3mm, like my first idea, if the result is not very acceptable, I will undo.

 

[savatage1973]

The PT heats the chassis more than the tubes, the temperature just beside of it is close to is core temp.

This was the question that I posed previously--is the amp running "in-spec"? I don't understand the how/why of the PT running hotter than the actual tubes.I have a number of relatively high power tube amps (ARC and Carver) and the PTs barely get warm compared to the KT88 and KT120 output tubes. That just seems "odd" to me. Is this a "home-brew" or a specific manufactured model?

 

[mroboto]

The transformer do not run hotter than the tubes, but the transformer is metal contact to metal, when the tubes have plastic bases that don't touch the stainless chassis but ceramic sockets, and there is still some venting around, even if not perfect.

The KT88 plates are about 300°F or slightly less, does not run too hot.

There still have a very very slight hum (at 10 inches from speakers) because of some mods, but soon I will replace the 180 ohms virtual center tap resistors I previously added because I had in hands, by 100 ohms resistors.

The PT heats less than before the 180 ohms center tap addition, but it's not perfect.

For the beginning of the story, the 6.3V KT88 heaters was not center tapped, there was a cathode feedback that I removed, adding hum, then I added a virtual center tap that partially fixed that issue, I have the 100 ohms resistors, but i have another mod to do prior to install then definitely, I want to add a triode operation switch, waiting for the switch.

But the amp works great, the only concern is thet this is my hottest amp, the cover on the top of the transformer of my 300B Yaqin is at about 92°F and my PP EL84 Raphaelite transformer core is about 125°F , but the chassis don't heat as much as the SE KT88, the Raphaelite have washers under the PT , maybe this is one of the reasons why the chassis is just warm not hot.

This amp is not branded or maybe Nobsound, not sure, amp from China.

 

[savatage1973]

Lifting the transformer off the chassis would definitely eliminate the chassis heating up excessively--I'd use teflon washers/spacers to do it, but the issue remains that the transformer is running that hot in the first place--that just seems awfully high. But really not surprising with Chi-Fi. Even when they use the best of components (as often advertised), they tend to run them at 100% (or even slightly higher) than their rated specs. I have worked on/modded a few Chi-Fi tube amps for some folks, and the caps were running right at (or a little above) their rated voltages. Sounds like that transformer is running at about the top of its operating range, in terms of acceptable demand. Even the best of components will fail if being continuously run at their max rating (or above) all the time. It's kind of like running your car at red-line all the time and expecting the engine to not fail for 10 years.

 

[mroboto]

5U4G and KT88 ask for a lot of current, with a not so beefy transformer it's normal, justt under 130°F is not critical, under 150° I am pretty sure it will survive, windings and varnish will be fine it's just to have a chassis near this temperature that is new to me.

 

[Lavane]

I've never noticed that much heat coming off the chassis. Modern quality electrolytic caps are much better than most made back in the day. Using a temp gun i find around 90 to 95° is the hottest any of my chassis get. And that being close to a power tube or rectifier. In one amp I have a very hot running dual regulator mounted to the chassis. It will burn and blister your finger if you tough it. Mounted to the chassis I get around 95° at the point where its mounted. The only time I use a plastic washer/spacer or rubber washer to mount a trany is if I get mechanical hum. Some electrolytics are mounted right behind the driver tubes on my ST-70. I only get a reading in the 8o° range on the caps. With some air gap around the caps you should be fine.

 

[FlaCharlie]

The power transformer can survive with a temperature that do not exceed maybe 160°F , but the caps under the chassis will dry faster if I don't change anything, even if they are rated for 105°C

If you're worried about the temps of the caps, don't. Assuming that they are quality parts and not Chinese counterfeits.

If the transformer is running at 128 F and some of that heat is being dissipated by the chassis, I can't imagine that the caps themselves are getting all that hot. If they are rated at 105 C, that is 221 F. I usually recommend 105 degree caps rated for 10,000 hours. The hour rating is for the caps running at that temp. If the actual operating temp is lower, the hourly rating goes up substantially. Even if they were running at the same temp as the transformer, their lifespan will be much much longer than their hour rating.

 

[mroboto]

If you're worried about the temps of the caps, don't. Assuming that they are quality parts and not Chinese counterfeits.

If the transformer is running at 128 F and some of that heat is being dissipated by the chassis, I can't imagine that the caps themselves are getting all that hot. If they are rated at 105 C, that is 221 F. I usually recommend 105 degree caps rated for 10,000 hours. The hour rating is for the caps running at that temp. If the actual operating temp is lower, the hourly rating goes up substantially. Even if they were running at the same temp as the transformer, their lifespan will be much much longer than their hour rating.

Capacitors lifespan rating approximately doubles each time the temperature is lowered by 10°C (From what I have read) , I did not calculate anything, but I just hope I will not need or feel the need to replace them before at least two decades.

 

[Retrovert]

Perfectly true.

Ripple current causes ohmic heating, and the ESR in electrolytic capacitors determines how much heating occurs.

The lifespan of an electrolytic exponentially decreases with temperature, as the electrolyte is evaporated. The capacitor eventually fails, causing the rectifier to short to ground which then arcs over (if not fused) and the B+ winding melts because the main fuse is sized for the substantial heater current, not the low B+ current.

 

[mroboto]

I am thinking about adding a SS full wave rectifier between the PT and tube rectifier inputs to diminish or avoid such failures.

Eventually, if there is a failure downstream, the SS rectifier can fail open instead of the PT.

 

[maxhifi]

As was previously mentioned, standing the transformer a bit off the chassis can help with vibration related buzz and hum. If it's running too hot and you want to cool it down, use a fan - it's incredible how effective forced air cooling is.

Don't need to be fancy, I've used a USB powered desktop fan I got at an office supply store, and it substantially reduced the temperature of electrical equipment. The hard part is finding a fan that's quiet enough not to interfere with enjoying your equipment, but they definitely do exist.

With larger electrical transformers, fan cooling with modestly sized fans can increase the rating up to 1.5 times the non fan cooled rating.. they're amazingly effective.

As for your transformer, if it's only 128F, it's rather normal, and really isn't very hot. That's only 53C, which is only a 33 degree rise over the ambient temperature. You will have to get lots hotter than that to damage insulation and stuff - sounds to me like that's just a transformer which is designed to run hot.

If you're used to solid state equipment with cold transformers, remember that most solid state amps are class B, and pull almost no current from the power supply most of the time. In tube amps, there's filaments and bias current which put a continuous load on the transformer, so having it warm up a bit is normal.

 

[mroboto]

As was previously mentioned, standing the transformer a bit off the chassis can help with vibration related buzz and hum. If it's running too hot and you want to cool it down, use a fan - it's incredible how effective forced air cooling is.

I thinking about that too, but noise, dust, and maintenance keeps me from going that route, for now.

But I done that with SS amps at full power.