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Old 14th March 2013, 09:12 PM   #1
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Default cooling traditional E-core iron transformers

I'm considering trying some KT120s in various bass guitar amps originally built for "lesser" tubes. I can add soft-start current-inrush limiting when standby mode is turned on for the sake of the power transformer, and plenty of capacitor energy storage to help average the demand more like a hi-fi amp. Which means the weak link may be the output transformer, which is also the most expensive part, and the whole point is to push it to its max (but hopefully not far beyond). Come on, we're all tinkerers, what can I do to improve its chances of survival? I can't just specify a heftier one.

I imagine that transformer failures are usually due to some spike in supply or load, insulation breakdown, etc., and a few degrees change in overall average operating temp might be irrelevant to what's happening momentarily deep within the hottest most vulnerable part of the windings. Yet running them cool probably does prolong life and improve performance. Look at how the power company uses liquid oil coolant and various radiators and tubes to improve performance and reduce cost. But I'm unknoledgeable about exactly what happens in the iron when it's saturated or carrying heavy eddy currents, but I imagine the heat generated in a transformer is not lnear with load wattage when it approaches its limit. Can I push that milit at all, with protective measures?

Of course a fan comes to mind, but what more can I do, and is there any value in the pursuit?

I have some deep-finned aluminum heat-sink. I have some heat-conductive electrically-insulating "loaded" potting epoxy. Is there any value to trying to cool a conventional E-core power or output transformer better? Cooling the bell-end covers probably isn't going to help very much, but even that can't hurt; I'm kind of surprised they're not finned. Can't really cool the copper windings directly very well. If I glue some heat-sink onto the iron core, it's going to be very difficult (to say the least) to disassemble in the future, not that that's likely anyway.

I could easily weld steel fins onto the bell-ends or glue on aluminum heat-sink, fill the bell-ends with potting epoxy, and bolt them on...forever. I could remove the bell-ends and put the entire transformer in a finned aluminum box or can filled with potting epoxy, like some Harmon-Kardon, McIntosh, and military equipment. Their intent seems more to prevent transformer vibration, prevent wire movement and insulation cracking, improve insulation, prevent flash-over, etc. rather than improve cooling. Sometimes high voltage trasnformers are potted in rubber that seems a better insulator than condutor of heat (and electricity, which is likely the point).

It seems both obvious and a little nuts. Is it pointless, futile, or just expensive? Any creative ideas or advice for a born tinkerer who just can't leave anything alone?
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Old 14th March 2013, 09:21 PM   #2
DF96 is offline DF96  England
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I suspect you can't make much difference. Ensuring good thermal contact between the transformer and a cool metal chassis may help a little. If the chassis itself is hot, as can happen with valves, then it won't help. A fan may help, if you can put up with the noise, but really only at the margins.
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Old 14th March 2013, 09:59 PM   #3
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Having some sinks already, I would put them on three of the four sides of the laminations. Cut the heatsinks to exactly match the dimension of each side, a generous amount of heatgrease....thick enough to get into the irregularities of the lams, a nice colored Zip-tie for the sides, gravity for the top, ....perhaps cut the ends at 45 degrees????? cept for the bottom of the sides which will contact, or nearly contact the chassis.


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Old 14th March 2013, 10:17 PM   #4
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I did something similar to what Richard is describing. Different scenario, as this is the power transformer of my tube amp, but same principle. I stripped some 14 gauge solid core electrical wire and used it secure the heat sink to the back-side of the transformer. Used some CPU thermal compound I had in the parts bin. Twisted the copper wire with pliers for a nice, tight fit.

Oh, it works, by the way. Transformer used to run at about 170 or so, now stable for hours at around 148-149 (still pretty hot, really).

As you can see, I had to modify the cage to accommodate the heat-sink, but nobody sees any of it unless I specifically show them.

Click the image to open in full size.

Last edited by cogitech; 14th March 2013 at 10:19 PM.
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Old 14th March 2013, 10:28 PM   #5
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If you wanted to go "hardcore" with the idea above, you could get something like this...

Click the image to open in full size.

...and strap a quiet-running 120cm fan to it. Rig up a 12v DC supply and Bob's yer uncle.
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Old 14th March 2013, 10:33 PM   #6
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Like this, if you wanted to go with one straight out the back of the amp, like I did...

Click the image to open in full size.

I have considered adding a fan and possibly going with a big heat-pipe. It gets very hot here in the summer and the amp is right by a huge window.
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Old 14th March 2013, 10:49 PM   #7
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Some random thoughts:

Do you know the secondary ht winding current spec? Are you wanting to run the idle at or near a level which is effectively the 'max rating' of the HT supply?

You may find that HT voltage regulation with increasing continuous sine output will mean a leveling off of output power capability, rather than any form of output valve overdrive limitation of rms output power.

The thermal lag of a few hundred watt transformer is going to be many tens of minutes - that is the outer measureable temperature of the core at steady state with a constant load on the transformer. The risk is that the winding temperature at the 'heart' of the core/winding will ramp up to exceed insulation limits in a time frame that is quicker than the capability to move the heat from the outer core surface.

Using a high C filtered diode rectifier forces high crest factor current waveform through te windings, making them less effective at transferring max power than if say you used a choke input filter.

Your heater winding is capable of a change in tube?

Unless your core is in a postion to allow airflow over an added heatsink to get in and escape, then the benefit of a heatsink can be quite restricted. Is the increase in power dissipation of transformer and valves going to cause a headache for the existing cooling scheme used in the amp, or is the transformer an independant part (with respect to cooling)?
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Old 15th March 2013, 12:47 AM   #8
popilin is offline popilin  Argentina
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I know something that cools better and works with any transformer.
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Old 15th March 2013, 05:14 AM   #9
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TROBBINS - My first concern was the output transformer, but also the power transformer to a lesser degree. I haven't even decided which amp will be the guinea-pig yet; I suspect the SVT will have the biggest transformers. OOps, I mintioned guitar amps at risk of being shuffled off the the intrument forum. But I was thinking I'd get an aftermarket filament transformer for the preamp heaters, then the stock one should handle the KT120 heaters fine. Still lots to consider in issues of bias and driving 6 of them. Heck, I don't have coherent objectives and a balanced/proportional design yet. I'm still all googly-eyed over big tubes...
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Old 15th March 2013, 05:20 AM   #10
AJT is offline AJT  Philippines
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if you want a cool running traffo, overdesign it, meaning get a traffo with much bigger capacity than your intended load...

one tip, if using 120 volt mains, compute the primary winding as if you are using 200 volt mains...
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