Cooling for hot running TM and GM units.

A few people have posted saying they are worried about the effects of overheating with the packed lower chamber of the TMs and GMs.  I have my doubts as to whether this will be a real worry but, as always, it seemed to be something I could have a bit of fun with and was easily helped.

I already have a home made base unit for the amp.  A simple bit of MDF with rings stuck on it to locate and hold the feet and a slot cut out to locate over the cabinet handle makes it a rock steady platform.  I just extended this idea to include my cooling setup.

I found a pair of compact twin 12V HDD fan units on ebay for £0.99 each including p&p so I snapped them up. These had two ball bearing fans mounted on a thin black aluminium frame so all of the hard work was done.  I did a rethink of the base to include a layer with cutout for mounting the fans and another below it as a space to allow air in.  I made deep depressions for the amp feet to stabilise it and to lower the bottom to almost touch the fan unit.  I then mounted a 2.1 x 5.5mm DC socket on a small aluminium plate on the back to take a 12V 1A wall wart, and a couple of plastic mesh dust screens on the underside and voila!  I was ready.

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The image is not very clear but it does show the idea. I can take more detailed pics of the parts if anyone is interested and wants to do something similar. I will say a router (as in "now" not "who") was essential to making up my own idea but I'm sure I could have been less fussy and done it all with plain old hand tools and a bit more time, and other ideas may be simpler.

It still needs tidying up and finishing off properly but its not too ugly to look at, and it works a treat.  I have some black P-section draught excluder coming to stick in place on the top surface and make a seal around the outside of the bottom vents on the amp. It will then sit directly on that and give even more throughput of air. Even at this stage though, you can clearly feel the current of forced air coming out of the top rear vents.  I hope it's not too much, those EL84s are meant to run fairly hot. Now as to the idea of temperature sensing and fan speed control....

I cool my GM36 with 55mm pc fans - they move just enough air so I can comfortably rest my hand on the top - makes a nice hand warmer for my old hands

Some neat ideas here. I love that platform with the built-in fans!!!!

From what I've gathered, the inside of a TM/GM is no hotter than a regular amp. In fact it might actually be cooler than a conventional amp thanks to the metal chassis that acts as a heatsoak. A lot of folks get freaked out when they touch the top of their TM/GM amp and notice how hot it is, but that's just the heatsoak doing it's job. Regardless, I think it's a great idea to use some sort of active air circulation, just for peace of mind. Especially if you use the powersoak which generates extra heat. I'm no engineer but I can't help but think you are extending the life of some of the electrical components by keeping the temp down.

My GM is rackmounted - it certainly isn't crammed in, but still, it's somewhat enclosed, and so I have a 6" clip-on fan mounted inside the rear of the rack, blowing onto the GM. I'm not currently using the GM's powersoak, but I want to at least make an attempt at allowing air circulation. It works, but it's just a temporary solution until I think of something more permanent.

Namklak, can you describe how you're using those 55mm fans? Do you have them mounted somehow?

Some neat ideas here. I love that platform with the built-in fans!!!!

Thank you kindly sir.  It was no big deal, it came about from a desire to stabilise and lock the amp on the top of the speaker stack, then the idea of cooling was added to the MkII when people here put the worm in my brain.

....it might actually be cooler than a conventional amp thanks to the metal chassis that acts as a heatsoak. A lot of folks get freaked out when they touch the top of their TM/GM amp and notice how hot it is, but that's just the heatsoak doing it's job.

Yes, that is a clever if simple and obvious piece of design from H&K.  Why stick to putting your amp in an insulating wooden box just because that was what was used 60 years ago?  It's done all the time elsewhere.

Regardless, I think it's a great idea to use some sort of active air circulation, just for peace of mind. Especially if you use the powersoak which generates extra heat. I'm no engineer but I can't help but think you are extending the life of some of the electrical components by keeping the temp down.

Right on both counts.  The idea was just to offer something simple which would assist the amp in its natural process as you say for peace of mind.  (Mind you, how long before there is a £250 H&K version on offer, maybe with the WiFi MIDI in as well at £450? )  In fact the powersoak is going to dissipate less heat than you would think.  The output of guitar amps is ramped up as high as it can be for the spec sheets by things like quoting RMS values, (the only level playing field so it's quite right to do so).  But music, even guitar music, is much more transient based and rarely fills out the signal to the point where it is as heavy as a constant sine wave just clipping the supplies.  The powersoak is only dissipating a real world fraction of the fraction of power that is left when the currently selected amp output is taken from the 18W quoted, (remember a pair of output valves is turned off with the soak on).

And as you say, some components can last a lot longer if they work in lower temperatures but it must be seen in perspective.  That doesn't mean they can't take heat or it is a highly problematic area, up to 55degC is fine and that's too hot to comfortably touch.  For example electrolytics, (today's incorrectly painted bad boys), are usually specified to 85degC for standard types with high spec at 105degC.  Now an expected lifespan is quoted for running at those temperatures and it is upwards of 1000hrs continuous use for even the cheapest ones.  It doubles for every 10deg below that level the component runs at.  So at our 55deg the standard type is up over 8000hours of continuous use.  That's playing your amp without a break for a year!  And it doesn't mean that the cap dies at that point, usually its value just starts to slip out of spec.  Most capacitors will give 2-4 times that lifespan without a grumble.  Semiconductors, being metallic, will generally work fine at 100degC.  The danger in not keeping temperatures down is that you are getting close to risking hot spots in isolated insulated areas so you are absolutely right, it makes sense to attempt to do something.

My GM is rackmounted - it certainly isn't crammed in, but still, it's somewhat enclosed, and so I have a 6" clip-on fan mounted inside the rear of the rack, blowing onto the GM. I'm not currently using the GM's powersoak, but I want to at least make an attempt at allowing air circulation. It works, but it's just a temporary solution until I think of something more permanent.

There are 1U rack mounting fan units on sale, I have used them in my job and they are good, but you are talking about £30-£50.  Have a look on ebay then make one up yourself!  

Anyone who is worried about fried PCB's need only look at the American built Peavey Classic 30s, many of which have outlasted much more expensive amps, despite using a PCB and having the tubes and transformers mounted underneath the PCB, where the heat rises into.

More of a concern with the TubeMeister is the cheasy power and output transformers, if anything, those will be the components that go first, regardless of what fans are used.

Not sure what you mean about cheesey Txs CSG? I always thought that, for a little 36W job, these were pretty over engineered devices. I think they are a part of the way the amp can drive a range of 8-16ohm impedance rather than uase taps.

I have a suspicion that they may even be too big for the job and be involved in that characteristic slightly shrill quality the H&Ks have. It doesn't lead to good tone to just put in a bigger output Tx. The low volume tone suffers if the core area is too big for the low flux generated, and being close to maxing out in magnetic flux at top power is a part of the rich quality of driven valve amps.