A cooler GM, no fabricating

We have discussed at length the idea of heat in our beloved amplifier and whether or not it`s a "safe and durable" operating range, and pretty well agreed that H&K knew what they were doing. And as there are a few electronic minded folks on here that know a thing or two about the mechanics of an electronic device, and we can all agree that reductions in operating temperature, even if not outside of "projected standard operating conditions" will increase the life expectancy of any electronic device.

I wanted a simple way to keep my two GM`s cooler without having to fabricate anything. I went and got two 60mm computer fans (these particular ones are 60x25, are sub 18db noise and flow 18cfm of air). I have an infrared thermometer, and tested with and without fans on with the power soak at 1 watt to generate the most heat.

Without the fans I got a max temp on the cover right over the tubes of about 125 degrees F (which is really nothing in the big picture). With the fans on, the same spot is right about 103 degrees. That`s roughly 18% less heat, a substantial reduction. What is more exciting was the difference in chassis temp. I measured right next to the DI out, and without the fans was at roughly 110 degrees. With the fans it`s down to 98. That means that everything inside the chassis has experienced a 10% drop in operating temperature. And mind you this was with only one fan per amp, and a small one at that. The 50mm fan actually fits the vents better but the 60`s flowed significantly more air than the 50`s that were available.

I got both fans and a power supply for $20 and it required no fabricating or even tools. The fans came wit these little rubber insulator mounts that fit perfectly between the vent slots. I might take one of the fans and move it to the other amp to test what difference two will make, but I`m honestly 100% satisfied with what I`m working with now.

A cheap and very simple solution for those who just want to keep their amp a little cooler for safety`s sake. The folks at H&K were tickled with the setup.

Love it Jeff! I'm the fiddler who can't resist making a "product", you're the pragmatist who goes beyond that and just gets a result with the least effort!

The thing that surprises me is that you can get such significant results with so little additional gear, just a 60mm fan through the existing slots. Is there any way that you can get readings from say the surface of the PCB in the base through the slots underneath or is your IR thermometer too broad in the beam to measure that? And readings for a single amp with both fans fitted to it would be good too.

I`ll do two fans on one amp tomorrow and post up results. As far as the IR goes, it`s a direct ratio distance to width (1 foot away = 1 foot wide beam etc) so I likely can`t get in there with any accuracy. Now if I still had my thermocoupler and needle pointed test rod.....

2nd fan got ten more degrees out of the top, no change in chassis temp. What that tells me is that only so much air can be drawn through the amp with a certain amount of pulling force. I could put a higher pressure fan on it and it might make a difference, but I`m already happy with an average of 13% reduction in operating temp.

Great work Jeff. I have to say that your findings are just what I would have expected. Removing air actively from the topside will mean that the fan is probably moving more air from its small part of the vent grid than the whole grid allows out by natural convection without it. As soon as the fan starts working and slightly reduces the pressure inside the enclosure, air will be drawn back in through the rest of the back panel vent grid as well as what can be drawn up through from the bottom side. That's inefficiency at work.

If you're up for it, here's a counterintuitive thought to test. Try fitting your two fans to the back and block off the rest of the grid with something like tape! That way you prevent any air from sneaking into the top enclosure through the "unfanned" vent area and almost all that is extracted will then have to be drawn through from the bottom. There may be other effects which come in, (like reduced conduction in lower pressure air for example), but that one is part of the picture I'm sure, and my bet is that other effects have less overall significance.

Anyway, just as you say, you have a significant reduction with only a small outlay and no bother in terms of fitting. Result!

one step ahead of you, BB. I taped a file card over the available uncovered vent grids just to see what an increase in internal negative pressure might bring. No measurable change indicates poor volumetric efficiency of the design and a broad level of diminishing returns. The fan is only designed to support a max pressure of X (don`t recall what it is), so I suspect that tape or not it`s pulling as hard as it can. With a max volume of 18cfm, if it were working at 100% efficiency it would exchange the entire volume of the case 6x per minute (give or take, and I bet it`s half that if I`m lucky). If it`s location has it operating at 50% max through the case (being that it`s max pressure only allows X amount of air through the bottom chassis vents), then sealing it all up taps it out at 60% because it can only physically drive that much air without a bigger inlet......we only net a 20% improvement, or roughly 4 more degrees and a harder working fan making more noise.

**whew**.

**edit:

I did only experiment with just one fan on that endeavor. Two would have most certainly made a more significant difference.

Hahahahahaha, you little rascal you!  Hoist by my own petard!  You're a closet engineer aren't you Jeff?  Or maybe a real one from a past life?  That incomparably complete explanation puts the whole issue to bed I think.  Great piece of deductive work and a really useful way of helping the heating thing.






EDIT: Now if you were to just manufacture a manifold which would close couple to the rear face covering all of the rear vents and funnel them back to the fan, then drill some relief holes in the chassis plate between the two chambers........

Cool (all puns intended) - actually objective measurements.

I have two 60mm fans with magnets that I pop on the back, one on each side, as I'm setting up in my practice space at home. With those fans, I can touch the top of the amp. Granted, I don't use the internal power soak, but I "felt" an improvement. I put that together soon after buying this amp... But I had no objective measurements of actual temperature reduction.

I rarely bring my amp to band practice (we practice in a very small space and I just use an old digitech modeler there), so the gm is at home with these two fans most of the time.

My gig cabinet is a gutted Vox combo cab and I set my GM on top of it - the cab has vents on its top that line up right were the bottom vents on the gm are placed - lucky me. I put two 100mm fans inside the cab blowing up into the gm. These are permanently attached and plugged into a power strip also mounted inside the cab, so I don't have to set anything up. With my nominal understanding of cooling, I knew forcing air in to as space is not as effective exhausting air, but it does cool the amp slightly, so that on gigs on cold days I can warm my hand on the gm top plate.

Neither of my fan setups make any appreciable noise, audible or electrical.

Cooling is cool

One thing that should be pointed out here for people coming across this without having the background of previous info......


The maximum temperature that Jeff has mentioned is 125degF which he measured directly above the valves. That is 51degC. That's NOTHING as far as electronic components are concerned.

They are made to work all day every day at temperatures well above that. The worst of the bunch, electrolytics, are usually specified at either 85degC/185degF or 105degC/221degF, you can usually see it on the can. Their expected lifetimes are specified as a number of hours, (usually over 1000 in continuous use), and they behave according to Arrhenius's Law where the quoted lifetime is doubled for every 10degC reduction that they work at. So at 50degC they would have expected lifetimes of over 8X the quoted period. This amounts to many 1000s of hours continuous working. And of course it is much higher if the 43degC/110degF in the lower chamber is true.

Jeff really has shown that the amp is well designed and it has no obvious inherent weakness in temperature terms. Now that said, it won't hurt to reduce that temperature in ways such as we are discussing. But it isn't a deal breaker in terms of the amp being reliable in any way!

Great work guys analyzing this heated subject (pun undeniable).  I use a CPU fan in my server cabinet in the master closet of my house and it does the same thing.  Works like a champ. This helps us in the winter months, giving us a place to warm our hands up before we play.

Great thread Jeff!

Glad you guys found this useful. This particular experiment accomplishes exactly what I wanted: a measurable decrease in operating temp without overcooling the tubes or having a noisy fan broadcasting it`s voice through the amp. I will go on record with the opinion that tubes do not need to be "as hot as possible", simply heat soaked at whatever temperature gets them there to sound their best.

VoodooJeff wrote:tubes do not need to be "as hot as possible", simply heat soaked at whatever temperature gets them there to sound their best.

Amen to that Jeff.  Checking it up shows that, as a rule of thumb a 10% increase in operating temperature of the cathode gives about 3X increase in electron emission!  And just because the valve's external temperature only rises by 10deg doesn't mean that is what the internal temperature will do, it may increase even more.  That's a pretty unstable situation.  Now that would be fine if you knew that the working temperature would be higher at design time and could account for it but when you design for the norm and that then gets changed significantly....  Funny how we always seem to know better than the original designers isn't it?

And I have to say "useful"?  That's not the word Jeff, it's the piece of practically derived info we needed to prove our fiddling to be significant.  Great stuff.  "Kewell bro'" as you youngsters would say!  

This is a 6" clip-on fan which I got on ebay for like $15. My GM is racked and the fan neatly clips onto the top rear rail as you can see...



Truth be told, I've rarely used it, I bought the fan during the Heat Craze of 2015 when we were all freaking out about how the amp was going to go molten I decided to err on the side of caution and pick something up just in case... also factoring my GM sits in a rack and perhaps could use a little extra ventilation... I figure it won't hurt to run it if/when I ever find myself playing a hot summer gig, but for home use and the odd live show I don't use it. Just thought I'd share another fan option for ya's. If I had a therm gun handy I'd do a temp comparison for ya.... maybe I'll pick one up...

Nice setup Gravy! Can you show us what the front looks like?

Rarely used it but piece of mind is good insurance. Works for me and is cheap to fix.

gravydb wrote:This is a 6" clip-on fan which I got on ebay for like $15. My GM is racked and the fan neatly clips onto the top rear rail as you can see...

Nice! I may have that exact same fan. It worked wonders until I started recording with a mic instead of direct (and clips onto a mic stand very well). Of course now my cab is in another room and even the AC is blocked to it so the only errant noise I get is when there is a crew of landscapers or roofers in the neighborhood LOL.

Ok here's a full frontal



This is the simplest rig I've ever run, thanks to the GM's built-in FX. At the top of the rack is a 1U power conditioner, and then a Korg Pitchblack tuner which sits right before the GM in the signal chain. Not shown is a Boss FV30L low impedance volume pedal which sits in the amp's fx loop, and a FSM-432 foot switch. That's all there is to it! Oh the cabinet, it's a H&K CC212 loaded with stock greenbacks YEAH BABY!!!

Nice setup!

K.I.S.S. works for me always!  That's why I love my GM36 so much.  Thanks for sharing the frontal!

Not to totally derail this thread... but here's a couple shots taken at a recent live show...



Mr Bill and I have a hand-signal system worked out but lately he just makes any needed adjustments for me on the fly...

Jeff,
Did you do the fans as intake or exhaust?
And with the 2 fan setup were they both intake, both exhaust or one intake one exhaust?

Depending on your chosen setup, (fans on the top back or fans underneath), you should always be trying to draw cooler air in through the slots in the base over the circuit boards then up into the upper chamber over the valves before pushing the hotter air out of the top back. It's not the valves you are trying to cool it's the other components on the boards. If you do the reverse you are simply heating the air up over the valves before pushing hotter air down into the lower chamber with the circuitry. So in this case they really should be set up for exhaust. If you choose the underneath route they should be blowing.

kherman wrote:Jeff,
Did you do the fans as intake or exhaust?
And with the 2 fan setup were they both intake, both exhaust or one intake one exhaust?

Exhaust all the way. The idea is not so much to cool the tubes, rather the internals of the amp as BB said. And it`s really just a precaution. My laptop runs several degrees hotter than the amp does with no fans on it and we`ve all pretty much established that the amp is well within it`s operating range of temps. The fans just keep it cooler to extend the life expectancy of the internals in a device we`re not likely to see the end of anyway.