Heat build up in stainless steel vs. aluminum chassis - diyAudio
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Old 17th October 2013, 08:38 PM   #1
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Default Heat build up in stainless steel vs. aluminum chassis

Hey, so I built a combo amp with a 0.060" thick aluminum Princeton 5F2A styled chassis (controls up top, power tubes hanging underneath). It's a single ended 6L6 doing about 15 watts, which is pretty close to the old Deluxes that also have the tubes hanging upside down and controls up top. The thing I'm noticing is that the chassis gets warm pretty quickly. The tubes are all running well within their max dissipation limits and of course the 6L6 and GZ34 put out a bunch of heat but I was surprised by how quickly the heat transfers through the somewhat thick chassis and up to the pots/controls (I'm using aluminum knobs). Talking like less than 15 minutes or so and you can physically feel the heat up top on the chassis. Fender never ventilated or used a fan in any of their combos like this and as far as I can tell is the only real difference is they used stainless steel chassis. Stainless steel is significantly less thermally conductive, so is that all that's going on here? The stainless steel chassis just take that much longer to heat up? Or do those old amp chassis get hot as well but go somewhat unnoticed because of the plastic chicken head knobs? It's not excessive heat (it isn't painful to touch) which means all of the components inside are operating within their specified temperature range, I guess I just don't really like it and wondering if switching to a stainless chassis might help. Thanks for any input.
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Old 17th October 2013, 08:48 PM   #2
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Quote:
Originally Posted by howe0168 View Post
Stainless steel is significantly less thermally conductive, so is that all that's going on here? The stainless steel chassis just take that much longer to heat up?
That'd be my guess. Stainless steel is a terrible thermal conductor.
You could try putting an aluminum heatsink near the heat source to capture and dissipate much of the heat before it flowed to the pots/controls. It may not solve the issue completely, but it should extend the time significantly over the 15 minutes currently experienced.
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Old 17th October 2013, 09:05 PM   #3
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Cool, thank you.
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Old 17th October 2013, 09:18 PM   #4
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Mesa Boogie use a fan for cooling in a few of their combo's. The fan blows on the valves to direct the heat source away from the chassis. The Vox AC30 gets very hot!
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Old 17th October 2013, 09:36 PM   #5
Pemo is offline Pemo  Mexico
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304 Stainless do not transfer heat (almost)
430 Stainless transfer heat (a lot), not as copper or Al.

Use always 304 for chassis, it is also no-magnetic.
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Old 18th October 2013, 02:12 PM   #6
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Very good point - thank you.
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Old 18th October 2013, 04:12 PM   #7
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AFAIK Fender never used stainless steel on any chassis. The only Fender SE 6L6 amp I can think of was something called the Super Champ from the Rivera years. Almost all of the Tweed era amps with 6L6s were designed around the old 6L6 {nothing}, 6L6G, 6L6GA and 6L6GB, not the higher powered 6L6GC. Weber now uses stainless on the chassis they sell because there were problems with consistant chrome plating. Mission Amps also used stainless for a time.

I built a PP 6L6GC amp on a 5E3 Deluxe chassis with an upgraded power transformer. It ran hot even with a solid state rectifier. I eventually changed out the power transformer for an even bigger one you can still touch after an hour. Fender never pushed power to the high levels common today in those old tweed amps. They ran warm, but not excessively so.
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Old 18th October 2013, 04:21 PM   #8
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Quote:
Originally Posted by Loudthud View Post
AFAIK Fender never used stainless steel on any chassis.
chrome(plated) soft steel

btw, you can use the 'trapped' heat for convection cooling of the tubes
commonly seen as a circle of small holes around each tube
when the air 'escapes' through the holes they create air flow removing heat from the tubes faster
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Old 18th October 2013, 04:26 PM   #9
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Many years ago, amplifier manufacturers copper-clad their steel chassis for improved heat transfer. Back then, they didn't use aluminum because you couldn't solder to aluminum. It was also weaker than steel (it couldn't support the heavy transformers).
Many manufacturers had included steel ground tabs which were punched from the chassis and bent upwards. They used these tabs as chassis ground points.
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Old 18th October 2013, 05:33 PM   #10
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Interesting point about the trapped heat idea. I was actually thinking about doing that too but then wouldn't the heat just go up inside the chassis? Or are there also vents within the chassis to dissipate that heat?
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