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.
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.
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.
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.
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
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.
Many manufacturers had included steel ground tabs which were punched from the chassis and bent upwards. They used these tabs as chassis ground points.
yes, ofcourse
need to take in fresh air to work
anywhere its convenient
sides, back, or bottom
I'm not entirely sure that I can contradict that statement but I have had a dickens of a time working some of the chassis they used in the 60-70's... it may not have been stainless but it was incredibly tough stuff!!
I'm a great fan of medium cost and workability materials such as zinc plated mild steel... if you get your earthing strategy right you needn't worry about eddy currents (I haven't for a long time - I'm willing to share that info)... 1.5mm is a good thickness/strength/weight ratio... and it's cheap. If I need to mod it it's easy enough to drill or punch... it's a no brainer for me
After punching my own first 5 or so aluminum chassis I decided that I'm never doing that again! I couldn't imagine having to drill/punch a stainless chassis - I currently get mine done on CNC so workability isn't really an issue for me. I'm also getting them powder coated and silk screened at a 1.5mm thickness and they look awesome. And thanks but I've got a pretty decent earthing layout going - I'm just following the valve wizard's recommendations and it's pretty much dead silent.
I think the aluminum is doing your tubes good, but not your hands. Guitar amps are known for burning up output tubes versus hifi's or organs. The melting point of plate steel is constant, and if the rest of the tube is really hot, the tube red plates quicker and burns holes in the plate.
You need to punch some holes in the side somewhere of your aluminum chassis and insert a baffle slanted to make the hot air waft out the back or side vent, away from the speaker and controls. Standup feet and holes in the bottom would also increase air flow. Don't forget to put steel or aluminum screen in the holes, to keep out spiders, mice, and RF emissions from lamps and radios (cell phones).
You need to punch some holes in the side somewhere of your aluminum chassis and insert a baffle slanted to make the hot air waft out the back or side vent, away from the speaker and controls. Standup feet and holes in the bottom would also increase air flow. Don't forget to put steel or aluminum screen in the holes, to keep out spiders, mice, and RF emissions from lamps and radios (cell phones).
I was merely going to offer some stuff I've learned from others... I've never turned down any help or advice in the business of building amps and have progressed in it fairly well because of all the folks who have shared their knowledge... you may have your grounding scheme nailed but someone else might learn from our discussion?
And to keep this on thread - transformer orientation, isolation and location play a massive part in heat dissipation... It's easy to assume that bolting a through chassis PT good and tight to the chassis is a good idea... but it can be more advantageous to isolate it with something like phenolic bonded paper tubing (Tufnol, Paxolite) which if cut to the right length to alter the axial alignment of the windings/laminations can actually reduce the interaction of stray magnetic fields between the power and output transformers -not to mention reduce thermal coupling between the PT and chassis and creating an airgap for further ventilation...
Oh, sorry - I definitely appreciate the help. Basically my grounding scheme is grounding all the cathode grounds/bleed resistors to their associated power cap ground, connecting the power cap grounds together via a bus wire and then making one ground connection next to the input jack. The PT's center tap is connected to the reservoir cap and it seems to be working well so far - if you've got any suggestions or recommendations I'm all ears (eyes).
And that's an interesting point - I hadn't really given any thought to the transformers yet since the power tube's heat dissipation seemed significantly higher than the transformer. When I've been messing with the circuitry and had the chassis out of the enclosure (tubes lying horizontally) the chassis doesn't heat up at all so I assumed the majority of the heat dissipation was that hot air rising from the tube and hitting that flat bottom surface. The OT doesn't seem to warm up at all but putting something between the PT mounting surface and chassis could definitely be worth checking into.
And that's an interesting point - I hadn't really given any thought to the transformers yet since the power tube's heat dissipation seemed significantly higher than the transformer. When I've been messing with the circuitry and had the chassis out of the enclosure (tubes lying horizontally) the chassis doesn't heat up at all so I assumed the majority of the heat dissipation was that hot air rising from the tube and hitting that flat bottom surface. The OT doesn't seem to warm up at all but putting something between the PT mounting surface and chassis could definitely be worth checking into.
Yeah, Tinitus...the holes around the tubes are for a right-side-up chassis config; he said this is Fender-style with the tubes hanging down from a chassis at the top of a combo.
But as far as I know most tube combo amps have an open-back cabinet that's huge compared to a "head" cabinet, so I don't know what you mean by "Fender never ventilated ...any of their combos like this" (upside-down tubes with control pots at top). I think maybe the Fender Studio Bass combo with 6 6L6's had a sealed bottom but still a ventilated head, but otherwise ALL Fender combos had large open back cabinets with excellent chimney-like characteristics that let the air in near the bottom of the cabinet and out near the top so they cooled better than a "head".
Your beefed-up single-ended amp will be much less efficient than the push/pull Deluxe you were comparing it to though the push/pull Deluxe would have twice as many tubes. It will make much more heat! You really need a comprehensive plan to manage the heat! Yours will also make more heat from just one tube even at idle, so for long life your one tube needs better ventilation. You output transformer also has to dissipate a bunch of DC.
For longest life you really can't have a sealed cabinet, and you need air inlet near the bottom and outlet near the top, and preferably you should be able to "see" the output tubes when looking at the amp from behind; that means that the tubes will radiate any higher frequency heat (which behaves like light) directly from the plate thru the glass and right out the back into the room until it hits a wall or something.
It really helps if the bottom of the chassis (which the tubes hang from) is angled up toward the back so that rising hot air moves out the back instead of being trapped.
You might use a trick from the Sound City & HIWATT amps: a shiny metal reflector behind the power tubes to further direct radiated heat out the back.
You can also do like many Vox amps and ventilate the inside of the chassis with small vents in the top of the cabinet. In any upside-down fender-style tube amp not only does the heat rise to the chassis, the chassis can become like an oven and cook anything inside. If you do NOT do like Tinitus says and instead seal the chassis bottom, and have some ventilation holes in the sides of the cabinet (with some creative grille or screen) and additional ventilation grates on the top it will do a lot to both make the air inside the chassis cooler and cool the chassis itself measurably.
Also, an aluminum face panel can help radiate significant amounts of heat, so choose your panel faceplate material carefully. And the rear of the chassis (where you might have fuses and power cord entry, speaker jack, etc,) also should radiate directly to the air outside the back.
Long story short: Fender did only a marginal job of heat management in their upside-down tube amps, and you will need to dissipate more heat than the Deluxe you mention. Having trapped air inside the chassis, exposure to the rising hot air from below, and wood insulation on top and sides, changing the chassis material to a metal that conducts worse will not reduce the chasiss temp when it reaches balance, changing to stainless will only change how long it takes to get hot (reach stasis balance of heat generation and radiation) and make the bottom panel eventually get even hotter because it doesn't conduct to the front and back as well so the front and back don't get to help radiate as much. So stainless may make it take a half-hour to get hot instead of 15 minutes, but it might eventually get even hotter than aluminum would. Any single-ended class-A design should start with heat management, and using more powerful tubes or parallel single-ended means you've got to dissipate a LOT of heat.
I don't like fans for a single-ended, I only like fans for really loud amps.
I generally resort to a large open head cabinet instead of a combo, but usually the reason I'm making a beefed-up single-ended amp is so it can push a sealed speaker cabinet and I won't put a tube amp in a sealed cabinet. Then, on the "head" I put on tall rubber feet, large openings with grates in the bottom, vents in the sides, open airflow thru a grille cloth front, a opening in the bottom of the back, a steel grate in the back by the tubes, and an opening near the top by the chassis, and vents into the sides of the chassis, and vents out the top to cool the inside of the chassis.
Good luck. Let us know what you come up with.
But as far as I know most tube combo amps have an open-back cabinet that's huge compared to a "head" cabinet, so I don't know what you mean by "Fender never ventilated ...any of their combos like this" (upside-down tubes with control pots at top). I think maybe the Fender Studio Bass combo with 6 6L6's had a sealed bottom but still a ventilated head, but otherwise ALL Fender combos had large open back cabinets with excellent chimney-like characteristics that let the air in near the bottom of the cabinet and out near the top so they cooled better than a "head".
Your beefed-up single-ended amp will be much less efficient than the push/pull Deluxe you were comparing it to though the push/pull Deluxe would have twice as many tubes. It will make much more heat! You really need a comprehensive plan to manage the heat! Yours will also make more heat from just one tube even at idle, so for long life your one tube needs better ventilation. You output transformer also has to dissipate a bunch of DC.
For longest life you really can't have a sealed cabinet, and you need air inlet near the bottom and outlet near the top, and preferably you should be able to "see" the output tubes when looking at the amp from behind; that means that the tubes will radiate any higher frequency heat (which behaves like light) directly from the plate thru the glass and right out the back into the room until it hits a wall or something.
It really helps if the bottom of the chassis (which the tubes hang from) is angled up toward the back so that rising hot air moves out the back instead of being trapped.
You might use a trick from the Sound City & HIWATT amps: a shiny metal reflector behind the power tubes to further direct radiated heat out the back.
You can also do like many Vox amps and ventilate the inside of the chassis with small vents in the top of the cabinet. In any upside-down fender-style tube amp not only does the heat rise to the chassis, the chassis can become like an oven and cook anything inside. If you do NOT do like Tinitus says and instead seal the chassis bottom, and have some ventilation holes in the sides of the cabinet (with some creative grille or screen) and additional ventilation grates on the top it will do a lot to both make the air inside the chassis cooler and cool the chassis itself measurably.
Also, an aluminum face panel can help radiate significant amounts of heat, so choose your panel faceplate material carefully. And the rear of the chassis (where you might have fuses and power cord entry, speaker jack, etc,) also should radiate directly to the air outside the back.
Long story short: Fender did only a marginal job of heat management in their upside-down tube amps, and you will need to dissipate more heat than the Deluxe you mention. Having trapped air inside the chassis, exposure to the rising hot air from below, and wood insulation on top and sides, changing the chassis material to a metal that conducts worse will not reduce the chasiss temp when it reaches balance, changing to stainless will only change how long it takes to get hot (reach stasis balance of heat generation and radiation) and make the bottom panel eventually get even hotter because it doesn't conduct to the front and back as well so the front and back don't get to help radiate as much. So stainless may make it take a half-hour to get hot instead of 15 minutes, but it might eventually get even hotter than aluminum would. Any single-ended class-A design should start with heat management, and using more powerful tubes or parallel single-ended means you've got to dissipate a LOT of heat.
I don't like fans for a single-ended, I only like fans for really loud amps.
I generally resort to a large open head cabinet instead of a combo, but usually the reason I'm making a beefed-up single-ended amp is so it can push a sealed speaker cabinet and I won't put a tube amp in a sealed cabinet. Then, on the "head" I put on tall rubber feet, large openings with grates in the bottom, vents in the sides, open airflow thru a grille cloth front, a opening in the bottom of the back, a steel grate in the back by the tubes, and an opening near the top by the chassis, and vents into the sides of the chassis, and vents out the top to cool the inside of the chassis.
Good luck. Let us know what you come up with.
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