Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alerich on April 21, 2013, 11:31:04 am
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I built a JCM 800 50 watt clone in a practice amp combo chassis: http://www.el34world.com/Forum/index.php?topic=15286.0 (http://www.el34world.com/Forum/index.php?topic=15286.0)
It's a cozy fit. Two EL34 bottles, three 12AX7 bottles and 50 watt iron in a tight space. Being a combo the chassis hangs upside down so all of the heat rises. After 5-10 minutes of idle time the chassis around the power and preamp tubes is so hot you cannot touch it for long. Even more so after playing it. I have what is essentially the same amplifier in my modded Sovtek Mig60 head but the parts are more spread out on the chassis and the chassis is below the tubes so the heat rises away. Even after hours of hard playing the chassis of the Sovtek gets barely warm.
I became concerned about the heat in the JCM 800 clone. My instinct just tells me that all that heat cannot be good for the caps and resistors and sockets and all that. I fitted a 12 VDC computer fan in the bottom of the cabinet. It doesn't blow on the tubes - it blows across the bottom of the cabinet but moves enough air around that the chassis now stays comfortable to touch. It's a quiet little fan. You can hear it but it's not objectionably loud.
Exhibit #2. I have a Goodsell Super 17 combo built from a Hammond AO-35 chassis and iron. Two EL-84 tubes, two 12AX7 and a 5Y3 rectifier. Another tight fit. Hot cathode bias and the chassis hangs upside down. It gets nearly as hot as the JCM 800 clone did before the addition of the fan. The PT however doesn't get overly warm. Before I bought the amp I contacted the builder Richard Goodsell and he assured me that this was the nature of those amps and that this amp had been running for seven years at that point with no issues (and 45-50 years prior in a Hammond organ albeit with the chassis mounted upright). He said he would continue to warrant the amp and told me to buy with confidence which I did. He's a good guy to deal with. Be that as it may, I am considering also installing one of these fans in my Goodsell given my instincts noted above.
What are your thoughts? I know Fender built like a trillion amps with the chassis inverted and the tube heat rising up without significant problems but I don't recall any of my Fender amps really getting too hot to touch. Maybe they did and my memory is failing me. Is a really hot chassis all that bad of an environment for passive electronic components? What about the tubes themselves? Do they benefit from dissipating this peripheral heat? I haven't really had a chance to A/B the JCM 800 clone fan versus no fan to detect tone differences. It was late last night when I buttoned it back up and I like to play it loud. Am I just over-thinking this?
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I know Fender built like a trillion amps with the chassis inverted and the tube heat rising up without significant problems but I don't recall any of my Fender amps really getting too hot to touch.
Marshall did too, in the JCM800 combo.
But the key is your amp seems to be a very small footprint compared to these other amps. Bigg chassis area + open-backed combo cab = room for air to circulate, even if the tubes-down position isn't ideal.
To answer your question though, yes, if the chassis is too hot to touch, it's too hot and needs the fan. Heat that's not moved out can reduce the effective dissipation ratings of the tubes. In other words, they're trying to lose heat from the plate, but if that's not radiating to the air and being moved from the tube envelope by convection of the air, the tube tends to overheat.
When it's 100 outside, doesn't it feel less-hot when you're driving down the road with the windows open (pretend it's 1950 and your car has no a.c.)?
Resistors and caps get cooked to, causing them to drift or shortening their life. It may take 10 years to observe the effects of this. Or maybe less time. Either way, heat's not great for the passive components.
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Use good caps, add a coffee-cup holder.
I knew some Fishers run 24/7 too hot to touch. No problem.
OTOH fans tend to be problems. Sound, dust, and if you actually *need* one, it always cuts-out on the hottest evening just after Radio Shack closes.
"Excess space" is always good. For cooling and for repairs/mods. Too-small always becomes a problem down the road.
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add a coffee-cup holder.
is this a joke or a real technique of some kind?
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Reminds me of the first time someone suggested using a Hot Plate to tame my amp a bit. :l2:
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add a coffee-cup holder.
is this a joke or a real technique of some kind?
It helps keep your coffee warm! :icon_biggrin:
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oh, I get it - amp's so hot you can keep your coffee warm on it - you just need a holder for the cup.