Chassis ventilation - Worth the effort?

[Gaz]

I'm planning a 4 x KT88 Hiwatt DR201 style build, and was wondering if it'd be wise to add some ventilation to the top of the chassis around the tubes. Here's a link to a picture of an original Hiwatt slave unit with such ventilation:

Hiwatt Internals

I really don't understand the logic of the venting to be honest. Firstly, I don't know why having a bigger output section would cause the inside of the chassis to get hotter, compared to say a 100W amp without the venting.

And I get how hot air should exit through the vents, but it's unclear to me how efficient this would be seeing that there is no way for air to actually flow through the chassis like there'd be if there was additional venting on the bottom or sides of the chassis.

Is it totally pointless to adding venting like that? Would it be more worthwhile to add a fan?

Thanks!

[Jazz P Bass]

Personally, I think vents on the tops of amps are dumb.
As opposed to a fan, that is.
Why would you make a way for stuff to get in the amp.
Opps, I spilled my (add favorite beverage here)

[tedmich]

the tubes are what get hot, so you only need active cooling if you go all rackmount like some of John Chamber's builds
Champ 3HMV 300 Watt Power Amplifier Unit

everything does last longer if it stays cool but beer and/or greasy dust are a pain on the internals.

[Gaz]

Thanks for the replies. Jazz P Bass, did you mean the vents in the picture or vents on top of the head cab? I'm guessing the former, but just wanted to check.

[exclamationmark]

If it's anything like my PC, fans will tend to accelerate the build up of dust, which increases the chance of arc over in anything non-ptp.

For non-rackmount builds, most all the heat generating components (tubes, transformers) are situated outside the chassis anyway. The chassis will still get hot, and so along with anything inside it, but usually this isn't of concern. It probably has more to do with the fact the air is stagnating inside a giant heatsink (ie, the chassis), than the actual heat generated. I guess in the right circumstances, slots in the chassis would allow convection to work it's magic without the drawback of dust accumulation from fans.

Though in saying this... some part of me just wants to dip everything in epoxy and add about 10 fans to my builds

[d95err]

Some things inside the chassis will generate heat, such as the cathode resistor in cathode biased amps. The first dropping resistor in the power supply (if a choke is not used) will also generate heat. Some of the heat from tubes and transformers will spread into chassis as well, adding to the ambient temperature. The problem is worse in combo amps with the tubes hanging down.

[Gaz]

Yeah, I agree with everything said so far. Seems like the chassis slots around the power tubes may just be there to look nice. I know that Morgan Jones mentions chassis venting quite a bit in his "Building Valve Amplifiers" book, but never mentions top vents alone. Thanks again for the replies.

[Jazz P Bass]

Marshall has had vents forever, no.

[Gaz]

Marshall has had vents forever, no.

Did you look at the pic in the original link? I'm talking about vents in the chassis, not in the cabinet!

[Jazz P Bass]

No I did not.
Now I did.
Oops.
Seeing that the idea never caught on, I would chalk it up to an engineers whim.
It would certainly help 'move' air up over the tubes.
Whether or not it is worth the effort......

[soundguruman]

The cooler the parts run, the longer the amp will last. The airflow is produced by convection.
There are, these days, resistors and capacitors that are rated 105C, compared to the old 85C parts of yesteryear.
For every 10C that you can reduce the temperature, you have increased the life span by double.

[Gaz]

Haha, you must have thought I was pretty dumb, asking about whether or not to ventilate the head cab!

[Gaz]

I understand that 'heat is the enemy of electronics', but what do you think about the Hiwatt method, soundguruman?

[soundguruman]

I think that what somebody has been trying to say is that:
'hiwatt engineers got it wrong'
Now let's see this same person design an amp that lasts even half the time. Good luck.
You think you know better than the person who originally designed it? That person was pretty smart, it seems.

[Gaz]

I can't figure out if you're trying to be a dick or be helpful