300b tube Temperature?

[ddsbuilder]

Currently building a child protector for a friends amp, can someone tell me the approximate temperature the 300b tubes get? The amp is apart right now.
warm?
Hot?
very hot?
approximate Temperature?
Thanks

[kevinkr]

Hard to estimate without knowing things like plate dissipation, but they do get very hot - as in contact will result in burns, hot.. Anything you do should not prevent adequate ventilation..

My 300B which run at 400V/80mA or 32W Pdiss + 6W(fils) for 38W total are 109 degrees C (228 degrees F) on the shoulder at this very moment, room is 26 degrees C.. More than hot enough to cause a serious burn.

[ddsbuilder]

Just what I need to know, thanks much

[ChrisA]

Originally Posted by ddsbuilder

Currently building a child protector for a friends amp, can someone tell me the approximate temperature the 300b tubes get? The amp is apart right now.
warm?
Hot?
very hot?
approximate Temperature?
Thanks

Why is it that today, amateur HiFi builders rarely bother to build enclosures? I guess it's just extra work that does nothing for the sound. Back in the tube era, even when big companies built death-trap hot chassis tube radios no one even thought about leaving the tubes exposed. Even in the 1930's it was such an obvious safely issue.

And this thing seems to be mainly in the HiFi hobby. Ham radio builders and certainly people who build musical instrument amps and studio equipment mostly do build enclosures.

With audio HiFi gear in the 60's people used at least perforated metal cages but at some point it became fashionable to omit the cage. Does anyone remember when that was? Maybe in the 80's??? I don't know.

To answer the question about temperature. What matters as much is the total heat. Heat and temperature are not the same thing. The simplest encluse that makes the amp reasonably safe is a 1930's style wood cabinet. Just a five sided box withthe back open.

Remember that tubes cool by _radiation_ (the hot parts are in vacuum after all) so to keep them cool leave a path for that. There should be a lot of space between the tube and the first thing the radiation will hit or that thing it hits will get quite hot. I'd say about 2.5 tube diameters at least. the "inverse square law" works for you that way

Cooling tubes in counter intuitive. Even if you place the tube in water (people have tried this) it does not cool the hot parts of the tube. Again, these are _vacuum_ tubes.

[kevinkr]

Depends on who made the project - I've seen a lot of diy from the 1950s and most of it does not have cages. Many but not all kits did, but I suppose hobbyists even then found the cages quite expensive.. I use them only when I need electro-static shielding, (or when they might be used outside of my lab/listening space) we have no kids, and the listening room is a pet free zone. The Hammond covers are pretty expensive and come in limited sizes which is another disincentive.

[Adipocere]

Throughout history a large percentage of tube amplifiers, especially homebrew, have had no protective cover. In fact, they didn't even use to insulate the wiring. (Photo from uv202.com)


Hams often don't build fancy cover either. I know I don't - why waste the money?

[rknize]

Yes, I really do wish there were some better solutions here. There are the new production Dynaco ones, if they happen to fir your application. The Hammond cages are decent (they look a lot like the Dynaco), but like you said...the size selection is really limited and they are quite pricey. I've tried to use other things, like those steel mesh organizers that you can get at places like Target. But again, you are at the mercy of what you can find and they often have handles or are in weird shapes. Plus, that steel mesh looks seems to be going out of style now.

[Corvus corax]

The bulb temperature of 300B is typically well over 400 degrees F. Hot enough to melt most plastics, and cause wicked skin burns faster than you can pull away during an accidental contact. The center of the glass envelope is the hottest part, where the radiation from the plate passes through the glass, followed by the top of the envelope.

[Pano]

Originally Posted by Adipocere

I know I don't - why waste the money?

Your answer is in the first sentence of the first post.