forced aircooling

[makeb4break]

hi there

a thought that's been on my mind for some time. how much would you be able to push a tube with some serious aircooling? say, a 300B with some massive noisy fans (i can put my amps in another room, or even outside), would you be able to push it to 50-60W? more?

i know it's common with big **** transmitter tubes etc ..

cheers,

mb4b

[nhuwar]

You could try and push it a little but not to much, maybe a few extra watts.

Nick

[Richard Ellis]

No not really .JJs' 'hot' 300B is rated at 40 watts.
You are assuming there is a direct connection between heat generated and power, there really is none. Seen the insides of a tube? Lotsa fine delicate stuff inside, easy to make fireworks with.
______________________________________-Rick...........

[nhuwar]

I agree will Richard. It's been common practice for ham operator to push tubes but it's always a gamble. And that is with old quality made tubes, most new stuff I doubt could handles this. But when it all comes down to it, it's your call, just remember most fac glass tubes have a chimney and ducting for the air to be routed around the pins.


As you push a tube harder past it's rating you will start to lose the vacuum.

Nick

[m6tt]

Air cooling in glass envelope tubes only allows the envelope temperature to be kept low. The plate radiates heat, and so it doesn't matter what you blow at it, it's in a vacuum. That being said, it might allow you to push it in tubes where the bulb will fail before the plate starts outgassing bad. The transmitter tubes that are forced air cooled have *external anodes*, which means some of the plate structure is out "here" in plain air. This allows the direct cooling of the plate with convective means, but also means you have a big piece of high conductivity metal with a couple thousand volts on it hanging out nearby.

[nhuwar]

Originally posted by m6tt
Air cooling in glass envelope tubes only allows the envelope temperature to be kept low. The plate radiates heat, and so it doesn't matter what you blow at it, it's in a vacuum. That being said, it might allow you to push it in tubes where the bulb will fail before the plate starts outgassing bad. The transmitter tubes that are forced air cooled have *external anodes*, which means some of the plate structure is out "here" in plain air. This allows the direct cooling of the plate with convective means, but also means you have a big piece of high conductivity metal with a couple thousand volts on it hanging out nearby.

I don't see that if you pushed a tube's dissipation by a few watts would it cause the envelope to fail, I don't think it wouldn't happen. If you kept the seals cool it would be fine besides damage done to the internals, ie plate outgassing. Of course the other problem would be the grid dissipation.

Ask George From Tubelab and he will be able to give much more info on the sudject then I can though.

An example I have pushed a 3c24 to a dissipation of 100 watts and all that happened was the getter material on the anode started to sublimate of and condense on the inside of the glass envelope.

And believe it or not the 3c24 is still functional
Nick

[m6tt]

I didn't mean to imply that the seals or envelope are going to blow if you push the current/voltage a bit . I just imagine that aircooling a glass tube would really only affect the envelope and not the anode so much...maybe you could mount the fan under the tube to cool the seals & maybe the plate a little through the pin. Probably wouldn't give you much.

I guess my example referred to tubes like the 6aq5, where I have a feeling the maximums are derated from 6v6 for reasons other than cathode size or arc-over, but because of bulb temp. It's just a hunch, maybe tubelab will blow some up someday and we'll see what the failure mode is

[makeb4break]

thanx gentlemen, interesting stuff

[Miles Prower]

Originally posted by m6tt
I guess my example referred to tubes like the 6aq5, where I have a feeling the maximums are derated from 6v6 for reasons other than cathode size or arc-over, but because of bulb temp.

It is envelope temp, since the 6AQ5's wrap around anode is quite close to the glass.

It's just a hunch, maybe tubelab will blow some up someday and we'll see what the failure mode is

He already did that, getting a 6AQ5 to put out some 30W. The glass melted.

[wakibaki]

Valves operate by virtue of a temperature differential.

If the anode were as hot as the cathode then all other things being equal, their emissions would be equal.

The anode (in a glass valve) loses heat by radiation, and to a degree by conduction in some cases.

If the envelope is hot, it radiates heat back to the anode.

Not only does reduced envelope temperature reduce noise, it also improves valve life. A corollary of this is that it permits operation at higher anode dissipations. The problem is, how much?

You can get push-on valve heatsinks, you know.

w

Don't do it. Accept the improvement in valve life. If you want more power, build bigger.