Maybe this is why 300b's dont last as long

[duderduderini]

Hi
I contacted an Australian supplier of 300b's (Shuguan 300b-98's) and asked him about a few things re reliability etc. What he did say that compared to say old production 300b's the new ones dont have thoriated filaments and a) dont last as long and b) arent as emmisive because of this.
He mentioned that thorium kind of killed people in the factorys so I looked it up and well he may have a point there.
Thorium is indeed radioactive and can be used instead of uranium for reactors. It has a half life of 4 billion years or so.
So it seems that thorium may not be used in new tubes and if that is the case they wont emit for as long.
Dont break your tubes when they burn out as inhaling Thorium is not on my list of things to do.
It also begs the question... Tubes must give off radiation.
Hazard suit anyone?
Nick

[SpreadSpectrum]

Tubes must give off radiation.

That's how they cool themselves.

new ones dont have thoriated filaments

Are they oxide coated, then?

[duderduderini]

He didnt tell me what the new filaments are made of but one must assume they are oxide coated. With what I wonder?

[overdrajv]

they could be just tungsten without thorium. thorium is added for emitting electrons at lower temperatures.

[OzMikeH]

You can still buy thoriated tungsten TIG welding electrodes.
The main thing is to use dust extraction when grinding them to a point.

If that is the case it might still be used in filaments. I expect you have to pay a lot extra.

[Miles Prower]

Originally posted by duderduderini
I contacted an Australian supplier of 300b's (Shuguan 300b-98's) and asked him about a few things re reliability etc. What he did say that compared to say old production 300b's the new ones dont have thoriated filaments and a) dont last as long and b) arent as emmisive because of this.
He mentioned that thorium kind of killed people in the factorys so I looked it up and well he may have a point there.

He's putting one over on you. Thorium isn't all that dangerous, and the fact is that they still use it to make the W/Th filaments for new production 845s, 813s, and other high voltage, DH RF types. As far as I can tell, 300Bs have always used oxide filaments since the type isn't rated for 750V+.

Thorium is indeed radioactive and can be used instead of uranium for reactors. It has a half life of 4 billion years or so.
So it seems that thorium may not be used in new tubes and if that is the case they wont emit for as long.

More B.S. W/Th filaments are still used for new production 845s, 813s, 811As, and probably other types as well. So far as I can tell, 300Bs always used oxide filaments since they aren't rated to operate at voltages of 750V+, where you definitely need the W/Th since oxide can't stand up to that.

It also begs the question... Tubes must give off radiation.
Hazard suit anyone?
Nick

Keep the operating potentials below 10KV, and you don't have to worry about it.

Whoever that supplier is he's putting a story on you, probably to unload inferior products so's you won't raise a fuss with him. Don't trust him, find another supplier, or, better still, a design that avoids the audiophool expen$ive boutique VTs.

[Tweeker]

Pure tungsten filaments run very hot, if they had them the tube would be like a light bulb.

There are some, but it is usually for tubes that run at above 15,000V.

OSHA doesnt have a cow if I TIG weld with thoriated tungsten rods, and thats with a naked fuming arc, rather than sealed in a glass bulb