Bright tube heater glow when applying power?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

tk@halmi

Well-known member
Joined
Jun 3, 2004
Messages
999
Location
Oregon, USA
I got a box of used tubes, mostly compact triodes like 12BH7. When I plug them into the tube tester some of them (10%) emit a bright glow for a second or so. Most of the tubes don't do that though. As I understand the filament has very low resistance when cold so it will pass whatever current is available until it heats up a little. What makes the majority not glow?

Thanks,
Tamas
 
This is because the resistance of a cold heater is much lower than when at temperature. Hence, the inrush current is really high. Basically, you get a higher equivalent power delivery for a moment.

The solution is to drive the tube with a current limited heater supply. Or go current mode instead of voltage.

jh
 
Jim,

Thanks, I was aware of that. The question was why most of them do NOT glow bright at start up.

Tamas
PS: More mistery to me, the tubes are of the same brand.
 
I have some amperex that glow and some others that not glow.
maybe different manufacture, even different fabrication country.

glow not seem to be a problem tubes don´t die, don´t sound worst
and look really cool :grin:
 
[quote author="12afael"]
glow not seem to be a problem tubes don´t die, don´t sound worst
and look really cool :grin:[/quote]

NOt always true. If the glow is blue in color that is bad very bad. It means the vacum inside has a leak and that oxygen is getting inside. If this is the case eventually the tube will die...
 
[quote author="tk@halmi"]
The question was why most of them do NOT glow bright at start up.
[/quote]

This is a theory: In the flashing tubes the filament is not completely covered by the cathode. The part sticking out obviously has a way lower heat capacity than the rest of the filament. When voltage is apllied this part heats up faster than the rest/the resistance rises faster. As the resistance of the rest of the filament is still low/the current is higher than normal, the exposed part gets more power than with a fully heated filament, resulting in the flash.
Sorry, i sound like a idiot writing this in english ;)

;Matthias
 
I am pretty sure that tubes that glow very brightly at turn-on are designed to reach their operating temperature faster and hence form a space cloud around the cathode more quickly. This was probably a military innovation. I have also seen this 'feature' in a 12AX7. They are probably OK. Also, I am sure the blue glow as seen in vacuum tubes is due to the rare gas Argon being ionised; not oxygen. It is usually not a good thing but there will always be some gas in a tube since it leaks between the metal pins and the glass envelope due to the difference in the thermal expansion ratio of these materials.
 
> filament is not completely covered by the cathode. The part sticking out obviously has a way lower heat capacity

Something like that. It can also be more/less insulating ceramic around the heater, or heater wire diameter, many things.

> designed to reach their operating temperature faster and hence form a space cloud around the cathode more quickly.

I doubt that, because I never saw a heater/cathode tube warm up much faster or slower whether it "flashed" or not. You certainly saw it a lot in TV and radios, which didn't warm up any faster than the tube manual says. (Heater should stabilize in 11 seconds; some low-current circuits may start to pass small signal in 9 seconds, no big innovation.)

If you NEED a tube to warm up fast, for military urgency, you get a directly heated filimentary cathode. They never totally vanished, for this reason (walkie-talkie, mobile radio, etc)

Blue Glow: WHERE is it???

Various pretty colors on the inside of the glass is normal and utterly harmless. There are always some electrons that miss the Plate and hit the glass. The glass is pure-enough, but not perfectly pure. Many tube glasses have trace amounts of phosphorescent material, and glow blue or purple when electrons hit. In power amp stages, big signal changes the electron pattern and the glow.

But glow between the cathode and plate (where it is also hard to see) is a very bad sign. You almost never see it: by the time this gas-glow is strong enough to see, the vacuum tube has turned into an uncontrolled gas tube and something is melted or smoking. Pretty much: if you can stand there and ask "what is that glow?", and the amp keeps working, then it is probably the harmless kind.

Orange glow inside the cathode is of course normal and essential.

Orange glow on the outside of the plate, on the tubes we use, is overheating. Many power tubes can live with a trace of "color", and big transmitter tubes are sometimes rated to run this way. The plate does not mind, it is a long way from orange to melting. In fact the factory ran the whole tube red hot for minutes or days to drive the gas out of the metal. Cheap modern tubes don't get fully baked, so a little plate glow may liberate more gas than the getter can absorb; back when TungSol made tubes, this didn't happen. Receiving tubes can also get in trouble because their getters work best when warm, not hot; also if the pin-seals get so hot they crack and leak. So you normally do not want to see glow on the plate, but it is not always a panic problem.

> The solution is to drive the tube with a...

It doesn't need a solution. Except in the worst abuse (TV sets with series-strings of low-bid tubes), heater failure is extremely rare. Not counting cheap TV sets, I've seen it just twice: once when I was bored and ran a tube up to 3X rated heater voltage (it lived a while, I had to go 5X to get quick death) and once (frustratingly intermittent) in a 1958 sig-gen that had a very hard life and then got thrown in a dumpster.

Tube heaters are NOT light-bulbs. Receiving tube heaters on hot-start constant-voltage supplies will probably out-live all of us. Any "fix" more complicated than a rugged resistor will actually reduce the box's reliability.

If you "need a solution" for a "military" reason: every time your tube flashes, the enemy shoots at you, then use metal tubes. As far as I can tell, this is why many 1940s glass 6V6 have that black carbon coat on the inside. After the early thrill of having radio, the next fashion was to hide the distracting glow (hard as it is to understand today).
 

Latest posts

Back
Top