The AM Forum

I discovered last night that all the tubes in both of my ft-901dm's glow blue when they are drawing plate current. i know that tubes can glow blue when they get gassy, but these only glow when they draw plate current, and they are good tubes. The amount of glow changes with the amount of bias on the tubes. I thought that maybe it was due to impurities in the glass causing the glow, but when i look closer it appears that the glow is coming from outside the plate. it's not that i am worried about it its just interesting and would like to know what causes it because i've never seen other tubes do it except for a 35C5 in the audio stage of an old zenith am/fm radio that i have and the glow from that tube was similar to what i saw in the 901, and another radio that had the same kind of glow but was a lighter blue glow and i know that was due to the tubes being bad, and an old sylvania 6k6gt that i stuck in my crosley, but when it first came on it would not glow, but if you tapped the bulb of the tube the glass began to glow. I also discovered that the glow is still there if you key the rig and then shut off the filaments, it stays for a while but dims as the tube begins to cool. My idea is that it is due to electrons flowing inside the tube, but i don't know if that right or not. but it is something interesting to see.
shelby

Does this help?

http://www.jacmusic.com/html/articles/blueglow/blueglow.htm

In a few short words, if the blue glow appears at the interior of the tube, in the vicinity of the grid/plate structure, that indicates that the tube is very gassy.

If it appears at the surface of the envelope, it is due to fluorescence from impurities in the glass, caused by bombardment of those minerals by stray electrons.  If the plate and screen current and voltage are within manufacturer's specifications, this phenomenon is completely harmless if the tube is working OK .  I have seen it occur with brand new tubes right out of the box.  It is exactly the same phenomenon that causes the screens of CRT's in oscilloscopes, TVs and computers to glow.

I remember hearing about this before. i wasn't sure exactly what caused it but i knew it had to do with the impurities in the glass.

Don said
 In the article some blue glow is OK. I have been working with 807's-6550 and many other tubes in amplifiers and in fact I made a living for about ten years repairing organ amplifiers, PA amps and guitar amps. In every case when I took the time to look at the grid and plate structure, there always was a blue glow in the elements. To me this is normal. When you see a tube that has enveloped itself in a blue glow it has always meant two things to me, one the tube is gassy or the tube was drawing excessive current due to a component failure. Since most small amplifiers have no meters on them, a glow test can reveal a lot.

Don I respect you opinion greatly but this is what I have found in my days as a repairman.

Wasn't there a thread here several years ago about purple glow on 1625's? Someone stated that even new ones demonstrate this phenomenon but pretty much all of them do it. I think the original poster was going to deep six his 1625's thinking they were bad.

What i thought was strange about it was that i had the bias in both rigs set close to the same(according to the meter) but the much more beat up 901 had a brighter glow than the good 901. I knew when i posted this that the glow wasnt bad because ive seen a gassy tube glowing before but i just wanted to know what caused non gassy tubes to glow as well.

When I talk on em they'll be gassy in seconds. Even if they was good before.

When I first fired up my push pull 100th rig,(JAN 100ths) Both tubes had blue glow...Around the plate element and on the envelope...At one point the stem that runs from the plate  to the plate cap had blue doughnut torrid shapes running up and down when transmitting....After 4 years of use  (maybe 40 hrs key down time,probably 200 hrs filament time) the glow is almost gone except for a slight blue around the perimeter of the glass about mid tube...I still get full output.....I miss the blue however...

Are you sure the glow comes from the space surrounding the elements, or from the mica insulation used with most tubes?  I have seen the mica insulation light up a bright bluish purple.  Again, it is from  electron bombardment.  If the empty space between elements appears to be giving off light, that means the tube is gassy.  The vacuum of empty space can't  glow unless there are molecules of something there to give off the light.

I have seen known gassy tubes work fine in the circuit.  I once had a cracked audio output tube in an old radio.  You could see the hairline fracture in the glass envelope. I made an effort to seal it by putting model airplane glue over the crack.  The tube glowed blue inside when powered up.  But the radio still played normally.

Roger 10-4 on the nice blue glow.
That's normal for it to be inside the elements.
In audio amps it's cool to see the glow flickering and dimming to the bass beat in the music.
My parents thought I was weird looking inside EL84's or the Knight Kit that had EL37's in the finals

Fred

Don said
The glow is between the screen and the plate, maybe even toward the filament? You only see this in indirectly heated filaments. Maybe it happens in 813's but the filament is too bright?

I guess a very slight amount of gas is OK but when someone says to me a tube is GASSY, that to me, means a defective tube. so to reiterate, just because a tube has a little gas in it that may make it glow does not mean it's shot or gassy. I am, however  not trying to be smug about any of this ! I'm just a poor little old, over the hill, past tube puller who thought at one time, knew something about these glowing attributes that we are so fond of :-* :-*

Maybee the tubes are lonely?


klc

klc said:
I know I'm lonely when I'm gassy.  :D