Tube with purple hue

[K4RT]

When applying plate voltage to this tube, a 27, a purple hue appears inside the tube. Does that indicate that the tube is gassy!

[W7TFO]

Wat'cha got on the plate? 

It does look like gas, but that can be induced by high plate voltages over design specs.

73DG

[Pete, WA2CWA]

When applying plate voltage to this tube, a 27, a purple hue appears inside the tube. Does that indicate that the tube is gassy!

Generally not a bad thing:
https://www.google.com/search?client=firefox-b-1-d&q=purple+glow+inside++Vacuum+tube
My EL-34'a in my Apache have had a similar glow for the last 40 years. When a modulate the rig, it also modulates the purple glow.


Pulled this off the web:

[W7TFO]

If the tube in the photo (center with a metal base) is an 83, it glows cause it is a mercury vapor rectifier.

Some tubes, especially audio power tetrodes can show blue ion patterns on the inside of the glass envelope.

Typically, gassy tubes have a fuzzy blue glow all around the elements, and some orange glow between the elements at times.  A gassy tube will draw heavy plate current with less control by the grid.

73DG

[WBear2GCR]

it's fine.

[Opcom]

Just opinion based on observation, YMMV:

You can look closely into the interior of the tube, at the space between cathode and plate.

This can't be easily observed in all tubes, but some brands or types like 6CA7/EL34 have 'tabs' or 'vents' type features on the sides of the plate which can be peered into, and some with fairly transparent mica at the top can also be observed.

A gassy or just plain overheating tube may show a bright or sky-blue ionization-looking glow between the screen and or other elements. I've seen this on an ultralinear amp with questionable EL34s. The tubes tested good, but when they got really hot because of stupid-high screen voltage, the phenomenon appeared. Anyway, the biggest gas glow is usually inside the space/volume of the elements where most of the power is going. Tubes at low powers and low voltages might not even have a visible glow to be found.

The usually darker-tinted or deeper-blue blue fluorescence on the inside of the glass, with no other 'glowing spaces' is caused by electrons hitting the glass and should not be an issue. I believe the 27 shown in the original post does not show a problem because the glow appears to be on the inside of the glass, but a 2D view isn't the best.

[K4RT]

Thanks for your replies. The plate voltage was 252 VDC, and it's in a Hartley oscillator for CW. The spec plate voltage is 250 for continuous service I think. My other 27's don't do this in the same circuit, same plate voltage.

[wd8das]

Look at the tube from various directions.  If the glow is in the seemingly empty space inside the tube, then I'd say it could be gassy. 

If, however, the glow is coming from the glass envelope then that could be normal.  Some tubes do that, others do not. My theory is that it depends on the materials mixed into the glass.

My old 807 breadboard project I deliberately selected a pair of 807s that lit up like that.   I studied the glowing pattern on the glass and realized that it was the overspray of electrons that were sneaking around the plate and exciting something in the glass to glow.  The non-glowing area were in the shadow of the plate.   It was fun to watch the glow cut on and off as I keyed the rig in CW.

Here's a writeup on that transmitter project:

http://www.wd8das.net/dream.html


Steve WD8DAS

[wa2tak]

THAT 2 x 807 was my first novice transmitter...1964.

Had a key relay...even grommeted to the chassis...sounded like a small hammer hitting the chassis...rather loudly too.

And...from dusk to dawn...my parents in the next room put up with that!

After some time...wanted to get a DX-100....Dad had no problem paying for it.

Not too long ago, saw the same homebrew rig ( not mine, naturally ) for sale on eBay...have it!

[KD1SH]

Used to run a Tram D201 back in my 11 meter days in the 70's; 6L6 modulated by a 6L6. Always loved to see that blue glow through the louvers, pulsating with my modulation.

[Opcom]

Now THAT'S what I'm talkin about!

Fond childhood memories of much time spent observing the fluorescent glow on the glass of a 50L6 through the holes in the back of a wooden RCA tabletop set with the Golden Voice.

[KD1SH]

Add the smell of the hot glass envelopes and you've got the magic of radio.

Now THAT'S what I'm talkin about!

Fond childhood memories of much time spent observing the fluorescent glow on the glass of a 50L6 through the holes in the back of a wooden RCA tabletop set with the Golden Voice.