Do Sweep tubes Glow Blue?

[wrenchone]

Some beam power output tubes exhibit a blue-violet glow as a result of stray electrons striking the glass envelope. This is an incidental effect, but much appreciated when it happens, as the glow often dances with the music (you get a light show as well as the cool tube sounds). I've seen this effect in vintage 7591A output tubes as well as (faintly) in old 1625s. I have built two sweep tube amplifiers, but have not seen the effect in either. Has anyone seen the glow in their sweep tube amp (screen driven or otherwise)? Just curious - I'm wondering if it's the difference in construction, envelope glass, etc.

[Colt45]

i think it's just impurities in the glass fluorescing... so I guess... roll tubes until you find one that glows

I think I've played with sweeps that had some before, can't remember which , but I don't see it helping much anyways.

[m6tt]

My sweep tubes were glowing blue a bit...I was running a few in push-parallel as a vocals amp at a show we did recently. Little glow at idle, higher glow at full out. The amp is 6av5 push-pull parallel, with screens at about 250v pentode. Glows blue on glass more with no load or with oscillation, as I discovered during design & testing. EH6CA7s glow for sure too. Actually, I haven't used any sweep tube that didn't glow blue on the glass when operating near limits.

I've always heard that's the sign of a good vacuum (as opposed to the mica-illuminating interior plate glow of gas) since nothing is impeding the electrons till they hit the glass. It's never a general glow, unless it's gas. Mostly, it's where the electrons can get around the plate. The fun part is when you take an old speaker basket magnet near the glowing glass, it's entertaining for a good 5 minutes or so .

[SY]

Blue, blue, my tubes are blue. (6JN6 and 6LF6)

[tubelab.com]

Blue, blue, my tubes are blue. (6JN6 and 6LF6)

I must be doing something wrong, my tubes glow red!

I have been exploring the upper limit of power available in my trusty old pair of Chinese 6L6GC's. I can get over 90 watts at 3% distortion from these guys in AB2 without red plate. I noticed last night that they too have some blue on the glass when cranked hard. It seems that the blue glow might be related more to the operating voltage than the power being dissipated. Currently 500V on plate 400V on G2.

Some serious sweep tube testing is coming.

[SY]

Well, figure that the operating voltages will affect the electron velocity...

[jlsem]

Well, figure that the operating voltages will affect the electron velocity...

...toward the plate. Isn't the energy of the electrons striking the envelope related to the temperature of the cathode? I was under the impression that most of the phosphorescence on the surface of the glass was caused by the electrons that escaped through the inspection holes in the plate.

Wait a minute, I just answered my own question.

John

[w5jag]

I had some RCA 6BQ6GTB that would strongly fluoresce.

I think fluoresence is directly related to voltage, and somewhat to brand.

In my experience, most GE tubes fluoresce as do most Russian tubes. I even had a GE 6201 that would glow blue. A lot of Sylvania tubes do it. The above noted 6BQ6GTB are the only RCA made tubes I have ever seen fluoresce, and I have never seen a real Tung Sol glow.

My Chinese coke bottle 6L6GC glow blue, but only above about 425 volts.

Bendix 5992 has a great glow, can't speak for the others in their line.

Win W5JAG

[SY]

John, you got it. This is also why Deketh found an increase in linearity when power tubes had the inside of their envelopes carbonized.

[tubelab.com]

The above noted 6BQ6GTB are the only RCA made tubes I have ever seen fluoresce,

Last night I was exploring the upper limits of a pair of RCA 6BQ6GTB's (the 98 cent ones from last years AES sale) in screen driven P-P. I had the B+ voltage set on 600 volts. With a 6600 ohm load is was seeing reliable and stable power outputs in the 90+ watt range. There was no glow either red or blue!

Then I got greedy and switched to a 3300 ohm load. I saw the power output display on the audio analyzer hit 125 watts before the runaway started. It seems that I have found a new way to blow up a tube. In extreme screen driven situations (125 watts from 6BQ6's IS extreme) the screen grid is going highly positive while the plate is approaching zero. This makes the screen grid glow. When it glows is can emit electrons which travel to the positively charged plate. The PowerDrive circuit uses a resistor to return the output (screen or control grid) to the negative supply. As the screen grid starts to draw a lot of current it is no longer under mosfet control and the screen voltage will runaway. The screen voltage rose quickly causing the tube current to shoot toward 1/2 amp. I was observing this effect and managed to catch it 3 times, but on the 4th time the fireworks erupted inside thge tube and it doesn't work so good any more. At any rate there was various shades of red, orange and white, but no blue.

My Chinese coke bottle 6L6GC glow blue, but only above about 425 volts.

I was exploring the limits of these guys too. You can get 90 watts from a pair in AB2 with 500 volts on the plate. The only color visible is heater glow and blue!