Tube Fluorescence

by on Feb.03, 2009, under Electron Tube Audio

Night Shot - Amplifier
These are the output tubes on my Thomas amp. You can see the blue glow against the glass from stray electrons. This is very common with Sylvania tubes. Behind the blue glow, is the red glowing line from the electron beam hitting the plates (I can’t get the bias to go any lower with this amp). The two together look purple to the camera.

I have read contradictory discussions about blue glow against the glass envelope. Blue glow coming from inside or near the anode or other structures is normal, but some say that blue glow outside of that means a soft vacuum. Sylvania claims the opposite and that fluorescent glow inside the envelope is normal for some tubes. It results from stray electrons bombarding near the glass envelope and will change in brightness depending on the intensity of the signal passing through the tube. This is exactly what I see with these Philips JAN tubes, which are basically Sylvania. In fact when I push the amp really hard well into cut-off, the tubes will flash brightly signaling me to back-off

The “Winged-C” SED EL34s in the Dynakit seem to fluoresce at the screen behind the plate:

Dynakit VTA-70 - SED EL34 Fluorescence

2 comments for this entry:
  1. Avatar

    The color suggests ultraviolet spectral emissions.
    We used to call this ‘Black light’.

    I’m not sure if you want to spend the time understanding
    spectral emissions from vacuum audio tubes,
    but you’d have to start here:
    Just to get a feel for the math involved and
    vacuum tube history and behavior.

    You might also do a search on wikipedia for
    spectral emissions, electron tube.

    You also have to understand, that one aspect
    of audio amplifier tubes was their ability to
    amplify harmonics, which might have an impact
    on the visible spectrum, specially in low/no light
    photography. Did you use UV sensitive film or
    a Digital Camera that is more
    sensitive to UV to make your photo?

    Hope this helps-


  2. Avatar

    Cool, thanks for the info. It was just done with a compact digital camera set to manual exposure. The fluorescence is very blue to the human eye, but the plates do have a red line on them from the beam. You are probably right in that the camera was picking up more of the blue-ultraviolet light than my eyes could.

    I just thought it made a cool picture. 🙂

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