# Cherenkov Radiation in a Vacuum Tube

1. Apr 12, 2014

### kq6up

Someone on another forum shared with me that a 6L6 vacuum tube exhibits a blue dot when in operation that is supposed to be Cherenkov radiation. I say nay, as I have worked out the relativistic velocity using $$KE={ m }_{ 0 }{ c }^{ 2 }\left[ \frac { 1 }{ \sqrt { 1-\frac { { v }^{ 2 } }{ { c }^{ 2 } } } } -1 \right]$$. The plate voltage is 500V max, so I used 500eV for the KE. This works out to $$8x10^{-17}$$Joules. Solving for v, I get v=$$1.3x10^{7}m/s$$. If the speed of light in glass is $$\frac{c}{n}$$, where n=The index of refraction of glass (I will assume 1.5). Then these electrons are not fast enough for Cherenkov radiation. Am I on target here?

Thanks,
Chris Maness

2. Apr 12, 2014

### UltrafastPED

Its more likely some sort of ionization - and the spectrum is blue because the photon energies are around 4-4.5 eV.

3. Apr 12, 2014

### nsaspook

Most of the blue in good power tubes is from FLUORESCENCE, sometime near or surrounding the anode plate or glass envelope.
http://pitchperfectaudio.files.wordpress.com/2009/12/corton-charlemagne-glow.jpg

If it's more pinkish and near or inside the plate structure then it's ionization and that's usually the sign of a gassy power tube.
http://i195.photobucket.com/albums/z109/davethebass1/IMG00208-20120220-1316.jpg
http://www.amplimo.com/index.php/electron-tubes [Broken]

Last edited by a moderator: May 6, 2017