On Tue, 23 May 2006, Russell McMahon wrote:

> Re Chernkov / Cerenkov radiation.
>
> Long ago I noticed that when using 807 transmitting tubes (!!!) (2 tubes good 
> for 150 Watt AFAIR) they produced a (nice) blue glow at certain points on the 
> envelope. These corresponded to certain "windows" in the metal valve elements 
> and it seemed highly likely that accelerated electrons were striking the 
> glass and producing the glow.*

That is glass fluorescence. It is not caused by ionising radiation.

> The magnitude of the glow varied with RF output and seemed to be maximum at 
> maximum RF output in any given situation. This was slightly away from the 
> plate dip point which was the usual way to tune a final stage. Others in the 
> school radio club ridiculed this method but I subsequently found them using 
> it. I don't recall ever hearing about this being used or reported by anyone 
> else but it must have been common enough knowledge.

Other tubes that did this were PL506/509 EL34 EL36 and similar beam 
tetrodes and pentodes, as well as DY87/807 etc kenotrons. Any glass tube 
will do this with enough anode volts.

> At the time I wondered about the nature of the glow and whether it was 
> accompanied by a degree of irradiation with harder invisible 'stuff'. Odds 
> are it was :-). I'm not at all sure what the electron velocity is in a 
> transmitting tube, but Cherenkov radiation seems a plausible explanation for 
> the effect.

The 807(B) was a much liked tube for ham radio. PL506/509 was H output 
in B/W valved TVs. DY87/807 was the HV rectifier in same. EL34 and 36 
were used in audio amplifiers (100V line and such, running 600V plate 
voltage).

There are actually two kinds of visible effects in a hard driven tube, 
electric blue light that seems to appear in the tube and is likely 
caused by residual ions, and pale green fluorescence in the glass 
proper. The latter may mean you have X rays ;-)

Peter
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