Re: EL84 variety and screen grid glow

On Fri, 25 Apr 2008 02:07:46 +0000, Alex wrote:

<snip>
> 
> However, a new Russian "Sovtek" 6BQ5 tube I purchased needed -9V bias to
> run at the same standard operation point. I am wondering why the
> difference so great (20%)? Is it just a faulty tube? I also noticed,
> looking inside the anode box, a strange orange glow on the screen grid.
> It was not because of excessive dissipation on the grid (Ig2 was only
> 5mA). It was *not* a reflection of a cathode glow -- much brighter and
> more yellowish than the cathode.
> 
> The screen grid glow was only on the inside surfaces of the grid wire,
> facing cathode. It immediately disappears when anode current is shut
> down. It looks like a sort of phosphorence on the grid surface caused by
> the electron bombardment. It has nothing to do with this blueish glow
> often seen on the inner surface of the anode, which is quite common.
> 
<snip>


It could easily just be a bit out of spec and/or there may be some 
impurities in the material causing the phosphorescence. Set the bias to 
get the thing happy and forget it. Don't worry about the glow - providing 
your voltages, currents & power dissipations are within the correct 
limits, that is.

-- 
Mick                      (Working in a M$-free zone!)
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