Re: EL84 variety and screen grid glow

"Alex" <apogosso@[EMAIL PROTECTED]
> wrote in message 
news:S1bQj.4761$ko5.242@[EMAIL PROTECTED]
> Mainstream EL84s are supposed to operate in a standard class A mode (Ua
=
> Ug2 = 250V, Ia = 48mA) at about -7.5V bias. I believe, 6BQ5s should do
the
> same.
>
> "Old reliable" Soviet 6P14P, similar to EL84, reached the same working 
> point
> at about -6.5V...-7V bias.
>
> 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.
>
> I have never seen (or have never noticed) such screen grid glow before
on
> any other tubes.
>
> Do you know what it is? And what those Russions have done to the tube?
>
> Regards,
> Alex

Hi Alex:

It sounds like the screen grid is getting pounded by electrons due to
being 
incorrectly aligned with the control grid during tube manufacture.

Best Regards : Doug Bannard