"Patrick Turner" <info@[EMAIL PROTECTED]
> wrote in message
news:4811A955.50FD47E9@[EMAIL PROTECTED]
>
>
> Alex wrote:
> >
> > 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.
>
> You have a dud tube.
> >
> > 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.
>
> The screen grid wires are not properly aligned with grid wires.
My understanding is that EL84 is a pentode, not a beam tetrode, so the
screen grig does not have to be aligned. It runs at a gifferent pitch and
with opposite incination (winding direction) with respect to the control
grid.
What grid alignment is required?
> >
> > I have never seen (or have never noticed) such screen grid glow before
on
> > any other tubes.
>
> I have.
So what is this glow phenomenon? Can yo describe the physics of it please.
It does not look like heating, though I can not be sure. It looks more
like
fluorescence as if the grid contaminated / coated with something. What
with?
By the way, blue glow on the anode has a tripy pattern with glowing
stripes
in between the windings of the suppressor grid. (Suppressor grid is very
rare.) This makes perfect sense.
>
> They belong in the bin, or to be sent back to the maker.
> >
> > Do you know what it is? And what those Russions have done to the tube?
>
> Tubes made in 1960 by western countries had a failure rate.
> Good quality control weeded out most crook tubes, but not all.
> Same goes for many products.
>
> A better opinion can be gained after testing a number of samples from a
> number of batches. I have never found bad EL84 from Russia with glowing
> screens.
>
> I have seen Tesla EL34 with 30% of 20 tubes being problematic, so i sent
> them back
> and the supplier replaced them with Sovtek '34, and all worked well.
I am not sure the supplier (WES Components) will accept it back, possibly
at
best it might be replaced. Who knows, maybe they bought of factory rejects
from Russia and selling them in Australia for A$13. I would think that if
a
bias point is 20...25% away from the typical value, the tube shall be
considered faulty. It is not a transistor where beta can vary 5...10
times.
Thanks for the advice.
Regards,
Alex
>
> Patrick Turner.
>
> > Regards,
> > Alex
|
