"boardjunkie" wrote in message
...
On Apr 16, 2:02 pm, "Arfa Daily" wrote:
Just as an aside. I know how much you love valves ... Today, I had a
Marshall on the bench. Pair of EL34s in the output. On the tester, it
sounded awful, and the 'scope showed a highly assymetric output waveform.
The drive to both outputs was fine, and the bias was correct on both
valves.
The waveforms on the anode of each valve looked pretty similar, but on the
back side of the output tranny, it looked lousy. Both valves had screen
voltage, fed via a 1k resistor each, but I noticed that the screen volts
at
the actual pin was 4 volts lower on one valve, than the other. When I
measured actually across each screenfeed R, one of them had 4.4 volts
across
it, and the other had nothing. Turned out that the screen grid was open
circuit on that valve. I hung a test one in there, and the voltage drop
reappeared as it should, and the waveform on the output was now perfect. A
new pair of valves and a bias check finished the job off. How unusual a
problem is that ? In all my years of working with valves, I don't think
that
I can remember ever having had an open circuit screen grid.

Arfa

Screen failures are more common with the newer production tubes/
valves. Its either mechanical stress or over dissapation that kills
'em.

It was a fairly 'new' tube as in 8 years or so, but an original Marshall
factory fit. As far as you could see looking through the glass, it was a
'genuine' EL34 pentode, rather than a 6L6 look-alike beam tetrode
masquerading as an EL34. The screen grid connection ribbon looked intact.
Just for sport, I tried measuring the capacitance from the screen grid to
the anode, and it measured exactly the same as on the opposite tube which
was working, so that would indicate that the screen was in fact intact.
Never-the-less, the bad tube drew no discernible screen current at all. The
emission was pretty poor on both tubes, so about the only other thing that I
can come up with is that the emission was so low on that tube, that the
standing bias had it basically cut off, so the screen had no source of
current to draw from.

Arfa