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.