"Eeyore" wrote in message
...


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.

Can't say I have either. But then again have you ever seen 4 output valves
go
zero emission (this was in a Fender Twin Reverb). They were those dreadful
Z&I
Aero Services (Zaerix) Russian valves from the 70s/80s. Quite bizarre, all
drive
waveforms and bias voltages correct but no output at all.

Incidentally I got it working because it was needed urgently by fitting 4
EL34s
(I didn't have any 6L6s - you didn't see them much in the UK in those
days).

Graham

Yes Graham, I too have fitted 6L6's and vice versa. I have read on Wiki I
think that EL34's turn up with 6L6 guts in them. I think that the beam
tetrode structure was actually created to get around the patent that MO had
on the EL34 pentode structure, so I guess it's reasonable that they should
have similar characteristics. Crikey ! Z & I Aero Services aka Zaerix. That
takes me back a bit. I still have a Zaerix shortform catalogue in pristine
condition. Had some useful data in it, and ex military valves and 'scope /
radar tubes ...

Arfa