[Central-Electronics] RE: 6AG7 self destruction on CE 10B

[Jim candela]

Steve,

    It sure sounds to me that the tube was the problem. Could be gas, or
maybe a flake of cathode oxide on grid1. If so, how did it get that way?

Hmmm maybe 50 years of a slow leak of air into the tube?

or a prolonged overload vaporizing trapped molecules in the metal?

or a sudden burst of overload like a parasitic as George described?

or a flake of cathode oxide on G1 causing a local hot spot and gradual
reverse grid current until the tube goes into a thermal runaway. I have seen
this with the 4D32 in the Viking 1. It is worse with higher frequency, and
higher filament voltage. If your running 7 volts to the 6AG7 filament, drop
it down a volt with a dropping resistor.

If you back off the screen voltage, the output will fall back. Look at the
class A specs at the following link:

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AG7.pdf


With 300 volts @ 30 ma (9 watts Pd), they get 3 watts rms output. If your
getting 6-7 watts out on 40 meters, I bet your are at least doubling the 9
watts Pd rating. I use a 12 volt computer fan in my 20a to cool the RF
compartment. Also notice the lower screen voltage too. If you drop the
screen voltage, the bias must be lowered to keep the idle current suitable
for class A or Ab1 operation.

Measuring the idle current is easy by measuring the drop across that 100 ohm
resistor. Since I = E/R, 3 volts drop across 100 ohms = 30 ma plate current.
You could replace that 100 ohm resistor with a #49 pilot lamp. These I
believe light up fully at 2 volts across at 60ma through. I can imagine the
bulb protruding through a hole in the Front panel, and glued in place. Might
make a nice tuning indicator.

The 20a has a 2 amp fuse, so maybe you can go down a notch to 1 or 1.5 amp
to speed up the fault time before too much smoke is released.


So steve, this OM would like to know what PSK is, and how does well doea a
10B do on this mode?

Regards,
Jim
WD5JKO


-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net]On Behalf Of Steve
Hobensack
Sent: Tuesday, January 03, 2006 7:28 PM
To: central-electronics at mailman.qth.net
Subject: [Central-Electronics] RE: 6AG7 self destruction on CE 10B


  Here are some observations on the 6ag7 tube shorting:

    The first time it happened was about two years ago. The rig was turned
on but not being used. A bunch of smoke poured out of the area near the main
transformer. The trouble was the 100 ohm 5 watt resistor going up on flames.
The resistor is located in the B+ line that goes to a 2.5 mh rf choke that
goes to a parasitic suppressor that goes to the plate of the 6ag7. I
replaced the resistor. The tube checked ok. I turned the unit on, after
about a minute the 2 amp fuse blew. I went ahead and replaced the 6ag7. No
more problems for a year or more until last weekend. This time the smoke
poured again. I was running PSK at about 3 watts output on 40 meters. The
rig will make about 6 or 7 watts out but I cut it back due to the continuous
duty cycle of PSK. Upon Key-up, the main power transformer buzzed louder
than normal and the 2.5 mh choke plus the same 100 ohm 5 watt resistor burnt
up. I replaced the resistor, choke, & 6ag7. No problem. I put the old 6ag7
back in and the 2 amp fuse blew. I put a new 6ag7 in, no problems (knock on
wood).
    Voltage readings on the grid, Plate and screen seem normal . The tube
runs hot. Grid bias is  minus 120 key-up and minus 13 key-down. There is
nothing in the screen circuit to drop voltage except a 560 ohm resistor, so
the tube seems to be run to its maximum ability. Plate and screen voltages
are the same at 300 volts. The tubes are RCA brand.
    Someone once told me that the 100 ohm resistor serves as a fuse. Would
lowering the screen voltage help the situation without hurting the rf
output? I'm open to ideas.
..73
..Steve..N8YE


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