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

[Steve Hobensack]

  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