6AV5 Tube Autopsy: Now What?

[hareynolds]

I cut away the bottom mica and cut the two plate wires, and the plate structure just slid up & off. Here's what's inside.

The only anomaly so far seems to be a little flaking of the white coating on what I guess is the cathode...

I was expecting something lot more dramatic.
Looks like the poor thing died in its sleep, Bless Its Heart.

What next, George?

PS I feel like such a wuss for not really COOKING these tubes...but I guess you've got to start someplace.

[Tubelab_com]

Originally Posted by hareynolds

OK, you blood-thirsty animals, here we go:

First photo shows the wire from the cathode pin. Nothing unusual-looking anywhere under the bottom mica plate.

Other photos show the top mica intact and removed.
Again, nothing suspicious-looking.

More in a minute.....

The white wire in the first photo is part of the heater that gets the cathode glowing red hot. There is a silver strap that connects the metal cathode sleeve up to the pin on the base. It's usually quite close to the mica.

I started ripping tubes apart when I was a kid to see how they worked. I would dig through the trash behind the two TV repair shops within walking distance of my house to find tubes. I also went to the trash dump with a neighbor to shop for tubes, transformers and other useful parts from dead TV's, radios and whatever. I tried to build stuff without knowing what I was doing, so stuff died.....a lot. I then took it apart to see how or why it died. I did the same with lawnmower engines too, and I had the fastest go-cart in the neighborhood.

Most tubes die slowly without leaving much of a forensic trail behind. The flaking part of your cathode was likely done during disassembly. The pictures all look somewhat normal, with the possible exception of the last picture. It looks like there is a darkish brown discoloration on the right side of the cathode. If that's not an artifact from the camera or lighting, it could be cathode poisoning.

The tube in the picture actually exploded from being asked to dissipate nearly 400 watts instead of 40. The plate pin passed 100's of amps for a few milliseconds causing it to vaporize shattering the glass. I did not need to hammer this tube. You would think that all sorts of stuff would be fried inside, but there was only a small area of damage on the cathode where the lightning bolt jumped to the plate during a tube arc. There should be a wire leading from the plate to a pin in the front of the base. What's left of it was permanently welded into the tube socket.

In a different experiment many years ago I stuck some ugly 6BQ6GA tubes in an SSE and decided to see just how far I could push them. How's nearly 30 watts at maybe 10% THD. You could read a book by the white light coming from the plates. Tube life would have been measured in minutes, but this excess caused the cathode bypass cap to explode, ending the fun. Even so these tubes didn't seem to work so good any more. I got over 100 of these tubes at an average cost of 78 cents each. All were sold as "new" but some were obviously used, some even dead. These were the ugliest good ones, so I "tested" them. The 6BQ6GT, 6BQ6GA, and 6DQ6 were the tubes that I had the most of as a kid, because they were the most common tube in the dead TV's of the 60's. The 6BQ6GA and the 6AV5GA have identical guts, the only difference is the glass and the pin arrangement.

After this experiment, both tubes met the hammer. Other than some discoloration nothing was obvious here either. The cathodes had just lost emission due to excess heat and probably "gas" though no gaseous glow was present in the tubes.

The most common failure I have found is melted screen grid wires or warped grid rods. i have seen a few warped cathodes too.

[hareynolds]

Thanks George.
Anything to be gained by further dissection?