6AV5 Tube Autopsy: Now What?

[hareynolds]

Here's one of the 6AV5s that I toasted earlier in an SSE amp (with auxiliary sockets specifically for 6AV5 experiments).

Easy Peasy with a towel and the ball-peen end of a ball-peen hammer.
I do wish George had warned me about the NOXIOUS GAS released.
Next time, OUTSIDE. Not in a closed heated shop.

I suspect I got my lifetime dose of mercury vapor, if I hadn't already gotten it from playing with globs of mercury from mercury switches as a Yute.

So, what's the procedure, Doctor George, TP (tube pathlogist)?

Remove the crooked mica top plate? Then what?

[wrenchone]

No mercury, just barium.

[hareynolds]

THAT makes me feel much better.
Now I have lifetime doses of BOTH Barium and Mercury.

[wrenchone]

The barium is in the shiny getter coating on the inside of the glass envelope. It doesn't have a high vapor pressure like mercury, and oxidizes very shortly after the tube is shattered and brought up to air. The shiny getter coating turns white... As long as you wash your hands after handling the glass bits, you should be ok.

[Tubelab_com]

If smashing tubes was hazardous, I would be dead already. I have been taking them apart for over 60 years.

First look at the bottom of the cathode sleeve in the center of the tube. There should be a thin flat metal jumper connecting it to the cathode pin, or to the beam plates (the silver ones inside the black one). In a tube arc situation these can blow like a fuse abruptly ending the tube's life.

If that's intact you need to remove the plate (the outer black metal structure). The black coating will often rub off on your fingers. It never hurt me, but YMMV. Definitely wash your hands after playing with all the parts. Every tube is different, and I don't have a similar Sylvania handy, but cutting the tabs off the top and bottom will usually let you peel back the plate, or separate the two halves.

Once you can see the grids look for burned, or melted grid wires. An overloaded sweep tube will often have fried G2 wires, the outer grid. The grid support rods should be straight and all the wires intact. Sometimes a long slow cook will warp the screen grid causing uneven current flow, and hot spots.

The cathode coating should be evenly white with no flaky, burnt, or missing spots. A poor cathode coating will kill emission making the tube "weak."

If the tube was run in the red zone for an extended time, the excess heat will liberate trapped impurities polluting the vacuum. This is called "gas" or a "gassy tube." This gas can "poison" the cathode reducing emission, and it can cause grid current, upsetting the bias leading to a runaway. If nothing appears damaged, this is the likely cause of death.

[JMFahey]

smashing tubes

Great name for a Rock band

[hareynolds]

it'll be tomorrow before I get to the autopsy. after I get my COVID vaccination.
So far, the only perk I've discovered to being over 65...well, that and the "Senior coffee" at McDonalds.

You might want to avoid this threat if you have delicate sensibilities; likely to be gory.

[hareynolds]

THREAD, not threat.

[overtheairbroadcast]

Just here for the autopsy and threat (no, it's funny reading that and still works either way).

[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.....