Will a cracked tube cause a fuse to blow?

[Perkinsman]

Does a crack (loss of vacuum) in a preamp or power tube cause current to rise enough to blow a fuse? Would it be correct to say that a crack causes the circuit to open?

[Enzo]

I guess. I don't think about it that way. I see a cracked tube, I think. "Hey, that's a broken tube." It needs to be replaced. it may or may not be causing whatever you are trying to fix, but it has to be replaced regardless. Install a new tube and FIND OUT if there is more wrong or not.

[Chuck H]

I guess. I don't think about it that way. I see a cracked tube, I think. "Hey, that's a broken tube." It needs to be replaced. it may or may not be causing whatever you are trying to fix, but it has to be replaced regardless. Install a new tube and FIND OUT if there is more wrong or not.

This^^^^^^^^^^^^^^^^^^^^^^

There are mechanical structures and connections in the tube that may or may not be open or incorrectly connected in the event of a catastrophic failure. There's always a likelihood that any temporary short could cause circuit failures in the components as well. A broken amp starts with troubleshooting 101. In this case, busted tube.

[Perkinsman]

I understand that a blown fuse could be caused by a short in a tube but why would a cracked tube cause a fuse to blow?

[pdf64]

Normally, HT current flow relies on the vacuum. So if the envelope of a functional tube breaks, thereby losing the vacuum, in the absence of any other faults, I can’t see why a regular fuse should blow.
Maybe if it’s shared cathode bias and each tube has a fuse but we’re well outside the remit of a generic query with that.

[Mick Bailey]

I've had white-topped power tubes that have lost their vacuum, shorted and blown the HT fuse. I don't know the mechanism, other than the heater depends on the vacuum to keep it from evaporating material and burning out. Maybe the vacuum leaks the heater gets too hot and distorts the cathode so that it shorts against the screen or whatever, depending on the tube construction. A preamp tube short won't pull enough current under normal conditions to blow the HT fuse due the plate load resistor.

[DrGonz78]

Had a Mesa Boogie mkiii where one tube lost its vacuum whited out and the other tube on the other side had zero conductance. The two middle tubes were still good and the customer claimed he had played a show just recently with no problems. He had switched the amp in multiclass and other modes while doing the show and the amp worked either way. No fuses were harmed.

[Enzo]

Tube failure isn't generic. It is possible the tube would just sit there and cause no problem other than not functioning. The heaters run at super high temperature. In air they can burn up. That can cause shorts between elements.

[pdf64]

I was thinking that with air in the tube, the heaters might run a little cooler, because in addition to radiation and conduction, they would then have convection to assist in dissipation.

[Mick Bailey]

Well, with light bulbs the filament oxidizes in the presence of air. The oxide is non-conductive so reduces the CSA of the wire and the resistance goes up. As this happens the temperature increases and it burns out. I was thinking a tube would do similar. Just my flat-earth theory.

[Helmholtz]

I understand that a blown fuse could be caused by a short in a tube but why would a cracked tube cause a fuse to blow?

Yes. The visible crack is probably a consequence of a thermal runaway. This is caused by a somewhat leaky/gassy tube (and/or by too high value grid leak resistors). The gas molecules get stripped /ionized by the fast electrons and and the positive gas ions produce a positive grid charge which in turn rapidly increases plate current. Increasing plate current and temperature lead to catastrophic failure.

https://archive.org/details/Tomer_19...ge/n1/mode/2up

See pp 28/29.

[g1]

I'd think whether it's running or not when it cracked would be a factor.
I'd expect the heater of an already cracked tube to burn up during warm up like Mick outlined above (say a tube that cracked during transport or a new tube with bad vacuum).
For a tube that cracked during use, I'd think many different things could happen, including faults that would blow a fuse.

[Helmholtz]

I'd expect the heater of an already cracked tube to burn up during warm up like Mick outlined above (say a tube that cracked during transport or a new tube with bad vacuum).

Yes, a cracked envelope might cause the heater to burn up, but this as well as atmospheric air pressure inside (electrons can't travel in air) would stop the plate current and not blow the fuse.
So I think that thermal runaway was the reason for blowing the HT fuse and the cracking of the bulb.

[Pedro Vecino]

Many times the fissure is generated by the continous rubbing of the metal protector with the glass. Typically in the contact area between the metal fin and the spring. I mean the protectors that hold the top of the tube. Using heat shrink tubing in that area prevents it.

[Perkinsman]

If thermal runaway is the reason for the fuse blowing, is it an anomaly or is it a symptom of another problem? When I replaced the cracked tube and the fuse, everything seems to work fine, voltages were in spec, and sounds good.