Will a cracked tube cause a fuse to blow?

[Mick Bailey]

You can't assume it's thermal runaway any more than you can assume any other reason. A tube failure in the absence of any firm evidence to me is just a random tube failure. I received an order of brand-new, boxed tubes where some had a partial thickness crack in the class envelope, about 4mm long. So that isn't thermal runaway but rather mechanical damage and I bet if they were used the heat cycling would have caused the tubes to fail. While I was wrangling with the supplier trying to get them replaced one began to turn white on the gettering.

I have some difficulty in visualizing thermal runaway in a preamp tube, or fuse blowing due to a crack. As previously noted, the current draw is limited by the plate load resistor and also the resistor chain that drops the main B+.

If replacing the tube and fuse fixes the problem and it stays fixed, you'll perhaps never know what the reason was.

[Helmholtz]

I have some difficulty in visualizing thermal runaway in a preamp tube, or fuse blowing due to a crack. As previously noted, the current draw is limited by the plate load resistor and also the resistor chain that drops the main B+.

You're right. I missed that the question included preamp tubes. As you say, a cracked preamp tube would not explain a blown HT fuse.

Of course there are several possible reasons for glass cracks. And thermal runaway may be started by a micro crack, which let some air get inside.

I was looking for an explanation for a blown HT fuse with a cracked power tube. Excluding other amp problems, I think it leaves essentialy 2 explanations for the excessive tube current: Either an existing crack blew the heater and caused some metal part to produce an internal short between electrodes.
Or the power tube initially had no (visible) crack but was leaky and over time developed a thermal runaway caused by increased gas content. In this case extreme tube dissipation and redplating might have cracked the glass.

[Helmholtz]

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.

Still not clear to me if you are speaking of a preamp or power tube.

Thermal runaway is typically caused by a damaged or poor vacuum/quality tube. But too high grid resistance increases the risk with power tubes.

[Chuck H]

But too high grid resistance increases the risk with power tubes.

As does a loss of bias. And in an old amp this is what I would suspect first. An intermittent grid connection would be both a high (or infinite) grid resistance AND a loss of bias. This could be a bad solder joint or dirty/oxidized tube socket or tube pins.

I think it's clear that we're talking about a power tube if only because the fuse was blown. Not to assume these aren't coincidental circumstances that just happened together. That sort of thing does happen. But as it relates to the opening question, if the cracked tube DID cause the fuse to blow then it's a power tube.

[Helmholtz]

As does a loss of bias. And in an old amp this is what I would suspect first. An intermittent grid connection would be both a high (or infinite) grid resistance AND a loss of bias. This could be a bad solder joint or dirty/oxidized tube socket or tube pins.

Good point! Loss of bias would cause another kind of "thermal runaway" showing the same symptoms. An open grid in a fixed bias amp corresponds to infinite grid resistance. (In post #17 I explicitely excluded "other amp problems" just trying to answer the original question.)

So even if the amp is working fine with the new tube it still makes a lot of sense to look for intermittent contacts.

[dugdiamond]

YES IT WILL.
The tube no longer has a vacuum and it will simply flashover and if you are lucky it will blow the fuse before it does any serious damage.
Simple as that.

[Chuck H]

YES IT WILL.
The tube no longer has a vacuum and it will simply flashover and if you are lucky it will blow the fuse before it does any serious damage.
Simple as that.

I think it's important to qualify this statement by specifying a power tube as the culprit.?. A preamp tube would be subject to enough resistance that it wouldn't stress the mains fuse.

[Enzo]

I find myself having a similar discussion about preamp tubes in other contexts. Pointing out that even if the plate dead shorts to ground, the 100k plate resistor limits current to two or three milliamps, which won't blow any fuses. Exception might be a cathode follower, but then plate to cathode shorts are exceedingly rare.

[dugdiamond]

I think it's important to qualify this statement by specifying a power tube as the culprit.?. A preamp tube would be subject to enough resistance that it wouldn't stress the mains fuse.

Thank you for recognizing what I had specifically had in mind, but failed to elaborate.
For whatever reason, I was indeed only thinking in terms of a power amp tube. I really didnt consider the case of other types of tubes or their functions within a circuit, audio amplifier or any other type of circuitry.
But yes loss of vacuum in a power amp tube or rectifier tube usually will result in some sort of flashover that likely will take out a properly rated fuse.
Also many amplifiers will have two separate fused circuits. The "HT" fused circuit being the fuse that would likely go open in the case of the power tube losing vacuum and flashing over.
At least in my always limited but ever expanding experience and understanding.
I am always learning and it is always my preference to be correctly taught why, rather than discover why not, all on my own! Experience can be a harsh lesson plan if unprepared. I am thankful for the many that share their experience so freely.