Can a tube short without blowing?

I had a Mullard xf2 with maybe 1000 hours glow bright red and left channel went out. Opened amp up it was a blown fuse. Did not have extra fuses so called Conrad Johnson to order some.
 

What shocked me was CJ told me to put the same tube back in that red plated when I get fuses in, spayed tube most likely still good. This was a little shocking I asked them a few more times are they sure this can’t be. My fuses came in put bad tube back in and it has been running just fine for last year or so. To this day I still don’t understand but all I know is I didn’t have to buy a new tube,

@atmasphere @paulcreed  Fuse weirdness in play!  I replaced the tube fuse and all was well for one day then the AC fuse blew. So I put the stock front-end (preamp) tubes back in and replaced the AC fuse.  All good now for a couple days. I need to put the tubes back in that started it all to see if I’m in a similar situation as you were. 

Yes, it is possible for a vacuum tube to develop a short circuit without actually failing or visibly showing signs of damage. A shorted tube can indeed cause a fuse to blow, as the short circuit creates an excessive current draw that the fuse is designed to protect against. When the fuse blows, it interrupts the flow of current to prevent further damage to the amplifier circuitry.

In your case, where the left channel tube fuse has blown, it's a clear indication that there is an issue with the left channel, which could be caused by a shorted tube, a wiring problem, or a component failure in that channel's circuitry.

As for the preamp section tube (6922), if it's at or near its end of life (EOL), it may cause issues such as noise, distortion, or instability in the amplifier's performance, but it's less likely to directly cause a fuse to blow unless there's a severe internal short circuit in the tube itself. In most cases, a tube reaching its end of life will gradually degrade in performance rather than suddenly causing a fuse to blow.

To troubleshoot the issue, you can try the following steps:

1. Replace the Blown Fuse: Start by replacing the blown fuse with a new one of the same rating. Make sure to use the correct fuse type and rating as specified in the amplifier's manual.

2. Tube Inspection: Inspect all tubes visually to see if any of them show signs of damage or arcing. If you suspect a particular tube may be causing the issue, you can try swapping it with a known good tube from the other channel to see if the problem moves to the other channel.

3. Check Wiring and Connections: Ensure that all wiring and connections, especially those in the left channel, are secure and not damaged.

4. Seek Professional Help: If the issue persists after checking the above steps, it's advisable to consult with a qualified technician or the manufacturer's service center for a more in-depth diagnosis and repair. They can perform detailed tests and measurements to pinpoint the exact cause of the problem and perform any necessary repairs or component replacements.

In any case, it's important to exercise caution when working with high-voltage equipment like tube amplifiers, and if you're unsure about any aspect of the troubleshooting or repair process, it's best to seek professional assistance to ensure safety and proper resolution of the issue.