I've used an F-1X faithfully for years and now there's an intermittent very loud popping that occurs. I know this preamp uses a tube. Is this a common symptom of the tube going out or something more serious? Thanks.

Have a couple of questions to understand what's going on a little better.
 
After the pop, does the signal return immediately or does it fade back in?
 
Need to get an idea of the frequency of popping. Does it happen right away and then once every x minutes? Is it truly intermittent and has no predicatable pattern? What's the most number of pops you've experienced in a given playing period?
 
The tube is easy to replace if it's needed, but I'm not entirely convinced that's the needed action in your case yet.

After the pop, which is quite loud, there is usually no signal, and then it will fade back in after a few seconds. The pops generally don't happen right away. Usually the unit has been on for a while. And I've had several instances where, with the preamp on but unused for some time, the pops get semi-rhythmic, and begin pulsing, although I don't think the time distance between them is consistent. I've had it pop just a couple times and go away, and then I've had it where it starts pulsing like I described. I have noticed, since the unit is loose in the rack at the moment, that if it starts the pulsing some movement of the unit sometimes stops it. Thanks.

Any chance you are using a Stewart Amplifier with your F1-X?
 
Alemboid

No, not using a Stewart amplifer.

Another question for remote troubleshooting: does the popping sound happen every time after warm up, or does it happen that sometimes you can use it and have no popping?
 
The fact that moving it sometimes stops the problem, it suggests a loose connection. I have some advice for checking for a loose connection, but first I'm going to take a picture to help illustrate the concepts.  
 
NOTE: there is dangerous high voltage in the F-1X. Do not attempt to troubleshoot if you do not have experience with electricity.
 
Ponder my warning for a while then I'll take some pictures and post when I'm back from lunch.

What's the problem with using a Stewart amp?

Nothing is wrong with using a Stewart amp as long as it is a 1.6 or a 2.1. I used to use a 1.2, and I used to get loud pops, loud enough where I thought I was going to blow a speaker. This happened regardless of volume, so I checked my F1-X through a different amp and it was ok. Eventually the Stewart pretty much blew up, so I called Stewart. They stated that the 1.2 isn't quite well enough equipped to handle a lot of low notes, especially if you are playing a low B. The 1.6 and 2.1 operate much better for bass guitar applications. We don't recommend the 1.2 for bass (yet, many stores will sell you one to use for bass). They were really cool about it. They repaired my amp, and I switched to a Mackie 1400 and never looked back. It was the eventual crapping out of the Stewart that manifested itself as several loud pops that I thought you may be experiencing- if you were using a Stewart 1.2....that's why I asked.
 
Alemboid
 
The Stewart sounded awesome, but I never knew it was a time bomb!

The popping does not happen everytime after a warm up. I may be able to use it one day without any popping at all. I do have electronics experience and a loose connection also seemed a possibility, but I wanted to check with you first to see if you've heard of this before. Any photographs or ideas on where to check would be appreciated. I have already popped the hood and checked for general tightness of connectors, but did this with power off.

Here's the picture I took right after lunch:
 

It illustrates using something non conductive, like a Sharpie pen to lightly tap on the connectors while the F-1X is plugged in. You'll hear it if something is loose in one of the connectors.
 
I also made a short movie of the tapping, to show only light tapping is required. Again, do be extremely careful if you have the F-1X powered and the cover off.
 

Cool movie!!!
 
(I'm so easily entertained )

Bill-  
 
I think Mica's point about voltage includes with the power off.  I don't know what's inside the F-1X, but capacitors can store power after they have been unplugged and let loose when touched.  Power off and unplugged may not guarantee no electricity present.
 
-Bob

I did as you recommended, tapping the connectors with the cover removed and the power on, and got no pops of any kind. I put it back together and used it for an hour and got a single pop out of it while using it. Just now it started popping and crackling continously. I bumped the case, and it stopped.

I'm guessing there aren't any more suggestions. How do I go about sending the unit in for service? Thanks.

I had the same EXACT problem with my (now returned) Stewart 1.2.
 
stoney