Tracking down a hum.

Sounds like a tube.

If hum occurs only in one channel, swap tube between channels and see the hum follow. if hum stay in the same channel, tube isn't the problem.

If there is hum in both channel, probably a power supply issue.

Try unplugging sources and see if the hum goes away. It did for me.

If it goes away as system warms up, it may be a power supply capacitor issue. 

I noticed a hum from my tube pre amplifier, a Rogue Audio Model 99 (line stage only)... I'm using the Rogue to send a signal to a Musical Fidelity X-P200.

BTW, the problem may be with the power amp, how do you come to the conclusion that the preamp is the culprit?

Possibly a ground loop. The only safe way to fix it that I know is to get an isolation device like an isolation transformer. 

I had a Tung-Sol 6sn7 that developed a very low level static sound well before 300 hours.  Just sayin'.

Hello dierksb.  It sounds like a failing electrolytic capacitor in the power supply to me. The hum starts out bad and eventually diminishes. An electrolytic capacitor is a can of goo that forms bubbles on the inside of the can which forms the insulation. Its taking a while for the bubbles to form and its not so bad that the capacitor begins to act like a resistor and allows dc current to flow through it. At that point they often "blow up."  I recently bought an Arcam receiver which erupted in flames when I turned it on. It blew a 15 amp fuse and smoked up the room for a while. a competant technician should be able to locate the bad cap in 10 minutes or so. Get it checked before it goes.

Change out the tubes, you may have a bad tube.

Try using a cheater plug on the preamp. Could be something else but that's the cheapest option to try. 

Have you placed any new electronic devices near your amp lately or moved any equipment around on your rack?  Cable boxes and satellite receivers are notorious for EMI and feedback.

Good suggestions but I agree with boomerbillone probably a capacitor if the tube swapping does not solve your issue.

Thanks for the repsonses. I think boomerbillone is on the right track.

Go with the tubes first. Always keep a spare set, replace when you suspect a problem, if that fixes the problem throw the old tubes away and buy a new spare set.

What you are describing, I would bet, is the 6SN7 heaters are not powering up together (one tube is heating up faster than the other) and that imbalance creates the hum until both tubes reach operating temperature.

If new tubes cause the same humming problem the next thing to check is the tube sockets and the wiring connection to them.. Sometimes tube sockets lose their grip as tubes get pulled out and reinserted. That would be would be where I start when I open the hood. The next thing I check (on a cold amp) is the continuity of the RCA jack grounds with the power inlet earth ground.

If that does not fix it, then you start swinging for the capacitors.  

Thanks for the responses. I swapped tubes out and powered up the system, same result. Looking at the capacitors, they all LOOKED okay (no bulges or anything). Their are no capacitors in the external power supply so it's something under the hood. Either tube heaters or a bad capacitor somewhere. Fortunately this should not be a prohibitively expensive fix. It goes to the shop tomorrow.

@imhififan  I have swapped out tubes, the condition still persists. I connected my headphone amp to the power (briefly) no hum coming through that I could hear. So I think it's in an output capacitor or in the tubes heaters. It isn't the tubes themselves. 
Thanks for the advice.

Post Script to all this.
I took the amp to a shop for repairs. The techs (2 of them both experienced) could not find an issue. Told me everything in the amp is "up to spec". Gave it back to me no charge. Get it home hook it up and the hum is still there. Only now it is constant. I call Rogue. They say try the dip switch in the voltage regulator. I found it and switched it from H to L and now everything is fine. Hum is gone amp runs nice and quiet. If it happens again. I'll send it off to Rogue.
Thanks again to everyone for your input.

Curious about this H/L switch in the regulator. The only one I can see, and the only one in the manual, is the phono board gain.

Mine is a line stage. Look for a small orange and white switch it's upright and small, near the transformer.
I was able to find it without too much trouble

Well after the weekend the noise is back. It's through the speakers and independent of the volume control. It goes back to Rogue this week with my contact information and a description of the problem.

That's a drag. Interested to hear what's up. 

Sending it to Rogue. I'll post here what they tell me the issue is.

@noromance and others. I got a call from Rogue today. It's repaired. They replaced 6 components. A bunch of capacitors in the power supply and a resistor in the voltage regulator. Cost $255.00 all told. Disaster has been averted no explosions.

I was on the money! 😉

So much for those two 'experienced' techs.

@noroamnce, kudos. I'll know more about which caps were replaced when I see the invoice on delivery.
@thecarpathian. The explanation given to me is they were unable to get schematics with values and were reluctant to work on it as such. Rogue would not provide them either.

Ah, thank you for clarifying.

Okay I got the amp back today. I was hoping for an itemized list of parts replaced. I didn't get one. It looks like the replaced all the capacitors in the board nearest the transformer, and one resistor was replaced. I'm listening now and so far so good.

Well after listening for a couple of hours out of nowhere the noise is back.Evidently the folks at Rogue missed something. This time the noise doesn't dissipate. The noise pulses. get louder then softer louder then softer. I have no idea what the cause is now. I'm thinking a faulty rectifier.
This is frustrating and starting to get expensive.

I had a hum that was caused by a plug in air freshener in another room.  When it cycled on I would get a hum or sometimes a weird buzz.   Thought it was my Amp, it was a tricky one to trace down.  

It would be something else in the room or also some DC on the line causing the transformer to hum.

@ jond I don't think its the line. I swapped out the amp with an old NAD C 340 using the preamp outputs and everything is fine, nice and quiet. No noise in any other components using the same outlet.

This time the noise doesn't dissipate. The noise pulses. get louder then softer louder then softer. I have no idea what the cause is now. I'm thinking a faulty rectifier.

Noise presented on both channel?

For a tube preamp, the power supply seems very complicated! Curious what's inside the external power supply and did you send it in for repair along with the preamp?

This is frustrating and starting to get expensive.

Send it back! Rogue Audio should not charge for a failed repair. 

Oh yeah I sent it back to Rogue this morning. Power supply and all. There isn’t much in the external power supply, a transformer, some wires and a fuse. I don’t remember much else.
Hum is present in both channels and pulses. Failing rectifier maybe?
People are trying to tell me its an issue with my power outlet or a compatibility issue with my power amp. I’m starting to think it’s a ground loop issue. I know it’s not my power amp or my local utility that’s causing the problem.
We’ll find out soon enough. I’m about ready to call in a Priest to do an exorcism!

Hum is present in both channels and pulses. Failing rectifier maybe?

Sounds like low frequency oscillations associated with humming! There could be many things causing this problem, faulty rectifier, diode, regulator IC, capacitor or bad solder joint... Hopefully Rogue will be able to fix it this time. Keep your fingers crossed!

A bad solder joint would not surprise me. I bought this thing used and it had a bad solder joint in the left channel variable output. Got that fixed no problem.Hopefully they'll get it fixed. If they don't I'm going back to a solid state integrated amp.

I had an intermittent noise problem on my main system that I originally attributed to a bad tube. I replaced all the tubes from phono stage, to the recherché rectifier for the phono, to the line stage (old stock DR 6h30s, no fun to source), to everything on the Lamm ML2s. Still the noise intruded. Sent one long interconnect back to Joe Kubala to be reterminated- XLR. That didn’t fix the problem either. Found a wanky tube socket on the Allnic phono stage and replaced it. Nope. Glad I did all this stuff, it was due, but still the noise.

Talking to Vytas from Veloce-- he said swap the battery backs in the line stage and see if the noise changes sides. Well, pulling up one pack, there was a feather of dust on the contact plate. I cleaned it, swapped channels/battery packs-- noise eliminated.

Moral of story for me- it’s usually something stupid.

Good luck- by the time you are done sussing it out, you will have given your system a good going over. Back when I was a car guy, they’d sort it out this way-- get the car to a place where you can open it up- floor it-- if something breaks, you’ll know. If it blows out black exhaust and then starts singing at full chat, you have performed what I learned to call the "Italian tune-up." (With respect). :)

Well today I swapped emails with the people at Rogue. They found a faulty SW 5 switch. This is in addition to replacing 5 capacitors earlier. I need to get a new battery for my multi meter, and a set of precision screwdrivers so I can get at the battery. This in order to check the voltage coming from the outlets. I should have it back early next week.Wednesday most likely.

Well I plugged everything into a different outlet. So far so good. I'll have to give it some time to see if that really was the problem. As whart said. "It's usually something stupid."   It appears jond was right as well.

Story has a happy ending. Since the source of the hum has been found and dealt with It was the power outlet I was using odd in that it didn't affect anything else in the system Combined with the new parts installed by Rogue. This almost 20 year old Model 99 sounds AMAZING! Got the best of both worlds right now.