can you please help me around how to use my first tube amp?

Best way forward is to call Rogue and ask for a rundown. They’re exceedingly helpful. But following is my rundown:

I assume the amp has been run with these KT120 for a while? The bias will have been adjusted for each specific tube, so you need to know the specific slot each goes in. But really, I wouldn’t assume that is correct. If you don’t "know" the right order then the safest procedure is as follows:

• Open up the hatch plate exposing 4 blue trimpots and 4 silver toggle switches. The tiny gold screws on the trimpots are what you’ll be adjusting.
• Crank DOWN the bias ~ 3 - 6 rotational turns (of the gold screw heads) on each of the 4 trimpots, using the supplied "pen" tool. Note that on these trimpots, clockwise rotation turns the bias DOWN; counterclockwise turns the bias UP. It’s safe to go extra DOWN when you are using new or unknown tubes for the first time!
• Insert the tubes (you can do this part before or after the prior step).
• Power up.
• Let it warm up a few minutes. Watch for a too-hot bias (should not happen since we turned the bias way down as a precaution): If any one tube has its large gray plate (not the heater filament inside the tube) start glowing with a large orange patch, that’s too hot. If it goes cherry red that’s WAY TOO HOT - shut the amp down immediately and seek further advice.
• Use the toggle switches to start measuring the tubes in sequence; only engage one toggle at a time when measuring. While you’re measuring a tube, start turning the bias UP (counter-clockwise) on its trimpot with the goal of matching 35mA or 40mA (check with Rogue - but the KT120 can handle 40mA and even 45mA no problem). Then shut the toggle off and move to the next tube, left to right. After you complete this sequence for 4 tubes you’ll need to go back to the 1st tube and adjust again (note: it will be lower since you adjusted the other 3 tubes!). It’s an iterative process.
• After 20 minutes of warm up (since turn on) the tubes should be fully stable - do your your final round of adjustments.
• The trimpots and toggles aren’t the most robust parts in the world, so don’t get into the habit of being OCD and adjusting them daily. It’s not necessary with good tubes, and a variance of 1 - 2mA in your bias readings doesn’t matter anyways.

You can use the 8 ohm taps on 4 ohm speakers - it won’t hurt anything and will probably sound good, but you will lose some effective power output and probably slightly raise distortion too.

Troubleshooting: Along with 4 trimpots and 4 toggles, you’ll also see a big glass fuse for each tube slot. These will blow if a tube suddenly draws too much current. This can happen, especially with new tubes which sometimes intermittently burn off impurities while they’re still new. The fuse will flash white, but you won’t see this if you’ve covered it with the hatch plate. On the Atlas, having 1 tube out should be very audible as distortion in that channel. When you go to measure that tube, it will register 0 mA. That’s how you know for sure. Replace the fuse and try again, or call Rogue. I have Apollo Dark, and when a single tube fuse blows it is NOT immediately obvious, because those amps have three pull-pull pairs of KT120 per side, so the other 2 pairs are covering for the third one!

hi, congrats on an excellent sounding amp!  i had a rogue cronus magnum ii and loved it.  

dont be intidated about bias adjust, it is easy.  just let the amp warm up for 20 min or so, remove the screws on the small window plate.  

then flip the switch for bias check of each tube, read the meter and use the small screwdriver to adjust the blue potentiometer screw until the meter reads the correct current per the manual.  

for the speaker taps you need to remove the tubes and chassis cover.  you can see the wires going to the terminals, just loosen the nuts with an open end wrench and put the wire on the other terminal.  8 ohms will sound best with most speakers but it is worth a try to see how the other tap sounds.  good luck!

this is awesome feedback

thanks so much

what happens if the tubes are not biased?

it may sound unhalanced left to right or not as strong and powerful as it could.  if they are over biased the tubes could burn out prematurely.  

also it is a good way to spot a bad tube.  a bad tube cannot be biased to the correct current.  

definitely warm up the amp and at least check the bias of each tube by flipping the individual switches on and off and read the meter.  the manual says that they do not have to be perfect so you might not need to adjust the bias but i recommend it

An imbalance in bias within a push/pull pair (the two KT120 on the left form one such pair; the 2 on the right are the other), or far too low of a bias, will cause audible distortion. It can vary, but expect either a scratchy or choppy kind of incoherence to the sound from the affected channel(s). It’s the opposite of sounding "clean"! Like I mentioned earlier, any biasing issue in the Atlas should be immediately audible - unlike in the Apollo (and to a lesser degree M180), which mas multiple redundant push/pull pairs! 

Careless biasing could also possibly cause a level imbalance between Left and Right pairs, but in my experience that’s far less likely to be severe enough to be very audible, versus an imbalance within one or both pairs causing obvious distortion.

It is a very NICE feature that the Rogue tubes amps allow individual biasing of each tube. Other amps require matched pairs or quads of tubes in order for the push/pull "balance" to be correct. Of course I still buy matched pairs/quads anyways haha.

As @avanti1960 mentioned, stability in the bias point, once set, is a good indication the tube is solid. You’ll want to check the bias periodically over the next few sessions to look for tubes which may not be holding bias (I mean a swing of more than 5mA, not the usual normal variance of 1 - 2mA) - they should be replaced immediately!