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!
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!
Under biased tubes in push pull designs will have a shorter useful life apart from audible distortion or any imbalances on imaging.
Under Biased (tube runs hot) you make your amp to distort and clip earlier at a volume control range.
Over biased (tube runs cold) you are somehow altering amp’s operation and crossover distortion can appear even at low volumes. May lead to slightly longer tube life but at the expense of sound quality.
So the perfect line for performance and tube life is somewhere in between.
Just to make things more complicated, some tubes like to be under biased, like 6550, kt66, kt88 and most likely kt120 and kt150, where you get good bass and good headroom. But rule of thumb is never exceed manufacturer’s nominal value more than 5-10%.
Besides that solid information from all above and especially this one from @mulveling and @yogiboy
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.
Make sure the speakers are connected . Just in case you didn't know that!
I just want to say, get the meter you need to bias the amp. It's not very hard at all and I'm a total dummy on electrical stuff. I'm sure the manual tells you how to do it. It's not something to be feared it's just something new to learn. You got this.
you will probably also need a cheap digital voltmeter - you can get them online - not sure of cost in Canada. Easy to bias the tubes, as the other posters have given great instructions. You can probably get the manual online and download and print it. And call them if you have questions.
Probably too late now. The first thing I would do is to Open It Up. Check for over heated resistors, leaking capacitors, burnt wires, dead roaches... And while you are in there change the output ohms tap. 10 years. Might start thinking of recapping the unit.
Test the Tubes. Find someone who has a Tube Tester. Or buy one. Mutual Conductance Tube Tester. You're getting into tubes. Mind as well start getting the tools to support your hobby. Write down the values of each tube. Check them every 6 months to a year. See if the values are deteriorating.
If it has been sitting or stored for a while, I would do a slow power up without the tubes. But you probably don't have a variac. Remove the tubes. I usually put in a smaller rated fuse in but eh, lets chance some excitement. Plug it in, get ready to jump away and flip the switch on. No pop, sparks or flames you doing good. Put the tubes back in.
Checking the Capacitors. I would have done this when doing the internal inspection with a cap tester. Hook up the speakers, turn it on and listen for a 60/120hz hum or buzz as the tubes warm up. Should be dead silent. If you hear anything, probably the filter capacitor. It can get destructively loud. If you see the plates inside the power tubes start glowing cherry red, shut it down. Probably a shorted coupling capacitor or internal short and can damage the tube.
The rogue amp comes with a meter already built into the amp. No need at all for a multimeter. Some of you guys make it more difficult than need be. Why would the person need a multi meter when the amp comes with one built into the chassis and has markings for optimal bias?
While a visual inspection of the inside of the amp is helpful, one sort of needs experience to spot a burnt resistor or a bulging capacitor or other visual signs of distress. Having access to a tube tester is also nice, but decent ones are not cheap.
I don't think testing capacitors makes much sense. Capacitors generally can be tested only if they are removed from the circuit. The only in-circuit test that might indicate a problem is ESR (equivalent series resistance), and this is at best a rough measure. Also, most capacitor testing that can be found on multimeters is essentially useless because they only make measurements at very low voltages. The proper meters are dedicated capacitor/inductor meters and these can be very expensive (common price for a used high voltage capacitor tester is in the $800 plus range).
It is best if you can find a dealer or someone in your area with experience with tube gear to check your amp out. At 10 years of age, your amp is pretty young. My tube amp is about 15 years old, but, it is built mostly with parts that are in the 70-year age range (only the power transformer is "new"). I have not had to so much as replace a tube during this time.
I have had 3 (Tempest, Perseus, ST-100) Rogue products, all excellent, so you should be happy.
With these products, they are simple and cleanly laid out, I think you need to get comfortable with the insides. You will need to check the bias every 6 months, if you are going to change the output impedance, it is a simple wire change (if it is like what I've had), then there is any tube rolling which is on top with yours but requires general comfort with the product.
Good luck with it, I have nothing but positive stuff to say about Rogue and the people at Rogue are some of the best I have ever worked with.
For his future amp purchase? Everybody needs a multimeter, unless you think a light bulb change needs a service call 😁.
@russ69 Quite frankly if and when he upgrades from the already excellent Atlas Magnum, it should be a tube amp with auto-bias or at least similar built-in biasing tools (all Rogue amps have the same scheme). VAC even had a cool color LED-based biasing scheme before they went to full auto-bias. How many tube amps these days actually require a multimeter? It’s completely unnecessary and borderline sadistic. Weren’t you the "time marches on" guy in the coaxial thread? (lol - just ribbin' ya)
Yes, I have a multimeter. As far as my audio gear goes it’s only been needed for extremely dangerous and finicky electrostatic headphone amps. And once or twice to measure a speaker coil I thought was busted (it wasn’t, thankfully).
Agree with others regarding Rogue’s customer service being amazing!
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