can you please help me around how to use my first tube amp?
Hi, I picked up on a whim an older tube amp, first I have ever had that is non automatic biasing and outside of that really no nothing about tube amps
Its a used 10 year old or so Rogue Audio Atlas Magnum power amp with KT 120 power tubes
the pre amp/dac will be RME adi-2 and speakers are Sonus Faber Lumina V's (4ohm)
I was thinking I would also also try it without preamp from a bluesound node
I believe the stock setting is 8ohm on the amp and you need to crack the lid to adjust down to 4
nervous about installing and biasing the tubes
can anybody advise what happens if I run the amp without the tubes being biased correctly?
also, can I run this from the 8ohm default so I dont have to open it up?
finally, how do I know when i need new power tubes and pre tubes?
any other advice will be appreciated
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. 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! |
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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
Enjoy your new tube amp. |
