Rogue Audio M-120 vs M-150

Well, I guess it’s about time I write my impressions of the amps. Unfortunately it was nearly two months between sending the amps in for the upgrade until I could really do any serious listening. UPS took ten days to get the amps to Rogue and then a week to get them back to me. Rogue took just over two weeks to do the upgrade which was faster than they said so I can’t complain there. And just my luck, just when I got them back I had an infection that constricted my left ear canal and greatly attenuated my hearing in that ear, especially the high end. So, it’s pretty hard for me to tell a lot of difference in the sound of the two amps, the M-120’s sounded great, as do the M-150’s.

Rogue was supposed to use KT88’s with the upgrade but installed 6550’s by mistake, so after making sure the amps worked OK I took the new 6550’s out and installed the ones out of the M-120’s. I talked to Mark and he said he would either swap the tubes free of charge or sell me the KT88’s at his cost. I should have the KT88’s on Monday. I’m using RCA clear tops for the 12au7’s and 50’s vintage Matsushita 12ax7’s. As I said, I’m hard pressed to tell any major differences in the sound but for sure the sound stage is marginally wider and has a little more depth. In terms of sound quality too much time went by to tell any difference, both version sound great. I have not tried them with the stock small signal tubes from the M-120’s yet.

These amps run MUCH cooler than the M-120’s did. I’m really amazed at the difference there. The M-120’s even got the top cover over the transformers to hot to comfortably hold my hand on for more than a few seconds. The cover on the M-150’s only get luke warm, even after playing them loud for a long period of time. The output tubes also run MUCH cooler, putting out very little heat compared to the M-120;s. In a darkened room the plates on the output tubes in the M-120’s always glowed a dull red near the “wings” but in the M-150’s there is no hint of any glow from the plates. My speakers are relatively efficient at 92db/watt so both amps in triode mode will effortlessly play louder than I care to play them with no sign of any strain.

The M-120’s didn’t take kindly to the very high impedance that the powered subs presented to them and the output tubes would start to oscillate at about 2hz when I turned them up very loud and would continue to oscillate after turning them down so I would have to shut them off to stop it once it had started. The cure was a 20 ohm resistor across the output terminals. With the resistor they worked flawlessly and still had more power than I could use. The M-150’s do not do this, no matter what volume I play them at so no more need for the resistors.

I ordered a set of JJ E34L’s to try in them and received them today. Obviously they have not burned in yet but I’ll give my initial impressions. I don’t know how much less power the amps produce with these tubes compared to the 6550’s but it’s still more than I will use. The high end is very noticeably rolled off. A lot of details on the very high end that were very prominent with the 6550’s are barely noticeable with the E34L’s. At first I just plain didn’t like it, sounded almost like my tweeters had quit working but after a couple of hours I got more used to it. The bass is still very strong and dives right down to the deepest notes without ever sounding muddy, just good and clean without overpowering the rest of the music. The midrange is very nice, making vocals sound very good with no hint of harshness even on higher pitched female vocals such as Dolly Parton. In the few hours I’ve listened to them I’ve played some of the tracks I’m most familiar with from some older Dolly Parton, Dave and Sugar, Dire Straits, Pink Floyd, The Kinks, C. W. McCall, Emmylou Harris, and The Who, all on LP’s. I also played a couple of tracks from a Mary Kathryn CD and a Annie Herring CD. The two CD’s have some extremely deep bass which sounded very clean and didn’t skew up the mids. In all cases the vocals sound a little more natural than with the 6550’s. However if the high end doesn’t come around I don’t think the trade off is worth it. I look forward to getting the KT88’s so I can try them out The hard part will be leaving the E34L’s in long enough to get them thoroughly burned in before trying the KT88’s. Then, once the KT88’s are burned in I can compare all three sets to see which I really prefer. Once I get that figured out I may try an alternate brand of whichever type turns out to be my favorite. Then it will be time to experiment with different 6SN7’s in my preamp...

Well, I guess I’ve rambled on long enough. Hopefully some of it made some sense...

Have a good day
Brad

Brad,
thank you for the excellent follow-up.

I'm expecting (hey, do I look radiant or what?) the amps back soon. They've been gone for three weeks.

It'd be interesting to see how the KT88's match against the JAN GE 6550A's.

I'd also like to try some el34's in there eventually.

I tried running the amps in triode, but I found them too mushy and lacking impact so I've been running them in ultralinear. I will try he new and improved 150's in triode and see how that works out. Can youth change the mode while the amps are on?

Funny thing with the 120's, I first got them they would run hot has h-e-double-hockey-sticks, then I switched from the EH to Svetlana tubes, and the heat dropped by at least 75%. It was so bad at first, I had to keep the AC on and the door open! Heck, standing with bare feet on the floor close to the amps would feel hot, and you would not want to put your hand anywhere too close to the tubes.

I have since tried the EH tubes, and they ran fine without too much heat. Not sure what changed to cause that difference. Maybe they had to "break in" first dot dot dot question mark.

KP

KP;

The wait is not much fun, is it? But is sure makes you appreciate them more when you finally get them back...

The 6550’s I’ve been using are the original tubes supplied with the M-120’s. They are Electro-Harmonix 6550EH. They have seen about 18 months of almost daily use and still test very good so I feel like they are holding up very well. The KT88’s will be the same brand.

I had planned to try some EL34’s but I found so many positive reviews about the new JJ E34L’s that I decided to try them instead.

Interesting comment you make about running your amps in triode mode, I always thought mine sounded their best in triode mode, maybe due to the speakers I use. My speakers are a little different than the norm, they consist of a pair of upright powered subs sitting under a separate pair of “mid/high” array’s. The mid/high array’s are internally high pass filtered at 80hz and the subs are driven with the speaker level inputs via jumpers from the mid/high array’s, so the tube amps don’t have to deliver any current from about 80hz on down. I love this setup, the bass is so effortless and natural sounding, nothing flabby about it. The -3db frequency on the subs is 26hz and they don’t drop all the way off until 18hz. When called upon to do so this setup will shake the room with bass so deep it’s barely audible, something I don’t normally expect from tubed amps. It seems to give me the best of both worlds (of course the sub amps are SS but driven by the tube amps).

I have no idea why you M-120’s started running cooler, mine never did. I live in Phoenix and in the triple digit temps I simply couldn’t keep my living room cooled off while running the amps. Once the temp got up past 85° or so in my living room I just couldn’t take it anymore and would shut the stereo down. In the winter I never needed the heater while the stereo was on and in fact had to take steps to keep the room cool enough, even when the outside temp dropped into the 40’s or lower (which isn’t often here).

I hope you will post your impressions when you get the amps back, I look forward to reading it.

Brad

Brad,

I got them yesterday.
I wish I could make a true comparison of before and after, but I made a major change while the amps where gone by adding a wonderful Audio Note DAC.

Like yours, mine came with EH6550's, instead of the KT88's I expected. I have a set of those tubes already,and I can't say I cared for them. Rather dry sound. So I will be sending them back.

Well, I guess I can say it's better since the 6550's didn't sound as bad as I remember them from before. They do run slightly cooler than the 120's (keeping in mind my odd experienced with the 120's as I mentioned above). The biasing is not difficult, even with the do-hickeys placed in the back of the amp they can still be biased through the front.

I found that after all the tubes were biased it was wise to check them all and do another round of adjustments.

Did yours have a lose screw in the front-middle of the case? It seems the face plates don't have the threading for a screw there, but they just dropped the screw in anyway.

I am using GE 6550's now, and I love the sound. But like I said, I can't really do "before/after" with any degree of credibility.

One thing that I really like, is the ultralinear-triode switching on the fly. It was pain before, having to turn off the amps and take out the tubes. That's something else that's different.

Before, UL was clearly better for me, but now it's not so obvious. I'll have to play around with that for a few days before I can decide. Early impression is that the bass seems a touch fuller in UL, but in triode the mids and overall textures appear to be slightly more detailed.

KP

I gave up on the E34L’s, the high end just wasn’t coming around at all and remained extremely rolled off. On songs that previously had very subtle sounds in the high end they completely disappeared. One Dire Straits song, “One of my Friends” where the percussionist is using the brushes very lightly, even that was barely noticeable whereas with the 6550’s it is very distinct and natural sounding. It’s the most drastic change I’ve ever heard from changing tube types. It was as if I had added a tone control and turned the treble all the way down. Maybe if my system had been overly bright before these tubes would have been OK, but my system is not overly bright sounding. I had the E34L’s biased to 39ma as per Mark Walkers instructions. I tried biasing them to 35ma but there was no change. I tried biasing them to 40ma and still no change in the sound but the plates were just starting to glow a dull red so I think that was pushing them a little too hard. I may have to try a set of GE6550’s, although I’m quite pleased with the sound of the EH6550’s, so I may just keep using them. I’m sure there is room for improvement but quite honestly I’m not sure what kind of changes would sound better to me.

I got the KT88’s a couple of days ago but unfortunately one of them didn’t survive the trip so I’m waiting for a replacement. It will be interesting to see if there is any noticeable difference in the sound.

The center front screw on the top cover is not loose on mine, but then I opted to keep the M-120 face plates. I didn’t like the huge “ROGUE AUDIO” engraved on the M-150 face plates plus it saved me a little money. They gave me a great deal on a set of RCA clear top 12AU7’s in exchange for keeping the old face plates.

I guess I had later model M-120’s than you did, the triode/ultra linear switch was on the back of mine just like the M-150’s. I’ve always switched them on the fly without any problems, much easier to compare the sound that way.

Enjoy your new amps, I know I am!

Brad