Well, I guess its 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 cant 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, its pretty hard for me to tell a lot of difference in the sound of the two amps, the M-120s sounded great, as do the M-150s.
Rogue was supposed to use KT88s with the upgrade but installed 6550s by mistake, so after making sure the amps worked OK I took the new 6550s out and installed the ones out of the M-120s. I talked to Mark and he said he would either swap the tubes free of charge or sell me the KT88s at his cost. I should have the KT88s on Monday. Im using RCA clear tops for the 12au7s and 50s vintage Matsushita 12ax7s. As I said, Im 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-120s yet.
These amps run MUCH cooler than the M-120s did. Im really amazed at the difference there. The M-120s 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-150s 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-120s always glowed a dull red near the wings but in the M-150s 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-120s didnt 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-150s 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 E34Ls to try in them and received them today. Obviously they have not burned in yet but Ill give my initial impressions. I dont know how much less power the amps produce with these tubes compared to the 6550s but its 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 6550s are barely noticeable with the E34Ls. At first I just plain didnt 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 Ive listened to them Ive played some of the tracks Im 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 LPs. I also played a couple of tracks from a Mary Kathryn CD and a Annie Herring CD. The two CDs have some extremely deep bass which sounded very clean and didnt skew up the mids. In all cases the vocals sound a little more natural than with the 6550s. However if the high end doesnt come around I dont think the trade off is worth it. I look forward to getting the KT88s so I can try them out The hard part will be leaving the E34Ls in long enough to get them thoroughly burned in before trying the KT88s. Then, once the KT88s 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 6SN7s in my preamp...
Well, I guess Ive rambled on long enough. Hopefully some of it made some sense...
Have a good day
Brad
