There are a fair number of Atman-Sphere amp owners among us, so I thought it might be good for me to add my observations regarding 6SN7s to those of @luisma
above. The M-60s require 4 6SN7 tubes per channel, with one of the 4 positions requiring a 6SN7 GTA/GTB tube. The tubes need not be of the same manufacturer and need not be matched. As others have reported, I've found it advantageous to use different tubes in different positions as long as the left and right channels are symmetrical. The M-60s amps are very revealing of upstream components. So it is likely that there will be some variability in what others may prefer.
I've used the following tubes and list them in my order of preference:
1. Shuguang 6SN7GT- These are the cheap tubes that come stock in the M-60's. I don't like these tubes. They have an aggressive top end with a lot of glare. Mid range is unremarkable. Hard to find any redeeming virtue in these tubes.
2. Sophia GTA (the older clear glass tube). I had used these previously in my ModWright Sony CD player and thought they were very good in that application, but they were nothing special in the M-60 amps. Certainly they had a less aggressive top end, compared to the cheap Shuguangs, but they did not distinguish themselves with respect to dynamics, bloom, or anything else. Rather boring, actually.
3. Tung-Sol New production. These tubes are much better sounding tubes than the cheap Shuguangs and the much more expensive Sophia GTAs. I probably could have lived with these tubes had I not also received a warning from Ralph regarding failure rate. Overall, they were smooth, had a bit of bloom, and had a nice extended but not aggressive treble.
4. GE GTA 50's vintage. Overall, these tubes were a disappointment. Similar to the Sylvania sound but not as smooth and grain free. They also seemed to go harsh when presented with high volumes.
5. 60's vintage RCA GTBs. This was a pretty decent tube with what I would call typical RCA sound. Nice rich midrange, average dynamics, relatively grain free. This is a pretty good NOS tube for about the same price as the new production Tung-Sols.
6. 60's vintage Sylvania GTB green label. These feature great dynamics and a really nice high frequency sparkle without being overly aggressive. It would be a toss up with the previously listed RCAs and are about the same price.
7. Sylvania Bad Boys. Similar in virtues to the previously listed Sylvania, just more of everything. Really nice tubes.
8. 50's vintage RCA grey glass. Nice rich sweet tube, average dynamics. This is a tubey sounding tube.
9. Shugauang WE plus. These are very large tubes, even larger than the Sophia Blue Glass. Consequently, I can only get these to fit in the front isolated position, which is least impacted by tube rolling. I can say that these are far better than the GE GTAs one this position and also better than the Sylvania Bad Boys. Too bad I don't have enough clearance in the back row of tubes that would allow me to better judge the sound.
10. Sophia Blue Glass. These things had better be good for the price, and they are. Balanced virtues. Smooth and grain free. Nice sweet treble. Fully fleshed out midrange. This tube, in this application, is the best I have heard.
For now, optimal placement of the 6SN7s in my M-60 3.3s is as follows:
Left rear, RCA grey glass. Center rear, Sylvania Bad Boy. Center right, Sophia Blue glass. Front, Shuguang WE plus. While I am very pleased with the current complement, I will probably pick up one more pair of Sophia Blue Glass tubes for the left rear position as I suspect it will improve upon the RCA grey glass.
The Sophia Blue and Shuguang WE plus are both about 6 months old. Can't say how long they will last, but if they have reasonable life, the virtues of both justify the cost.
above. The M-60s require 4 6SN7 tubes per channel, with one of the 4 positions requiring a 6SN7 GTA/GTB tube. The tubes need not be of the same manufacturer and need not be matched. As others have reported, I've found it advantageous to use different tubes in different positions as long as the left and right channels are symmetrical. The M-60s amps are very revealing of upstream components. So it is likely that there will be some variability in what others may prefer.
I've used the following tubes and list them in my order of preference:
1. Shuguang 6SN7GT- These are the cheap tubes that come stock in the M-60's. I don't like these tubes. They have an aggressive top end with a lot of glare. Mid range is unremarkable. Hard to find any redeeming virtue in these tubes.
2. Sophia GTA (the older clear glass tube). I had used these previously in my ModWright Sony CD player and thought they were very good in that application, but they were nothing special in the M-60 amps. Certainly they had a less aggressive top end, compared to the cheap Shuguangs, but they did not distinguish themselves with respect to dynamics, bloom, or anything else. Rather boring, actually.
3. Tung-Sol New production. These tubes are much better sounding tubes than the cheap Shuguangs and the much more expensive Sophia GTAs. I probably could have lived with these tubes had I not also received a warning from Ralph regarding failure rate. Overall, they were smooth, had a bit of bloom, and had a nice extended but not aggressive treble.
4. GE GTA 50's vintage. Overall, these tubes were a disappointment. Similar to the Sylvania sound but not as smooth and grain free. They also seemed to go harsh when presented with high volumes.
5. 60's vintage RCA GTBs. This was a pretty decent tube with what I would call typical RCA sound. Nice rich midrange, average dynamics, relatively grain free. This is a pretty good NOS tube for about the same price as the new production Tung-Sols.
6. 60's vintage Sylvania GTB green label. These feature great dynamics and a really nice high frequency sparkle without being overly aggressive. It would be a toss up with the previously listed RCAs and are about the same price.
7. Sylvania Bad Boys. Similar in virtues to the previously listed Sylvania, just more of everything. Really nice tubes.
8. 50's vintage RCA grey glass. Nice rich sweet tube, average dynamics. This is a tubey sounding tube.
9. Shugauang WE plus. These are very large tubes, even larger than the Sophia Blue Glass. Consequently, I can only get these to fit in the front isolated position, which is least impacted by tube rolling. I can say that these are far better than the GE GTAs one this position and also better than the Sylvania Bad Boys. Too bad I don't have enough clearance in the back row of tubes that would allow me to better judge the sound.
10. Sophia Blue Glass. These things had better be good for the price, and they are. Balanced virtues. Smooth and grain free. Nice sweet treble. Fully fleshed out midrange. This tube, in this application, is the best I have heard.
For now, optimal placement of the 6SN7s in my M-60 3.3s is as follows:
Left rear, RCA grey glass. Center rear, Sylvania Bad Boy. Center right, Sophia Blue glass. Front, Shuguang WE plus. While I am very pleased with the current complement, I will probably pick up one more pair of Sophia Blue Glass tubes for the left rear position as I suspect it will improve upon the RCA grey glass.
The Sophia Blue and Shuguang WE plus are both about 6 months old. Can't say how long they will last, but if they have reasonable life, the virtues of both justify the cost.