Atma-Sphere MA-1 tube rolling

For those who are interested, here is the link to another site, where tube rolling the 6SN7 voltage gain / driver tubes is discussed, as well as Ralph's input:

Atma-Sphere MA-2 Amplifier | What's Best Audio and Video Forum. The Best High End Audio Forum on the planet! (whatsbestforum.com)

• Mar 19, 2019

• #9

Doc nowack said:I’m roiling tubes to fleshout the lower mids and bass. I would be interested in hearing from others concerning there experience with tubes and placement in the amps.

The four tubes in the rear are the voltage amplifier tubes- the two 6SN7s closest to the front panel are the driver tubes.

So the driver tubes, if you want to try rolling, should have -GTA or -GTB suffixes as they are subjected to higher voltages which will shorten the life of a GT.

The voltage amplifier tubes are the most important to the sound of the amp. The voltage amplifier is where all the gain of the amp occurs. It is a differential cascode circuit aided by a constant current source, resulting in fairly high CMRR (Common Mode Rejection Ratio) over 100dB.

There are three 6SN7s in a differential and parallel arrangement which drive the cathodes of the 4th 6SN7 which is the top of the differential cascode. So those first three tubes have the most sonic impact. Yyou don't have to replace all three when auditioning a tube brand (although the cumulative effects will be easier to hear if you replace all three). The top of the cascode IOW has less effect on the overall sound than the bottom, so you replace the first three 6SN7s from the rear panel first, then the 4th.

We've got very good reports of the Psvane and TJ Music 6SN7 variants. Note that they should not be used in the 5th and 6th locations as they don't have the additional voltage rating of the -GTA and -GTB suffix. People report that these tubes have all the best character of the best NOS tubes (Ken Rad being the best, followed by the metal base chrome dome Sylvania after that probably the RCA red base or CBS Zalytron)) without any of the weaknesses! Pretty powerful statements IME.

On tip: have fun- don't drive yourself crazy. I go for a good musical presentation; if I'm getting that I spend the rest of the vintage tube money on more LPs

One more troubleshooting note: I had an awful buzzing/loud humming coming out of one of my Sound Lab speakers. I switched MA-1 monoblocks, and the buzzing switched to the other speaker. Then I proceeded to replace all the output tubes, first seven on one side, then seven on the other, and tested after each replacement. The buzzing was still there. Next, I proceeded to replace the most posterior (sonically important) driver/gain 6SN7 tubes, but the buzzing was still there. Lastly (almost) I replaced the main driver tube, which is in front in the Mk 3.3 model, and the buzzing immediately stopped. I was most surprised that this was the failed tube, as it was a new tube, with less than 30 hours on it, and it was a modern Tung Sol 6SN7 GTB. . . so it appears that currently Ralph’s suggestion to stay away from the Russian 6SN7’s seems to be rather accurate.

I was most surprised that this was the failed tube, as it was a new tube, with less than 30 hours on it, and it was a modern Tung Sol 6SN7 GTB. . . so it appears that currently Ralph’s suggestion to stay away from the Russian 6SN7’s seems to be rather accurate.

If you look on our website you'll find this warning, which has been there for nearly 20 years now

NOTE REGARDING SOME TUBES
We have seen on-going problems with 6SN7s marked as "Sovtek" and "Electro Harmonics" More recently we have seen similar problems in reissue tubes marked "Tung Sol" and "Mullard". We do not recommend these tubes in our products. While they will not damage our products, neither will they perform optimally or reliably. If you are experiencing a problem and any of these tubes are installed, replace them first before doing anything else.

@atmasphere:
Thanks for the reply.  I must admit, I haven't read much of your website, but only the instruction manual that came with the MA-1.  
The amplifiers are a phenomenal match for the Sound Labs.  I'm enjoying them immensely.  Thanks for your great work! 

Hello Ralph (@atmasphere),I'm posting this here so others can share your response; apologies if there is a more appropriate place to ask.I just bought a pair of M-60s, built as II.2s but upgraded by you in 2008 with a PS boost; they have just one switch and one pot on the front.  I cannot get the output to null on either unit, so I am wondering how to proceed.  I checked all output tubes for blown tiny fusible links near the base, and indeed found one ( but only one ) bad tube.  Even if I remove a tube from the other side of that unit, output will still not zero.  On both units, I have to turn the pot fully CCW to get a minimum.  On one unit it's down pretty close to zero, on the other one closer to mid-scale.So:  1) is it safe to run the amps unbalanced, at least for a little while?2) without a tube tester, how do you suggest I find bad tubes?  For example, could I start with just one tube on each side and juggle tubes until I get a balance, then try for 2 on each side, etc?Thanks!