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Atma-sphere M-50 vs M-60

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Posted on May 15, 2012 at 14:07:57
vince saunders


 
Would like to know what the differences are between the two. I want to say there was a difference in the input tubes and some power but not sure. Thanks for your help.

 

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RE: Atma-sphere M-50 history, posted on May 16, 2012 at 09:05:17
Ralph
Manufacturer

Posts: 2180
Location: Minnesota
Joined: April 24, 2002
 The original M-50 used a different power transformer for the output tube B+. Pretty quickly we found out that the B+ transformer from the MA-1 made it sound better, so that was offered as an option. Eventually all the amps made had that option and so it became the M-60.

Then the MkII series came along, the main difference being that they had the octal base driver circuit. It takes some doing, but this can be incorporated into the older amps. In fact we've updated them all the way to Mk. 3.1.

The M-50s have been holding up well, the biggest problem being that the bias and DC offset test switches seem to get gummed up. If they don't get damaged, they can be cleaned with Radio Shack spray contact cleaner and they seem to come right around. The other big problem is that the tubes go downhill and then the amp doesn't sound right. That seems to be biggest reason for them turning up used.

The amp was originally intended to be entirely circuit-board based. It took a while to get the PCB on line, so early units are hand-wired, and then it turned out that the cost of the board was high enough that it was about the same cost to do the hand-wiring anyway, so later units are hand-wired as well. The circuit boards hold up quite well- the traces are heavy and the board is 1/8" so there have been no problems with it overheating.

There turned out to be a problem with the original B+ transformer (an EI core made by Subtronics of Minnesota). The transformer was rated correctly and met spec on the bench, but began random failing due to a construction problem with the windings which caused them to short going around a corner in the core. Most of these were replaced 20 years ago using the MA-1 toroid. If we ever see any of these that come in for service we usually replace the transformer regardless; even after 2 decades a power transformer should not be dying without provocation! Obviously we've not done business with that vendor ever again.

In terms of production the M-50 is rare compared to M-60s. There were about 60 pair produced before morphing into the M-60 (the last M-50 is serial number 175). The serial number is stamped below the IEC power entry connector. About 70% were sold domestically with about 25% going to Europe and 5% elsewhere. The usual complement of internals is Resista metal film resistors and custom-built polystyrene coupling caps. Some have been optioned with Caddock resistors but this was much more commonly seen in MA-1s of the period.

Quite often when M-50s show up used they don't go for a lot of money compared to later M-60s. As such they can be quite a bargain as with only minor updates they can be real performers.

 

RE: Atma-sphere M-50 history, posted on May 16, 2012 at 12:59:43
vince saunders


 
Thanks so much for all the info on the M-50. I appreciate it very much Ralph. I have been keeping an eye out for a pair. They are beautiful amplifiers with the gold faceplates. You did a great job in the design. Maybe a pair will show up soon and if so I will get with you on a complete overhaul.

Are there still replacement parts for these amplifiers such as faceplates and other items ?

 

RE: Atma-sphere M-50 history, posted on May 22, 2012 at 18:21:07
baileyler
Audiophile

Posts: 217
Location: Chadds Ford, PA
Joined: January 17, 2007
 Not for sale, but here are a couple of pics.









May not be a better condition pair still in existence. Mine are #150.

I was told they were upgraded to M-60 Mk1 status many years ago, but I have never actually confirmed that (Ralph - do you have any record of the upgrade).

This does have a circuit board.

I upgraded the caps to VCap. Interestingly enough, the amps when I received them were absolutely identical (as far as I could tell) except for the original caps - there were different values in each amp for some reason.

Ralph is right about the switches - on one amp they work OK (after spraying with cleaner) but the other has issues (still useable but with a lot of effort).

Other than that, they seem to work perfectly and are very stable (and sound great).

The only other issue I can find is that one of the "Power Available" LEDs is starting to dim (they are more than 20 years old after all).

 

RE: Atma-sphere M-50 history, posted on May 23, 2012 at 01:18:22
vince saunders


 
Good morning Baileyler,

Thanks for the photos. Beautiful, just beautiful ! Got to love the brushed gold faceplates. I bet they sound great. Those are keepers.

How are your tubes holding up in them ?

 

RE: Atma-sphere M-50 history, posted on May 23, 2012 at 04:23:45
baileyler
Audiophile

Posts: 217
Location: Chadds Ford, PA
Joined: January 17, 2007
 Hard to tell about tube useage, since I only play them a few hours per month (I rotate between a few amps).

I got this pair about a 1 1/2 years ago (I previously had another set) and, at that time, the tube set in them had apparently been there for a number of years. I switched out a couple of lower testing ones (plus the small singal tubes) and have been using that same set (with one exception) since then.

I did recently test the power tubes in one of the amps, and they all were still good with the exception of one that had some slight intermittent leakage.

Good thing is that this amp is a LOT cheaper to retube (when needed) than a 2A3 parallel push pull amp that I have.

 

RE: Atma-sphere M-50 history, posted on May 23, 2012 at 12:04:35
Ralph
Manufacturer

Posts: 2180
Location: Minnesota
Joined: April 24, 2002
 The serial number log does not indicate anything about the upgrades to the amp but for one this old that is not surprising.

BTW your power supply covers are on incorrectly- the slots should appear on top rather than to the rear.

 

RE: Atma-sphere M-50 history, posted on May 23, 2012 at 12:24:47
baileyler
Audiophile

Posts: 217
Location: Chadds Ford, PA
Joined: January 17, 2007
 Strange on the covers - I have had two pair of M-50s and both came to me with the vents as seen in the photos.

At least that is something I can fix.

Anything I should look for in particular that might indicate an upgrade to M-60 Mk 1 status?

 

RE: Atma-sphere M-50 history, posted on May 23, 2012 at 13:53:10
vince saunders


 
Ralph can you tell us the probable tube life of these amplifiers if all is working correctly ? Thanks.

 

RE: Atma-sphere M-50 history, posted on May 23, 2012 at 13:54:18
Ralph
Manufacturer

Posts: 2180
Location: Minnesota
Joined: April 24, 2002
 If its got the toriod you will see it when you correct the cover. There will also be a mounting bolt head visible on one of the power supply side panels.

 

RE: Atma-sphere M-50 history, posted on May 23, 2012 at 14:04:35
Ralph
Manufacturer

Posts: 2180
Location: Minnesota
Joined: April 24, 2002
 There are a lot of 'ifs', if the tubes are clean, biased right, warmed up right, run on a moderate load with moderate powers, if there is good air circulation, etc., then 10,000 is the expectation. We have seen this happen a lot over the last 20 years. Sometimes a tube is marginal from the factory and does not hold out all that long, other times the tubes go well in excess of that- we've seen over 20,000 hours too.

 

RE: Atma-sphere M-50 history, posted on May 23, 2012 at 16:57:23
baileyler
Audiophile

Posts: 217
Location: Chadds Ford, PA
Joined: January 17, 2007



They look to still be M-50s, as they have the Sub-Tronics transformers. I guess I need to have a talk with the guy that sold them to me.

It does, however, look like someone has been under the hood as some of the caps look to have been replaced at one point.





The other block has an additional dark blue cap (bottom right one).

That would at least explain why the covers were on the wrong way.

I would consider sending them in for a checkup and possible upgrade, except that I am not willing to ship (they are in too nice shape). As much as I try, there is no more "handle with care" by any of the shippers.

They actually work well now and are stable, so I will probably keep them going this way as long as I can.

Ralph - ever come to the Philly area?

 

RE: Atma-sphere M-50 history, posted on May 24, 2012 at 07:50:40
Ralph
Manufacturer

Posts: 2180
Location: Minnesota
Joined: April 24, 2002
 In the bottom photo, I see that the older method of mounting the meter is used. This can result in the meter becoming inaccurate if the mounting is over-tightened. The meter should appear to be a little loose in the front panel. If not, loosen the nuts on the backside to make it so. A drop of locktight or fingernail polish will keep them from moving.

It does look like a cap got replaced in the output section. Otherwise the condition looks great!

 

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