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RE: Atma-sphere M-50 history

Posted by Ralph (M) on May 16, 2012 at 09:05:17

In Reply to: Atma-sphere M-50 vs M-60 posted by vince saunders on May 15, 2012 at 14:07:57:

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.

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