Just as I get everything cleaned up from my Fisher X-1000 project (and related projects), this sucker showed up on my doorstep yesterday. The mailman was none to happy to put it there, but exceedingly happy once he had. I had similar feelings getting it downstairs to the lab. This is one heavy sucker!
I've never seen or heard one of these amps in person, but have read all manner of glowing sonic and theoretical evaluations, have seen a thread with a rebuild or two, and have even seen a number of threads where work was at least started on one -- or on it's smaller MO-100A brother. But still, while the specs certainly imply there should be good performance, and the sheer mass of the transformers scream that this thing should absolutely get'er done (a little southern speak there for ya), no one that I am aware of has ever taken a stock unit, put it through its paces to document the base performance of the unit, and then developed a plan accordingly to make the best of what it is as separated channels for stereo operation. This thread should go a long way to address this effort, as that is exactly what was requested of this unit.
The initial plan then will be:
1. Thoroughly evaluate the tubes provided, and (hopefully) produce two matched quads of good tubes -- from the batch of 16 that were sent. If we can't get past this step, then the whole scope of the work immediately changes.
2. Evaluate the unit itself, and address any needs to produce proper operation of the stock design.
3. Develop a base line of information regarding power bandwidth, distortion, and frequency response, as well as other lesser informative pieces of information.
When this work is finished, then a workable plan can be developed to maximize the unit for what it is, based on the known performance of the stock unit. Until that is known, any effort of improvement is just shooting in the wind.
For those interested, some basic information might be of value at this point. This unit was designed with two principle objectives in mind:
1. To work as a 200 watt sound distribution amplifier, with the necessary 25 volt and 70 volt connections provided for the constant voltage speaker systems, or
2. To provide a 200 watt 115 vac source of power to operate variable frequency devices such as shaker tables and the like.
Also, a conventional 8 ohm output tap is provided as well. However, one of the greatest features of this amplifier is the duty cycle it was designed to operate under: It was designed to produce full power output on a continuous basis.
When you consider the tubes used, it only takes a pair to produce 100 watts of power output (of plate power anyway). Therefore, to produce (essentially) 200 watts, only 4 tubes are really required. In fact, in the production amp world, there are plenty of 200 watt amps using just 4 6550s, 4 KT88s, and so on to produce that power level.
By using 8 similar type tubes in this amp then to produce the same power level, the necessary current draw through each tube is halved, lowering dissipation levels in each tube under full power conditions, and extending tube life.
I don't want to get ahead of myself, but considering the revised operating environment and cost of these tubes, one possibility is to simply remove a pair out output tubes in each "channel". Each channel would still produce nearly the same amount of power output, because the OPT primary impedance is based on a single pair of tubes producing 100 watts. That's why a quad of tubes in each channel produces no more power than a pair does, but simply halves the current through each tube to produce that power level.
In a different thread, I also mentioned an alternate output connection possibility for a much better 8 ohm match configuration than the 6.25 ohm connection does that is commonly mentioned -- the same alternate connection that Tom Bavis also mentioned from his testing on one of the OPTs used in these units.
Tube testing is underway now, with initial results in the next post. For now, pics include:
1. Get a jack or reinforced table to work on this one. It gives you a workout just moving it around on the bench!
2. This one is in pretty nice physical shape. Plenty of cooling holes provided to allow it to do its intended job on a continuous basis.
3. Not too bad. But somebody has been here before me.
4. The power supply has had the can caps replaced recently, and has been rebuilt in general. But the work is of very questionable quality: For an amplifier of this size and power level, this work is inviting all manner of big and potentially expensive problems.
5. Clearly, something blew up at some point.......
So, my work is cut out just to develop a base line -- but it should be interesting. More soon!
Dave
I've never seen or heard one of these amps in person, but have read all manner of glowing sonic and theoretical evaluations, have seen a thread with a rebuild or two, and have even seen a number of threads where work was at least started on one -- or on it's smaller MO-100A brother. But still, while the specs certainly imply there should be good performance, and the sheer mass of the transformers scream that this thing should absolutely get'er done (a little southern speak there for ya), no one that I am aware of has ever taken a stock unit, put it through its paces to document the base performance of the unit, and then developed a plan accordingly to make the best of what it is as separated channels for stereo operation. This thread should go a long way to address this effort, as that is exactly what was requested of this unit.
The initial plan then will be:
1. Thoroughly evaluate the tubes provided, and (hopefully) produce two matched quads of good tubes -- from the batch of 16 that were sent. If we can't get past this step, then the whole scope of the work immediately changes.
2. Evaluate the unit itself, and address any needs to produce proper operation of the stock design.
3. Develop a base line of information regarding power bandwidth, distortion, and frequency response, as well as other lesser informative pieces of information.
When this work is finished, then a workable plan can be developed to maximize the unit for what it is, based on the known performance of the stock unit. Until that is known, any effort of improvement is just shooting in the wind.
For those interested, some basic information might be of value at this point. This unit was designed with two principle objectives in mind:
1. To work as a 200 watt sound distribution amplifier, with the necessary 25 volt and 70 volt connections provided for the constant voltage speaker systems, or
2. To provide a 200 watt 115 vac source of power to operate variable frequency devices such as shaker tables and the like.
Also, a conventional 8 ohm output tap is provided as well. However, one of the greatest features of this amplifier is the duty cycle it was designed to operate under: It was designed to produce full power output on a continuous basis.
When you consider the tubes used, it only takes a pair to produce 100 watts of power output (of plate power anyway). Therefore, to produce (essentially) 200 watts, only 4 tubes are really required. In fact, in the production amp world, there are plenty of 200 watt amps using just 4 6550s, 4 KT88s, and so on to produce that power level.
By using 8 similar type tubes in this amp then to produce the same power level, the necessary current draw through each tube is halved, lowering dissipation levels in each tube under full power conditions, and extending tube life.
I don't want to get ahead of myself, but considering the revised operating environment and cost of these tubes, one possibility is to simply remove a pair out output tubes in each "channel". Each channel would still produce nearly the same amount of power output, because the OPT primary impedance is based on a single pair of tubes producing 100 watts. That's why a quad of tubes in each channel produces no more power than a pair does, but simply halves the current through each tube to produce that power level.
In a different thread, I also mentioned an alternate output connection possibility for a much better 8 ohm match configuration than the 6.25 ohm connection does that is commonly mentioned -- the same alternate connection that Tom Bavis also mentioned from his testing on one of the OPTs used in these units.
Tube testing is underway now, with initial results in the next post. For now, pics include:
1. Get a jack or reinforced table to work on this one. It gives you a workout just moving it around on the bench!
2. This one is in pretty nice physical shape. Plenty of cooling holes provided to allow it to do its intended job on a continuous basis.
3. Not too bad. But somebody has been here before me.
4. The power supply has had the can caps replaced recently, and has been rebuilt in general. But the work is of very questionable quality: For an amplifier of this size and power level, this work is inviting all manner of big and potentially expensive problems.
5. Clearly, something blew up at some point.......
So, my work is cut out just to develop a base line -- but it should be interesting. More soon!
Dave
this is a thread I've been waiting for.
