MAC Autoformers?

In addition to Brooks, John Ruttan at Audio Connection used to sell Vandersteen and MR combos and IIRC correctly it was the RM-10 and 2s. Given the new cost of the RM-200 a used one for under $3k is quite a bargain. It should also be noted that the RM-200, unlike most vacuum tube amplifiers which lose power while the speaker load impedance drops, increases output power as the speaker load becomes more challenging. It is stable down to 1 ohm and includes a 1 ohm tap.

While light loading sounds better to me, Roger would also be the first to say use the tap that sounds best to you. There is no reason to not experiment. It's a cheap and easy tweak.

If Ralph (Atmasphere) picks this post up, ... what is the tube life is in your amps. Btw Ralph, …. Roger mentioned a couple of times that negative feedback, *if used properly*, is not all that bad a design feature. Not taking a position, just passing along what RM said.

10,000 hours is typical- so we warrant the power tubes for a year on this basis, and always have.


I agree about the feedback 'used properly' (which many designers do not) comment. Proper application of feedback is tricky to say the least, and may not have been possible until the age of personal computing, due to the number of variables involved. Here is a nice primer on the topic:http://www.normankoren.com/Audio/FeedbackFidelity.html

one thing I greatly value is a collegial, challenging, supportive and sharing attitude in the user AND supplier community....I did not know Roger did the bias forming....also..so I will add to my list of collaborative yet competitive firms RM, Vandersteen, Audio Quest, Aesthetix, Brinkmann, ......I am sure the list goes on....

10,000 hours is typical- so we warrant the power tubes for a year on this basis, and always have. 

Ralph, What is your dissipation figure as a percentage of rated?  How do you figure in high current peaks when the amp is used at full power?

I agree about the feedback 'used properly' (which many designers do not) comment. Proper application of feedback is tricky to say the least, and may not have been possible until the age of personal computing, due to the number of variables involved. Here is a nice primer on the topic:http://www.normankoren.com/Audio/FeedbackFidelity.html

I have spoken with DImitry and a group of people trying to improve the tube models for SPICE. He put the parameters of the RM-10 into their SPICE program and got results that did not even come close to what an RM-10 actually does, and this is only for the midrange where the output tranformer is considered perfect. He did not attempt any feedback analysis as the output transformer is almost impossible to model where it matters.

I know of no tube amp designer who uses SPICE.  can you name a few?

 Since the RM-200 produces about the same power on both taps, what possible reason is there for NOT hooking up a nominal 8 ohm speaker to
the 4 ohm tap? Thanks---Eric. 

This is something that needs clarification. The reason we have taps on tube amplifiers is to get the full rated power into different loads. That means when we test an amplifier it produces full power into a load of the same resistance as the tap. Indeed hooking a 8 ohm load to the 4 ohm tap results in reduced power. Usually though only a 30% reduction, not 50% due to other factors (reduced loss in the output transformer, power supply and tube saturation voltage). So a typical 100 watt amp puts out 100 watts into any matched tap and somewhere around 75 watts into a tap mismatched by one step. In this case the tube are loafing along, distortion is reduced and damping increased. But this requires that the load does not go significantly below the tap impedance.

However going in the other direction where the load is lower than the tap impedance bad things happen. In that case the amplifier puts out less power, works harder and the tubes get overly hot to the point or radically shortening their life.

What an RM-200 does is to go into AB2 mode in the above case The tubes stay happy and the reduced load gets extra power in the same way a transistor amp gives more power into a lower load.