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MAC Autoformers?

Someone is selling a MAC MA6500 Integrated claiming its superiority over the Ma6600 due to the fact that "it does not have the degrading autoformer design found in the MA6600". That is the first time I've heard a claim that the autoformer was a hindrance to better performance; I thought quite the opposite. What do you MAC Maves think?
pubul57
bdp24's avatar
bdp24
10,008 posts
 

Speaking of Richard and Roger, Roger says the idea to install the Capacitor "Forming" function into the MK.2 version of his Music Reference RM-200 amp was a suggestion from Richard. Brooks Berdan sold a lot of RM-9's and RM-200's to owners of Vandersteen 2's and 3's, a great matchup.

For the price of retubing an ARC Reference 150, you can just about buy a used RM-200! The four output tubes run in the RM-200 (producing 100w/ch) last about five times longer than those in the REF 150. How many 150 owners have compared their amp to the RM-200? I’m guessing none. You can buy a used PAIR of RM-200’s for less than a single used Ref 75! There’s one on AudiogoN right now for less than $3k, a ridiculously good deal. I have all the power I need from the one I already own or I’d buy it.

clio09
3,360 posts
 
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.
atmasphere's avatar
atmasphere
11,992 posts
 
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
tomic601's avatar
tomic601
13,111 posts
 
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....


ramtubes's avatar
ramtubes
460 posts
 
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?
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