MAC Autoformers?

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. 

Read this recommended article with caution or not at all.

:http://www.normankoren.com/Audio/FeedbackFidelity.html

It has many errors and misconceptions. Not a good lesson on feedback. 

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

Nope. Maybe one? I know I don't! Victor Khomenko was trying to model our MA-1 in Spice many years ago before he became part of BAT. He called me up saying that the amp didn't run in Spice, but I had to point out to him that the amp actually worked despite what Spice came up with :)

As you can imagine, the ratings of the tubes tend to vary with load- and the amp will draw less power and run less heat if a higher impedance load is used. Also, as you've pointed out, the class of operation is affected. In our case the amp is biased to be class A2 on the proper load, but will be class AB2 if the load impedance gets low enough. We don't get crossover artifacts on that account though.

As far as high current spikes, at full power the tubes still have some dissipation left over, so they can handle spikes, however as the load impedance is reduced, eventually this will cause the cathode coating to fail.

Ralph,
Thanks for your answer. I think most of us develop amplifiers in real space verses cyber space. SPICE, by its name, was indeed designed to help people design ICs not tube power amps. 

The thing that concerns me about 6AS7s, it that they emit cathode sparks in my Tektronix 530 curver tracer at zero grid.That is consistant with your statement of cathode coating failure as it appears the sparks are bits of cathode coating. They actually look like sparks off of a 4th of July sparkler.