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How does Mesa's Simul-Class work?

5/14/2004 5:47 AM
Cole Davis
How does Mesa's Simul-Class work?
SSIA. Couldn't find anything using "search" but I vaguely remember reading a huge thread on it before. Anyone have any pointers or directions to start?
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5/15/2004 5:01 AM
try the audiocycopedia.. (tremaine) there is a whole explanation of this kind of output stage. Decades before RS 'invented' it. Imagine that!  
Basically, the two different output tube types bias differently, and you take advantage of that. For full output, your input signal on the grid needs to equal the bias.. so if one tube is running -10V bias, a 20V peak to peak input signal should create max output power. If you have a 2nd pair of tubes biased into class B (for ex) maybe at -20V, you won't get their attention until a much higher signal level comes along.. (they'd start working above 40Vp-p)so at low signal you use pair "A", and at higher signal levels pair "B" come on line. That's a *very* general idea. Then you just find an output imp they both like to work into (a good compromise) and you're set... or use different primary taps.. then you get into the effects of having tubes running the same signal in different parts of their operating curve etc etc. . bottom line? IMHO if it sounded that great, everyone would do it and the Simulclass amps would be worth major bucks.  
FWIW I'd spend the money on a Leslie cabinet..
5/17/2004 6:18 PM
Greg Simon
Re: Speedracer?
Hey Speedracer, where could I find a copy of the Cyclopedia? I've never seen it before. Thanks!  
5/17/2004 10:54 PM
perhaps it's in a library near you! (we should all be so lucky!)  
I have 3 local-ish libraries that have it here in CT.  
Else scour ebay, used book stores, etc. An amazing text and worth pretty much whatever you have to pay for it. Seriously.  
5/18/2004 6:09 PM
Greg Simon
Re: thanks Speedracer!
Thanks Speedracer. I found it in a local library, but they don't allow checkout. Theres one on ebay right now so I'll see if I can get that.  
5/17/2004 2:01 AM
CC Re: How does Mesa's Simul-Class work?
Basically he uses 4 tubes in PP, 2 each side. One tube on each side is biased to run class A while the other one each side is biased to be AB1. In other words, on each side you have a class A biased tube in paralle1 with a class AB1 biased tube. Boogies use fixed bias but the same thing can be done using cathode bias as well.  
I have a simulclass boogie and you really don't hear any difference unless you run the amp at very high volumes. At low volumes the tubes biased for AB1 are actually running class A so for all practical puposes at low volume it is a complete class A amp.  
5/17/2004 3:23 PM
Dan McCue

I dont know about the new ones, but the old ones had two wired in pentode and two wired triode strapped.  

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