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Re: How does Mesa's Simul-Class work?

5/17/2004 4:29 PM
bob p Re: How does Mesa's Simul-Class work?
"I dont know about the new ones, but the old ones had two wired in pentode and two wired triode strapped."
in at least some of the amps (like the Mk IV) you can independently select triode/pentode modes as well as Class A/Simulclass.  
IMHO one of the frustrating things about the Boogie SimulClass arrangement in the Mk IV is that you're somewhat limited in what you can do with it -- you can have Class A or Class A + Class AB1, but you can't have exclusively Class AB1 with no Class A.  
If you do the tube swapping thing (putting EL34 in the outer sockets and 6L6 in the inner sockets), its possible to play EL34 only, or EL34 + 6L6, but its impossible to play 6L6 only.  
While the combination of SimulClass 6L6 or 6L6 plus EL34 provides some very cool tonal options, the amp does suffer from the lack of ability to get really loud and clean AB1 tones from the 6L6. You're always stuck with the Class A 6L6 or EL34 tone as part of the mix. IMHO, this is the achilles heel of this type of amp design -- even though it may have the same power tube lineup, its no substitute for a cranked Twin Reverb.
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5/17/2004 4:19 PM
bob p
"I have a simulclass boogie and you really don't hear any difference unless you run the amp at very high volumes."
I have a Mk IV and that's been my experience too.  
Many people think of Boogie amps like the Mk IV as having their character based on preamp distortion. While I like to take advantage of the preamp distortion to use my Mk IV as a versatile low volume practice amp, the Mk IV behaves as a completely different animal when the power section is opened up.  
Lots of people make the mistake of comparing Mesa amps at low volume to other amps at higher volumes. To make the comparison fair its important to open the amp up a bit, as many of the features in these types of circuits can't be appreciated at bedroom levels.  
Just my two cents...
5/17/2004 7:07 PM
Cole Davis

Thanks for all the info guys. I'm going to take some time to decode Speedracer's answer.
5/17/2004 7:37 PM
How to mod Mesa MKIII for bias adjust.
I have a MKIII simulclass with fixed (non-adjustable) bias. Has anybody got experience modding these amps? What are the issues. How hard is it? Are there any schematics or "how too's" out there on the web?  
5/17/2004 11:04 PM
Carl Gigun
Re: How does Mesa's Simul-Class work?
I remember reading the patent for it when I was first getting into this. I don't have the link, but it's out there if you search. Explains it in lots of detail and schematics.  
Pretty simple idea though- two pairs of output tubes, one pair biased class A, the other pair biased AB. The idea being the class A pair would become overdriven before the AB pair, making the overall overdrive character more like a class A amplifier. For this to seem ideal you probably have to believe the mojo that class A = good sound and class AB = bad sound, but that's another story.  
If that doesn't make sense yet, what you're really needing to know is "what does class A or class B mean?" and that can be answered with a visit to  
-Carl Gigun
5/17/2004 11:07 PM
Cole Davis

Unfortunately the free IBM patent site is gone, I hadn't checked it in years and found it got taken over by some other company. The cheapest search option is something like $99 a month!
5/18/2004 2:25 PM
bob p
USTPO is free.

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