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I don't think you're actually pulling tubes by switching between class A & Simulclass. At least I could not find anything in the manual which states an impedance change.

http://www.mesaboogie.com/manuals/Mark%20III.pdf
 
I agree with you souelle. You are absolutely right about the impedance changing with pulled tubes. I've pulled tubes in the past with my Marshalls and others, and set the correct impedance as needed. I just figured that Mesa took it into account (because of the lack of mention in the manual) and some way set the correct impedance automatically when switched. I've never changed anything when I switch back and forth between Class A and Simulclass. So far I've not had any issues with over heating, excessive hum, etc from the tranformers. Hopefully others with more knowledge of the design/setup from these superb quality amps will chime in and confirm things for us. I certainly would not want to blow a tranny either (been there, done that).

:D
 
souelle:

Good questions,

I just assembled an Allen kit 5F1+ (off shoot of a Fender Tweed Champ) which house a single 6V6 (Class A). I e-mailed David Allen since he claimed this amp could use any power stage tube (well 6L6, EL34) about using a 6L6. He sent me the appropiate cathode resistor and capacitor to change but then he said although 8 ohm speaker is fine, running 4 ohms with the larger tube (6L6) would be even better.
 
When you pull tube from a 4 tube power amp you should half the impedence from the tap. The only problem is that a simul class OT has two primaries. It is like two OT's in one so when you shut the inner sockets to standby, the impedence is still going to run safely at 8 or 4 ohms depending on the impedence of the speaker you are using.
 
You can't get 16 ohms out of an 8/4/4 output transformer. Where are you getting this 16 ohm idea? The only mention of 16 ohms is about a speaker load if using 6V6's in tweed mode. :?
 

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