Mesa Boogie Mk III Coliseum Simul-Class

[CJ]

kind of a rare animal here, made around 1986,  maybe one hundread made?

has the eq,supposed to be loud, no model name on the front or back no serial number, so figuring out what we had was a pain, they were still kind of a botique amp shop back,

 

[CJ]

six power tubes, four 6L6GC and twoEL34's,  wtf, over? EL34's are stung up triode style and there is a resistor to ground on pin 5 (bias) to drop the neg volts to the grid,  as the 34 wants less bias to conduct i the sweet spot,

 

[CJ]

nice build quality, except the plastic speaker jacks, with this much power we want metal switchcraft hardware, eh?

this thing has a half power switch but it runs in simul-class all the time (no sw like other simul-class)

what is simul class?  34's run in class A (kind of, no cathode resistor) and the 6L6 tunbes run in your standard A1 with fixed bias

werid chassis heat sink,has reverb and a fan squeezed intothe box,

 

[CJ]

no schematics online for this beast, but here is a mark III  simul class>

 

[radardoug]

So are the 4 6L6's in basic push pull parallel, and the EL34s connected to the output tranny on the same taps? With maybe that switch in the cathodes? Weird man! You sure get the odd ones CJ!

 

[CJ]

those 34's are always in parallel with the 6L6 tubes,

half power is achieved by switching in more resistance , about 22K, in series with the screen supply so the screen voltage drops about 30 volts,

they say the 34's are running in Class A because they are biased hot, so more current is flowing in them at idle,  how close to real class a they are running i do not know,
supposedly this gives you the best of both worlds as far as even and odd distortion, crossover notch, low volume detail, interaction with leakage inductance of output transformer,


Randall still does design work, i talked to a Mesa guy at the guitar show last week, and he says that the guy loves to lock himself in a closet and design tube amps,  most people would have sold out by now, so he must still dig what he is doing,

these tubes were seeing 510 volts of B+, they used ceramic tube sockets which is good for heat and arc over prevention, pots are kept off the pc board which is a good idea, but this makes working on the board a pain as you have to un-solder a lot of wires, however, nothing on the board need replacing so this was not a problem,

here is as blurb from Mesa:

Simul-Class™ magic comes from the pairing of two separate and different power amps working together simultaneously to produce one gorgeous tone. For each channel, one pair of 6L6s operates in Class AB delivering punch and headroom. Meanwhile, the other two impart the sweetness and warmth of mystical Class A circuitry. So if you’re seeking three-dimensional “magic” in your sound, you’ll want the Simul-Class 2:Ninety™ as the foundation of your rack tone.

wiki>
More importantly, it marked the introduction of Mesa/Boogie's "Simul-Class" system, where two of the power tubes (always 6L6s) run in class AB pentode while the other two tubes (either 6L6s or EL34s) run in class A triode. In a simul-class amp, running all four tubes generates approximately 75 watts RMS of power; running only the class A tubes produces about 15 watts

so with four tubes and ab1 and only two in class a, there is probably not a big mix of sounds in the coliseum amp, maybe this is why it did not survive long,

on this amp, the triode pair of tubes had the screens wired to the plate via 560 ohm 2 watt resistors,

there are some schematics floating around that show a transformer tap for the 34's, but this was probably discontinued for cost reasons,