Re: Power ratings in older vs newer Mesa SimulClass power sections

Joey -

I can see how you'd be a bit miffed that no one answered the question
below after putting all the time into it. You go into great detail
here. I've asked questions here before also - sometimes they get
answered, and sometimes they don't. Unfortunately I'm not a Mesa amp
kind of guy and approach amps more from the player's/musican's point of
view - so I'm not the guy to give a definitive answer.

Doug Roccaforte, Rich Koerner, Ed Blum, and other techs on this
newsgroup could probably answer this auestion if they wanted to. I can
guarantee you if willie were still around, he wouldn't touch it.

I suspect part of the problem is the length and complexity of the
question. A lot of people would probably zone out before getting to
the end - even amp techs who know the answer. Try to simplify the
question (i.e., cut it down to about 25% of the current length) and see
if that works.

Not a bad idea to try other newsgroups/forums as well if you really
need an answer to the question.

- KK


Joey Goldstein wrote:
SimulClass MKIIC's, MKIII, and Simul Satellites (the amp that I
currently own) were all rated at 75 watts when in SimulClass mode. I
believe that SimulClass MKII's had to have EL34's in the outer pair of
tube sockets and that the MKIII allowed for a choice of 6L6's or EL34's
as does my Satellite.

I've been reading on usenet that Mesa's "Class A" modes are not really
true Class A circuits. Supposedly, on a Mesa, "Class A" simply means
that the outer tube pair is wired in Class AB triode as opposed to the
norm which is Class AB pentode. In triode mode the tubes put out less
power and can be overdriven at lower volumes and sound somewhat more
like a real Class A amp.

Most of what I've read suggests that on an older Mesa SimulClass amp, in
"Class A" mode, the inner tube pair is shut off and the amp puts out
approximately 15 watts. So, I've always figured that this means that the
inner pair puts out 60 watts (60 + 15 = 75).

But this doesn't add up so neatly with some of Mesa's power ratings on
the newer SimulClass amps, the MKIV and the 2:90. I think a big part of
my quandry is that in Class A mode the amps that are capable of taking
EL34's put out roughly half of the power that 6L6's in Class A put out.
So 6L6's in Class A put out approx 30 watts and EL34's in Class A put
out about 15 watts. Does that seem about right, or am I making this up?

So the rating of 75 watts maximum in SimulClass mode, mentioned above,
was referring to the amps that were using EL34's in the outer sockets.
And the amps that allow for 6L6's in the outer pair actually have more
power than this. Does this seem plausible?

So then, the amps that can, or need to, use 6L6's in the outer sockets
actually put out 30 watts in Class A mode? So the output of my Simul
Satellite is really more like 90 watts in SimulClass mode when I use 4
6L6's? I.e. With 4 6L6's it's more like a 2:90 than the power section of
a Simulclass MKII?

Now, the MKIV's owner's manual has some numbers that make this even more
confusing. The MKIV has a Pentode/Triode switch for the outer tube pair.
On page 12 of the manual they discuss the power output depending on the
various switch settings. They claim that with switch settings Class A
and Triode the outer pair puts out 30 watts. (They don't say so
explicitly, but I assume they are talking about using 6L6's in those
sockets.) With switch settings Class A and Pentode they say it's 50 watts.

But then they say that with switch settings SimulClass and Triode the
amp puts out 70 watts, and with switch settings SimulClass and Pentode
it puts out 85 watts. These last two numbers seem low to me and suggest
that that inner pair of tubes are only putting out about 40 watts (70 -
30 = 40, with switches in Triode mode - SimulClass) or 35 watts (85 - 50
= 35, with switches in Pentode mode - SimulClass).

That seems a pretty inefficient use of the inner pair of power tubes.
Shouldn't the numbers in SimulClass mode be more like 90 watts in Triode
mode (60 + 30 = 90) and over 110 watts in Pentode mode (60 + 50 = 110)?

What am I missing?
Does the Triode/Pentode switch actually have some unmentioned effect on
the inner pair of power tubes also?
Does the SimulClass/Class A switch have some unmentioned effect on the
inner pair of power tubes too?

And for me my main concern is that I would like to know the actual
output of my Simul Satellite in SimulClass mode with 4 6L6's. ??

I'm thinking of getting a Triode/Pentode switch installed in this amp
for even more headroom. Amp tech's here in Toronto have told me it
should be no problem and that I should still be able to use either
EL34's or 6L6's in the outer sockets. A Mesa rep has told me that it is
possible to do too. But he advised against it for "stability" reasons.
Anybody know what he means?

If you've read this far, then thanks a lot. Any tips/info appreciated.


--
Joey Goldstein
http://www.joeygoldstein.com
joegold AT sympatico DOT ca

.

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