Power ratings in older vs newer SimulClass power sections

[joegold]

Hi

I'm new here. This is my 1st post. This seems like a good place to gather Mesa info, without calling them directly.

So...

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 off 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.

 

[Boogiebabies]

You may want to read this before you call Mesa's Class A triode Class AB triode. Class A does not have to be exactly like a 6V6 tweed amp.

http://mesaboogie.com/US/Smith/ClassA.htm

 

[joegold]

You may want to read this before you call Mesa's Class A triode Class AB triode. Class A does not have to be exactly like a 6V6 tweed amp.

http://mesaboogie.com/US/Smith/ClassA.htm

Well I appear to be right about some things and wrong about others, from Mr. Smith's POV at least. Here's what he says about Simul-Class:

"Many Simul-Class amplifiers also ran the Class A pair wired for Triode operation. This means that the screen grids of those two tubes are wired to the plates and are functionally disabled. Think of the screen grids as accelerator elements, working like a catalyst to increase the tube’s sensitivity to incoming signals. With the screens disabled, the resulting Triode response has a much more gradual reaction (a lower “power sensitivity”) to incoming drive signals, causing a softer onset of clip. The sonic response is somewhat comparable to using your fleshy thumb to pluck a string as compared to the harder edge of a pick. The note is still the same frequency and may be just as loud, yet the harmonic content, and hence the timbre, is entirely different. Triode operation also reduces the tube’s power handling capability by about half.

Simul-Class is found today in our Mark IV and Stereo 2:Ninety amplifiers and has its precedent set in the hallowed Mark II-C’s of yesteryear, some of Metallica’s favorites. Simul-Class power sounds smoother, warmer and less punchy than our standard Hundred Watt power. In short, it has more of the Class A characteristics."

So, the implication is that older Simul-Class amps had the outer pair of tubes in Triode mode. He doesn't say so above but the implication is that in the newer Simul-class amps, the 2:90 and the MKIV, the outer pair of tubes are in Pentode mode while still supposedly being in "Class A" mode as well. This is reflected in the higher listed power rating fror these amps, compared to mine which is rated at 75 watts, like the MKIIC and the MKIII.

I used to think this as well, until I had some guys on usenet swear that it wasn't true. That Mr. Smith's version of "Class A" as implemented in his simul-class amps, all of them, isn't really "Class A" power. That all he was doing for his "Class A" effect was running the outer pair in Triode mode. I thought the usenet "amp gurus" were making it sound like you can't have a Class A circuit in Pentode mode either. Now, I don't know what to believe.

But I know what I want, which is to mod my Simul-Satellite so that it puts out as much power as a 2:90 or a MKIV in Simul Class/Pentode mode.

But Michael Taylor, at Mesa, has told me that, although this mod is possible, they advise against it. They say there are possible "stability issues", but what exactly those issues are we didn't get into, yet. My guess is that it probably has to do with my amp maybe having a different transformer than the MKIV, which is odd because my amp was designed as a MKIV stereo slave. Oh well.

Thanks for responding.

 

[Boogiebabies]

This is not to say that the outer sockets of a simul are not really running Class A, but AB triode. This is always how I looked at it, but smith explains it in a way where it is like just on the verge of AB but is a technical version of class A. All in all, I think it was very clever and produces a tone unlike any other power section.

 

[joegold]

This is not to say that the outer sockets of a simul are not really running Class A, but AB triode. This is always how I looked at it, but smith explains it in a way where it is like just on the verge of AB but is a technical version of class A.

Well, that's not what I got from that article. I thought he was spending all that verbiage to make the true Class A circuitry of the Lonestar Special more widely understood. In the brief discussion of SimulClass he makes it sound like it too is true Class A power, by the RCA handbook's standards.

This business about Mesa's "Class A" in the SimulClass amps really being just Class AB Triode is something I picked up on usenet, and it doesn't really jibe for me anymore because it seems this would mean that Mesa is out-and-out lying about certain aspects of their SimulClass amps.

Lately I tend to take Randall Smith at his word. Even if it's not true "Class A" he's using in those tubes, it *is* something other than regular old Class AB Pentode. I've trusted Mesa over the years. They've always seemed like good people.

And if it were simply Class AB triode, and he's just calling it "Class A", then that would make the terminolgy used for the MKIV too weird to contemplate. Because in "SimulClass" Pentode mode this amp would just be Class AB Pentode in all 4 power tubes.

It would also mean that in the 2:90 he's running the "Class A" tubes in such a way that, although they are running in Class AB Triode, they put out twice as much power as the same tubes running in my Satellite. This is possible, of course, with different biasing and a different transformer, but again it means that there's some serious misinformation going on somewhere.

All in all, I think it was very clever and produces a tone unlike any other power section.

I tend to like the sound of SimulClass a lot for pop/R&B/rock. But for jazz I like all the headroom that 4 6L6's can give me.

 

[Boogiebabies]

Well, that's too bad.

Maybe you should read the US patent on Simul-Class and that will explain it for you. Even better yet, the patent has expired so you can build your own.

 

[joegold]

Well, that's too bad.

What's too bad?

Maybe you should read the US patent on Simul-Class and that will explain it for you. Even better yet, the patent has expired so you can build your own.

Well I actually did spend some time reading the patent(s):
4,593,251 and the other one, 4,532,476, which seems to be the same thing.

Of course, tech talk like this is well beyond me. But it looks like those patents are saying that Mesa's "Class A", as implemented in the SimulClass amps, is true Class A and that it depends on running a pair of power tubes in Triode mode biased a particular way in order to make the tubes operate in a Class A power mode.

That's all fine, etc. But then what is going on in the MKIV when it's set for *Pentode* operation?
I.e. If Mesa's Class A circuit requires triode operation then when set for Class A Pentode, is the MKIV just a poorly biased Class AB pentode amp?
Is the MKIV no longer a "SimulClass" amp when the SimulClass and Pentode switches are on, it's just a regular 4 tube Class AB amp with poor biasing on 2 of the tubes?

Also...
If this type of "Class A" amplification depends on triode operation then the 2:90's "Class A" tubes must be in triode too, not pentode as I had been assuming.
So how come the power ratings on the 2:90 are so much higher than the power ratings for the older SimulClass amps?
Are they just biasing these tubes differently to coax more power out of them, but still keeping them in triode?

Obviously, I'm confused about a lot of this.
I'll be calling Mesa tomorrow about some other things and I'll try to get them to explain to me what the heck is really going on in these amps.
Of course it's likely that I won't understand a thing they're saying. ':?'

 

[Boogiebabies]

When the MK IV is switched to pentode, it never really removes the triode wiring ( 2.2M from pin5 to pin 8) from the outer two sockets. It just feeds everything through the inner sockets primary like a 100 watt power section. When switched to Simul, it sends the outer sockets signal to a second primary and then blends them through the output transformer to the speaker taps.

 

[joegold]

When the MK IV is switched to pentode, it never really removes the triode wiring ( 2.2M from pin5 to pin 8) from the outer two sockets. It just feeds everything through the inner sockets primary like a 100 watt power section. When switched to Simul, it sends the outer sockets signal to a second primary and then blends them through the output transformer to the speaker taps.

Well I wish I understood that because it's probably being really helpful.

Are syaing that switching this switch to the Pentode position does not really run those tubes in pentode but does something else?

Thanks for listening to my rants and baby-sitting me, by the way.

So...Have I misinterpreted Randall's Class A essay and his patent by reading them as saying that the Class A tubes in a SimulClass circuit are always running in triode?
I.e. Can he run these tubes in Pentode also and still have the "Class A" sound/power clip he wants?

Do you think the Class A tubes in the 2:90 are in triode or pentode?

What do you make of my tech's assertion (see my latest post in the other thread) that the "Class A" tubes in my Simul Satellite are in pentode?

 

[Boogiebabies]

If you look at the outer sockets of a MK IV, the resistor is soldered right to pins 5 and 8. I do not see how they could switch them out, but I will take another look.

 

[joegold]

If you look at the outer sockets of a MK IV, the resistor is soldered right to pins 5 and 8. I do not see how they could switch them out, but I will take another look.

You can see the switch in the MKIV schematic found here:

<http://www.tubefreak.com/mk4-4.gif>

 

[Boogiebabies]

Yes, I know but it only moves the outer sockets from the Simul primary to the 6L6 primary. It does not switch out the triode wiring on the tube.

 

[joegold]

Yes, I know but it only moves the outer sockets from the Simul primary to the 6L6 primary.

I wish I knew what that means. <g>

It does not switch out the triode wiring on the tube.

Are you trying to tell me that all the pentode/triode switch does is to send a different bias voltage to the the Class A tubes, but they still run in triode?

Then why does he call it a "pentode/triode switch"?

 

[Boogiebabies]

No, there is no difference in bias. It runs the signal through one primary so it has the effect of a push pull power amp, not a Simul-Class.

On a MK IV Simul Class amp there two taps (inputs) to the primary of the output transformer. One is for the outer sockets and one is for the inner sockets. When running triode lets say with EL34's in the outer sockets it uses the first tap ( output). Running 6L6's in the middle use the second tap ( output). This combines them to the secondary ( speaker output) and blens the two power amps together running simultaneously. When the pentode switch is engaged in uses only the tap ( output) that the 6L6's uses and negates the Simul-Class. It is just like a 100 watt push pull, but with the 2.2M between pins 5 and 8.

I need to take a good look at the switching on my MK IV and see what the pentode triode switch takes out or adds.

 

[screengrid]

Hi all,

just reading this interesting thread. I bought my MK IV in Jan 1991, so I guess it's a 1990 model.
Regarding the possible power modes of the simul class power amp,
here is what the original manual says:

Class A , Full Power / Triode - around 30 watts before clipping or 50 watts in pentode.
Class A Tweed Power, Triode - nothing mentionend
Simul Class, Full Power / Pentode - around 85 watts or 70 watts in Triode

A German Mesa Boogie prospect from 1991 says the following:

Class A / Tweed Power - 10 watts ( power stage clear/vintage ), 30 watts ( power stage overdriven/ singing )

Class A / Full Power - 15 watts ( power stage clear ), 45 watts ( powerstage english )
Simul-Class / Tweed Power - 45 watts ( power stage warm ), 100 watts ( power stage overdriven / fat )

Simul Class / Full Power - 75 watts ( power stage clear and full ), 170 watts ( power stage overdriven / way too loud !!!! )

A review of the MK IV combo in the "Gitarre&Bass"-magazine from Jan 1991 mentioned nearly the same.


Regarding the Pentode/ Triode switch:

According to two versions of schematics from the simul class power amp

schematic from 8/1992

only two taps at the primary of the OT are drawn. The anodes (pin 3 ) of the inner power tube pair V6 and V 7 are permanentely connected to the taps.
Also the anodes (pin 3 ) of the outer tube pair V8 and V9 are permanentely connected to the mentioned taps.
The Pent./Triode switch connects the screen grids ( pin 4 ) of the outer tube pair via 2,7 K resistors to the anodes ( pin 3 ) of this tubes .

The simul class / class A switch only interrupts the connection to ground of the both cathodes of the inner tube pair V 6 and V7, when in the Class A position, so that the inner tube pair is out of the cirquit in Class A operation.


later schematic from ....... ?????, I don't know!!

the output tranny now has four taps on the primary, two separate taps for the inner and outer power tube pairs. Apart from this the schematic looks the same like the upper mentioned version.


regards

 

[joegold]

Hi all,

just reading this interesting thread. I bought my MK IV in Jan 1991, so I guess it's a 1990 model.
Regarding the possible power modes of the simul class power amp,
here is what the original manual says:

Class A , Full Power / Triode - around 30 watts before clipping or 50 watts in pentode.

Mike Bendinelli has told me that this is a misprint in the MKIV's manual.
The MKIV in "Class A" Triode puts out about 15 watts (like the old SimulClass amps, MKII, early MKIII).
In "Class A" Pentode it puts out about 30 watts (like the 2:90's "Class A" tube pair).
These ratings are for Full Power not Tweed.
Evidently my Simul Satellite has the "Class A" tubes in pentode, so it's much more like a MKIV in Pentode or the 2:90 than the earlier SimulClass amps.

Supposedly, the only SimulClass amps that really had the outer tube pairs running in the "Class A range" were the early SimulClass amps, the later MKII's and the earlier MKIII's. These were wired in triode, biased to run in the "Class A range" and used EL34's in the outer tube sockets. But tube reliability was poor back then and they changed the design. Newer SmulClass amps (like mine, the later MKIII's, and the 2:90) have the outer pair running in pentode. They are biased differently than the inner pair but they *do not* run in the "Class A range". They are just Class AB pentode and biased funny.

Of course the MKIV allows for triode operation of these tubes as well and my guess is that they probably do run in the "Class A range" when in triode mode.
The 2:90 does not have a triode/pentode switch as far as I know. But its "half-power" mode may involve switching to triode. Anybody out ther know for sure how "1/2 power" is acheived in the 2:90? I forgot to ask this from Mike Bendinelli when I was talking to him.

Class A Tweed Power, Triode - nothing mentionend
Simul Class, Full Power / Pentode - around 85 watts or 70 watts in Triode

A German Mesa Boogie prospect from 1991 says the following:

Class A / Tweed Power - 10 watts ( power stage clear/vintage ), 30 watts ( power stage overdriven/ singing )

Class A / Full Power - 15 watts ( power stage clear ), 45 watts ( powerstage english )
Simul-Class / Tweed Power - 45 watts ( power stage warm ), 100 watts ( power stage overdriven / fat )

Simul Class / Full Power - 75 watts ( power stage clear and full ), 170 watts ( power stage overdriven / way too loud !!!! )

A review of the MK IV combo in the "Gitarre&Bass"-magazine from Jan 1991 mentioned nearly the same.


Regarding the Pentode/ Triode switch:

According to two versions of schematics from the simul class power amp

schematic from 8/1992

only two taps at the primary of the OT are drawn. The anodes (pin 3 ) of the inner power tube pair V6 and V 7 are permanentely connected to the taps.
Also the anodes (pin 3 ) of the outer tube pair V8 and V9 are permanentely connected to the mentioned taps.
The Pent./Triode switch connects the screen grids ( pin 4 ) of the outer tube pair via 2,7 K resistors to the anodes ( pin 3 ) of this tubes .

The simul class / class A switch only interrupts the connection to ground of the both cathodes of the inner tube pair V 6 and V7, when in the Class A position, so that the inner tube pair is out of the cirquit in Class A operation.

later schematic from ....... ?????, I don't know!!

the output tranny now has four taps on the primary, two separate taps for the inner and outer power tube pairs. Apart from this the schematic looks the same like the upper mentioned version.

regards

First off...I wish I understood all that stuff about the output transformer. I'm going to print that out and show it to my tech because he's still having trouble getting the left and right tube pairs in my amp to have the balanced bias voltages they're supposed to have.

Now, are those typos in the tube numbers listed above?
If the 4 power tubes are numbered V6, V7, V8 and V9 then the pairs would be:
V6 and V9 (the outer pair running in "Class A")
V7 and V8 (the inner pair running normally in Class AB)

But if those are not typos then this might hold some clues for my tech. Because our problem is that the outer pair of tubes have different bias voltages and so do the inner pair. I.e. There are *4* different bias voltages happening in my amp.

My power tube pairs are numbered V3 & V6 (the outer "Class A" pair) and V4 & V5 (the inner pair). (It's a 6 tube amp...2 12AX7's plus 4 6L6's.)
Mike Bendinelli told me that the correct bias voltages for my amp are -47V for V3 & V6 and -52V for V4 and V5.
But the readings that my tech is seeing are more like this (I may have forgotten the exact numbers, sorry):
V3 = -47V, V6 = -55V
V4 = -52V, V5 = (Actually I forget the bias voltage on V5, but it was not matched to V4 like it's supposed to be. I think it was like -55V also, but I'm not sure.)
So, V3 and V4 are at the voltages that MB told me they're supposed to be at but V6 abd V5 are way off.
And neither my tech or MB seems to know what's going on. All the resistors that are supposed to be present are there and are working. We're suspecting the OT at this point. But one experiment my tech did involved unsoldering one of the taps and it didn't make the difference he expected it to. So it might not be the OT either.
I dunno. We'll see how this plays out.

And another weird thing is that with all these weird bias voltages both MB and my tech seem to think that some type(s) of audio distortion should show up on the scope, but it doesn't.

The amp actually doesn't sound all that bad, but I've always felt that it should sound a bit better. Mabe I should just take it home and forget about all this balancing the sides crap before my tech ruins it!

 

[RussB]

Rated Output...


Rated Output of amplifiers is pretty much a joke.

What wattage, at what frequency, with what input, with what THD, into what load?

What that all means is there are far too many variables to make ANY number, that ANY manufacturer gives you, pretty much worthless. It has more to do with marketing than with fact.

Class A? :lol: :roll: yeah, RIGHT!

 

[Boogiebabies]

Is there something wrong with this thing or is this guy just tinkering?

You have to undrstand that there could be a + or - 20% drift in these readings. Although, I have three 21 year old boogies and they are right on spec to the schematic that does not mean that yours is off. What would the OT have to do with the bias being fed to the tubes have to do with it? No matter how much the tube draws, the bias is being fed from the bias tap and should be very close to spec. IMHO the worst case would be the 47uf 100V bias filter caps may be going out or this tech has no idea what is going on.

 

[Boogiebabies]

Im out, going to watch Van Helsing. Maybe it will be better the second time.

 

[joegold]

Now, are those typos in the tube numbers listed above?
If the 4 power tubes are numbered V6, V7, V8 and V9 then the pairs would be:
V6 and V9 (the outer pair running in "Class A")
V7 and V8 (the inner pair running normally in Class AB)

But if those are not typos then this might hold some clues for my tech. Because our problem is that the outer pair of tubes have different bias voltages and so do the inner pair. I.e. There are *4* different bias voltages happening in my amp.

My power tube pairs are numbered V3 & V6 (the outer "Class A" pair) and V4 & V5 (the inner pair). (It's a 6 tube amp...2 12AX7's plus 4 6L6's.)
Mike Bendinelli told me that the correct bias voltages for my amp are -47V for V3 & V6 and -52V for V4 and V5.
But the readings that my tech is seeing are more like this (I may have forgotten the exact numbers, sorry):
V3 = -47V, V6 = -55V
V4 = -52V, V5 = (Actually I forget the bias voltage on V5, but it was not matched to V4 like it's supposed to be. I think it was like -55V also, but I'm not sure.)
So, V3 and V4 are at the voltages that MB told me they're supposed to be at but V6 abd V5 are way off.

OK. I'm a dummy. Just ignore my tube numbering system because I'm wrong. On my amp the inner pair of power tubes would be numbered V3 & V4, and the outer pair would be V5 and V6.

You'd think I don't know how to read a schematic or a tube use chart...and you'd be right.

Sorry for any confusion.