Modifying a Simul Satellite to be better voiced forTriaxis?

[joegold]

Hi

I know that back in the early 90's Mesa started modifying the 50/50 power amps to work better with the Triaxis. It's sometimes called the "Deep Mod".

I use my Triaxis with an old Simul Satellite which is supposed to match the power section of the MKIV in Triode mode but really doesn't IMO. (I owned a MKIV for a while too.)

I was just wondering if anybody knows of any mods that can be done to the Satellite to make it work better with the Triaxis. It's OK as is, but if it could be even better then that would be great.

Anything that could be done to make the Satellite sound more like one side of a 2:90 is what I'm after. The 2:90 manual says that they're using a new type of driver stage that is more "head-like". The driver on the Satellite seems similar to the 50/50. There is a pot on the back chassis that sets the level of the power amp input. Actually it's an effects loop with variable Return level patch point for the Satellite's built in preamp. The MKIV's Satellite send signal was fairly low level too as I recall. For line level signals like the Triaxis' main outputs this pot is set at approx 3. For lower signals like the triaxis' effects loop send the pots is set more like 7. Similar settings seem to work on the 50/50's Master Volume pots, as I recall. But the Satellite also has a Master volume on the front, as well as that Effect Ret Level on the back.

So, is there anything that can be done to the SimulSatellite to make it sound more like a 2:90? FYI The Simul Satellite can use either EL34's or 6L6's in the outer pair of tube sockets. I usually use 6L6's.

Also, any mods that would give me the switchable functions of the 2:90 (1/2 Drive, Deep, and Modern) would be great.

FYI
The Satellite has a "Modern" mode which is accessed by not pulling the Presence pot. Pulling the Presence pot is marked "Shift" but it's actually the norm. When pulled there is negative feedback and this acts like a normal Presence control. When not pulled there is no negative feedback, the amps gets louder with a boost in the bottom end and the top end, it distorts a bit more, and the pot has only a marginal effect as you turn it up.

Also... The Simul Satellite has a Class A/SimulClass switch, of course. The 2:90 manual doesn't say so but my guess is that its "1/2 Power" mode is basically just turning off the inner pair of power tubes for each side of the amp, just like the Class A/SimulClass switch on most SimulClass Boogies. But on my Satellite this switch makes a pop. On the 2:90 switching to 1/2 Poer is real quiet.

There is nothing on the Satellite similar to the 2:90's "Deep Mode" although I do have a 5-band EQ that is footswitchable, but can not be switched from the Triaxis' switching system.

Any thoughts/tips/info appreciated.

 

[Boogiebabies]

Deep mode is basically removing the negative feedback loop. Hence, the damping and bottom end are different. You would have to put an adjustable depth mod on the negative feedback loop on the Satellite. The less signal from the speaker taps to the PI means less negative feedback. On the Dual Rectifier, Bold basically uses no negative feedback.

 

[joegold]

Deep mode is basically removing the negative feedback loop. Hence, the damping and bottom end are different. You would have to put an adjustable depth mod on the negative feedback loop on the Satellite. The less signal from the speaker taps to the PI means less negative feedback. On the Dual Rectifier, Bold basically uses no negative feedback.

Hmm. I thought that's what "Modern" was.

My Simul-Satellite has a Pull Shift function on its Presence pot. The Shift setting (i.e. pulled) is the norm though and when pulled the Presence control works like the Presence control my old MKII had on the back. When the pot is not pulled there is an increase in the bottom end, the top end gets a little bit brighter, the volume increases, the power amp distorts more easily, and turning the knob has very little effect on the presence component of the sound. I always thought that this knob, when not pulled, took the negative feedback loop out of the power amp signal.

I rented a 2:90 a few months ago and didn't notice the Presence knobs becoming ineffective when Deep was engaged. But there may have been something wrong with this amp because engaging Modern seemed to do nothing at all.

 

[Boogiebabies]

IMHO, the presense pot on the MK IV lead sounds more like a IIC when pushed in. When pull it adds a .001 cap to the circuit and it seems to add more negative feedback or at least filter the high end to make it sound sweeter. As far as modern, yes, that what the Deep Mod would be like on Simul 2:90. No negative feedback.

 

[joegold]

IMHO, the presense pot on the MK IV lead sounds more like a IIC when pushed in. When pull it adds a .001 cap to the circuit and it seems to add more negative feedback or at least filter the high end to make it sound sweeter.

That may well be. It's been a couple of years since I sold my MKIV. Not sure how it's relevant though.
The Push-Pull pot on the Satellite's Presence knob is a whole other animal.

As far as modern, yes, that what the Deep Mod would be like on Simul 2:90. No negative feedback.

Well, like i said, when I rented a 2:90 I don't remember that being the case. I may rent it again to check that out again as well as some other things. I like the way the 2:90 sounds with my Triaxis (obviously) but it's just too impractical for me to use one of them on the gigs I tend to do.

The Satellite, being a self contained powered speaker cabinet, is almost ideal. If I could get Mesa to make me 1 side of a 2:90 (switches and all) housed in a single 12" cabinet I'd be happy. Eventually I'd buy 2 of them, but I'd only need 1 most of the time.

 

[Boogiebabies]

That may well be. It's been a couple of years since I sold my MKIV. Not sure how it's relevant though.
The Push-Pull pot on the Satellite's Presence knob is a whole other animal.



I am sorry, I must have missed that in your manifesto.

 

[joegold]

That may well be. It's been a couple of years since I sold my MKIV. Not sure how it's relevant though.
The Push-Pull pot on the Satellite's Presence knob is a whole other animal.

I am sorry, I must have missed that in your manifesto.

Just curious...
Do you actually own a 2:90 or have you spent a lot of time playing them?

If yes, and you're sure about the "Deep" mode on the 2:90 affecting the negative feedback then I'll just take your word for it.

But, like I said, I don't recall it acting that way when I was renting a 2:90.
That doesn't mean it doesn't work that way, just that I don't recall it doing so. And everything I've read and know about the "Modern" mode leads me to think that it's ther mode that switches out the negative feedback.
Of course I could be wrong. Maybe they both switch out the negative feedback?
It would be nice if the 2:90 manual was more specific.

 

[Boogiebabies]

Modern removes all of the negative feedback. Any other resistance and capacitance would result in a change to the amount of signal being fed from the output tap to the phase inverter.

 

[joegold]

Well...To make thing's even weirder, for me at least, and with the effect of making me look like a total idiot...

It turns out that the Simul Satellite may already have the "Class A" tube pair running in Pentode. At least this is what a tech I have looking at my amp right now is trying to tell me. He says that part of the evidence for this is that there is a 2.7k screen resistor on the outer tubes.

I found a bunch of schematics for Boogie SimulClass stuff (everything except the Simul Sat) on the web. And all the newer amps look like they too have this 2.7k resistor on the outer tubes while the inner tubes have different stuff going on. So my tech is probably right.

This means that my power amp section is more like the 2:90's or the MKIV's (when the MKIV's Triode/Pentode switch is on Pentode).
I.e. In "Class A" mode the Simul Satellite is putting out more like 30 watts than the 15 watts that the "Class A" tubes in earlier SimulClass amps were putting out.

It means, of course, that my little project of modding this amp with a Pentode/Triode switch so that I could go from triode to pentode in order to get a bit more headroom is totally a non starter. (I have no desire to add a switch allowing triode operation with even less headroom.) So this amp already has as much headroom right now as it's ever gonna have, unless I take the "Class A" circuitry out altogether. And that's probably impossible and/or would require a new/different transformer.

Again, whether these tubes are really running in "Class A" or not doesn't really concern me all that much. Still it's weird that Mesa's SimulClass patent specifically says that the "Class A" tubes are running in triode mode.

Maybe I'll get off this kick now and start practicing again.

My tech did find some things that he found strange with this amp, but he doesn't think they are affecting the way the amp sounds.

1. He says that 1 side of the amp's bias reading is about 8 volts lower than the other side. He says the bias splitting resistors are fine.

2. Screen resistor voltages are different on both sides, which means that the transformer has some sort of a quirk in the way it's operating.

For all we know, because we don't have a schematic, these anomalies might be the way the amp is designed. My tech seems hesitant to call Mesa and ask about these anomalies. But he doesn't seem to think that there is anything to be alarmed about either.

 

[Boogiebabies]

I do not think the screen grid resistor has much to do with it. It still has a 2.2M between pins 5 and 8 and runs the outer two sockets to the Simul primary. Most EL34 amps use a 1K 5 Watt screen grid resistor. IMHO the plate voltage should be the same to all of the tubes, meaning it should be pretty close on pin 3 of all 4 tubes, unless the outer tubes are really drawing a high natural plate current and sucking more voltage.


Oh, the simul sockets are usually biased at -39V and the inner 6L6's at
-52V. What the actual tube draws from these bias setting needs to be probed.

 

[joegold]

I do not think the screen grid resistor has much to do with it. It still has a 2.2M between pins 5 and 8 and runs the outer two sockets to the Simul primary. Most EL34 amps use a 1K 5 Watt screen grid resistor. IMHO the plate voltage should be the same to all of the tubes, meaning it should be pretty close on pin 3 of all 4 tubes, unless the outer tubes are really drawing a high natural plate current and sucking more voltage.

Oh, the simul sockets are usually biased at -39V and the inner 6L6's at
-52V. What the actual tube draws from these bias setting needs to be probed.

Let me clarify something I said in my last post.

It's not that the outer tube pair and the inner tube pair have bias readings that are 8 volts off from each other.
It's that the two left-most tubes and the two right-most tubes have the 8 volts difference. I.e. One side of the entire parallel push-pull circuit has a bias reading that is 8 volts lower than the other side.
My tech described this as sort of like using un-matched power tubes in both the outer sockets and the inner sockets.

But again, he didn't notice any weird distortion or clipping when he put the output on his oscilloscope. So he decided not to do anything about it, for now. Changing the transformer would be costly and I may or may not notice any difference in the sound.

But he did say that he could compensate for the voltage difference with some resistors and/or caps in various places. For some reason, and I'm not sure why, we passed on doing this yesterday. But I may take the amp back to him and get him to do this for me. After all, why am I paying all this money for matched power tubes if my amp is going to unmatch them?

And as to whether my outer tube pair is running in triode or pentode...
He's convinced that it's pentode Class AB. But I don't think he "gets" that Mesa is doing a non-standard sort of "Class A". He seems to think that with Class A operation there should be some output with only 1 power tube running.

Now there is also a hint from Randall Smith that his form of "Class A", as implemented in his SimulClass amps, does not necessarily rely on the class A tubes being run in triode.
"Many Simul-Class amplifiers also ran the Class A pair wired for Triode operation."
To me this implies that some SimulClass amps have the outer pair wired for pentode operation.

At any rate, my amp seems to be more like a 2:90/MKIV SimulClass power section than a MKIIC/MKIII SimulClass power section.

 

[Boogiebabies]

OK, you have an inner pair that are within 10%. Thats fine.
I have never witnessed a Simul-Class amp wired in A/B pentode.
The simul 2:90 has the identical power amp and OT to the MK IV.

 

[joegold]

OK, you have an inner pair that are within 10%.

?? I don't follow how the information I've told you leads you to make the above comment?
The inner pair are in within 10% of "what"?

Did you understand what I said about the 8 volts difference in the bias voltages? That V3 and V4 have the correct voltages but the voltages at V5 and V6 are different, respectively, by about 8 volts?
[My amp has 5 tubes. V1 is the preamp input. V2 is the driver tube. V1 and V2 are both also involved with the built-in effects loop. V3 is power tube 1 of the Class A pair. V4 is power tube 1 of the Class AB pair. V5 is power tube 2 of the Class AB pair. V6 is power tube 2 of the Class A pair.]
So, assuming the bias voltaqges you gave me 2 posts ago are the bias voltages that my amp expects to see too, then this is how it plays out (I think):
V3 is getting -39V.
V4 is getting -52V.
V5 is getting -38V (or -47V).
V6 is getting -44V (or -60V).
I.e. There is an 8 volt difference between the bias voltages on V3 and V4 compared with the bias voltages on V5 and V6.
At least that's the best my little brain can parse what my tech has told me.

I have never witnessed a Simul-Class amp wired in A/B pentode.
The simul 2:90 has the identical power amp and OT to the MK IV.

Oy. I hope this is just a terminolgy/semantics problem we're having here. Again...I don't really care whether or not Mesa's "Class A" is really Class A or not. My tech thinks it's just Class AB pentode in my amp biased in some weird way. But let's call it "Class A".
When the MKIV is switched to pentode are the outer tubes not running in "Class A pentode"?
If so, then isn't the 2:90 also using "Class A pentode" operation in its Class A tube pairs?
Isn't the way that Class A operation is implemented in the 2:90 (and in the MKIV in pentode mode, and presumably in my amp too) different than how its implemented in the MKIII and the MKIIC?
Isn't this difference based on the Class A tubes being in pentode on the 2:90 and potentially on the MKIV and in triode on the MKIIC and the MKIII?
If that's not what's different between these SimulClass amps then what is different that allowes for the higher power ratings in the newer Simulclass amps?

BTW My tech told me that in Class A mode my amp is putting out about 25 watts.

 

[joegold]

BTW

Mike Bendinelli at Mesa just told me that the bias voltages for my amp should be:
-47V on the outer pair (The "Class A" pair)
-52V on the inner pair (The Class AB pair)

[He thinks that my tech probably took a fauly reading with his "8 volts difference between left and right pairs" and that he was really measuring the different bias voltages of the inner and outer pairs, which is totally within normal specs. I'll talk to my tech and clarify this once more, because he swore up and down to me that it was a left-right issue, not an inner-outer one.]

He also said that all of the newer SimulClass amps do not really run the "Class A" tube pair in "the Class A range".
The earlier amps (the SimulClass MKII's and early MKIII's) did run those tubes in the "Class A range" but it was too hard back then to find reliable tubes to run that hot. So in later amps (the later MKKIII's and after) they biased the outer sockets for 6L6's running in pentode rather than for EL34's running in triode.

He says that my "Class A" tubes are running in pentode, as do the "Class A" tubes in the 2:90 and the later MKIII's. He doesn't recommend using EL34's at all in the Satellite or the later MKIII's.

He admits that these newer "SimulClass" amps really don't have the outer pair in Class A, they're just Class AB biased funny.

I didn't get into what the pentode/triode switch on the MKIV does or doesn't do. But if the 2:90 and my amp have the outer tube pair in pentode then my guess is that this switch does the same thing for the MKIV's outer tube pair.

 

[Boogiebabies]

Great.