Mc2300 Peak Limiter Theory - REVISITED

[SarnoMusicSolutions]

Cool stuff. Thanks for all that. I am VERY aware of the clear distinction between the Mac's Sentry Monitor (aka, VI Limiter), and the Power Guard (optical peak limiter driven by the output) that Mac incorporated with later amps after the Mc2300. These are two very distinct circuits. There seems to be difficulty in the conversation sometimes when discussing this with people at McIntosh and with others because the word "limiter" can make people thing that the reference is to the Power Guard and not the Sentry Monitor (VI Limiter). Both are limiters. To be honest, some of us are still hoping to get 100% clarity on this suggestion that Jerry's Mc2300 did indeed NOT have the Sentry Monitor (VI Limiter) because it's not 100% clear that the statement that their amps didn't have the "limiters" wasn't possibly a reference to the Power Guard instead. Still seeking clarification on that.

The Mc2300 service manual that I've seen shows a single channel of the Mc2300 actually delivering about 343 watts of fairly clean power, and at that point the power has reached its limit for sustained power, and the distortion rises rapidly. Jerry was well known to have pegged his McIntosh VU meter hard to the right as he played, and it was set to the meter setting designed to read the highest power levels. It's been pretty clearly explained by Dan Healy and others that Jerry definitely drove his Mc2300 well into clipping. The popular conversation (debate) among us Jerry-tone-heads has been where is the dirt coming from, and what proportions of the various sources contribute the most. Here are the 3 places where the clipping/dirt most likely come from:

1 - the McIntosh simply clipping by reaching it's power limits
2 - the saturation of the JBL's
3 - the preamp being cranked to levels where preamp tube clipping occurs
4 - in the case of the Sentry Monitor (VI Limiter) present, the artifacts from that activity

If it's true that the Sentry Monitor was removed, then that leaves out #4, obviously.
I've tried all kind of combinations using the right pickups and Tiger buffer clone and real Fender preamp or similar into a Mc2300 and a Mc250 into JBL K120's and E120's. The Fender preamp circuit with the Jerry guitar setup (dimarzio Super II set to single coil with a Tiger Buffer) can enter into a bit of dirt and squish if you really crank the Fender. As I recall it becomes fairly audible as you get the gain knob on the Fender up near 8 or so. With the McIntosh, you can trade off the two gain settings so that either the Fender begins to clip a bit and the Mac is clean and at low volume. Or you can back the Fender down below 6 and run it clean and then open up the gain to the McIntosh and get louder and louder until it reaches its max power and begins to clip. This gives two scenarios. To my ears the clean preamp and the clipped Mac sounds very Jerry. The clipped preamp with clean Mac sounds like a more traditional guitar tone with that class-A, asymmetrical "warm" tubey sound. It's a nice sound, but personally I just can't get it to pop and cut and slice thru in the same way as the clean preamp with the clean/clipped Mac power. I'll try Scarlet Begonias chord chunks, Sugaree low note riff, China Cat, Franklins, etc. and the clean preamp just seems to have that extra something. Too much tube pre distortion and the sound is too compressed, and the harder you pick into it, the darker and warmer the result. The clean pre into the clipped power amp does the opposite with harder picked notes gaining even more punch and sparkle and generally the notes just seem to pop out.

I encourage everyone here who can to do this experiment. Try the cranked up Fender pre into totally clean power. Then try the clean Fender pre into a Mac that driven to the point of clipping and listen to that. Beware, even a single 50 watt side of a Mc250 is loud. A 300+ watt side of a Mc2300 is deadly in a small space. I actually find that at low levels playing by myself that getting a bit of pedal or tube dirt definitely helps things sound musical, but when everything is cranked at stage-band levels, EVERYTHING changes and in a big way.

Brad

[waldo041]

Maybe when Healy said the MC2300 didn't have the Sentry Monitor he really meant to say it didn't have the Power Guard. His is getting old ya know. Why wouldn't McIntosh build a MC2300 without the Sentry Monitor anyway? Is there any confirmation from McIntosh that they released any MC2300s without the Sentry Monitor circuit or did Healy say that they removed it after they got it?

Jim

i totally understand the skeptism. as we were also, but you have to understand the topology of the mc2300 and the topology and the location in the circuit of the power guard to grasp and fully undestand what healy stated. the power guard is located at the actual input of the mcintosh and at the actual output or speaker terminals. that's where it needs to be to do it's job. when very directly asked about the sentry, healy was not confused by where it lived. he stated specifically that there was no limiter on the driver cards they used. that driver card is exactly where the sentry monitor is located, not the input or output of the mac where the power guard is located. so he was definately not confusing the 2. he knows the difference as he is very fluent in this subject. the mc2500 does have the power guard, but jerry never used it, phil did. so as i stated, we were also skeptical and have been thru all these types of questioning.

jim, you have a few macs, pull the sentry monitor off that mc50, and give it a whirl, i bet you'll like what you hear and guarentee a difference with and without. after that you'll want to "curve tracer" and match the output transistors and driver. plus do the same thing to the emitter resistors. that will give you the symmetry jerry played with, and get you that much closer. nah, as i stated, healy is very fluent at what he does.

~mike

[SarnoMusicSolutions]

Maybe when Healy said the MC2300 didn't have the Sentry Monitor he really meant to say it didn't have the Power Guard. ..... Why wouldn't McIntosh build a MC2300 without the Sentry Monitor anyway? Is there any confirmation from McIntosh that they released any MC2300s without the Sentry Monitor circuit or did Healy say that they removed it after they got it?

Jim

Exactly Jim.......

Hence the ongoing discussion. Like Waldo said, Dan did give a few strong clues that he knows exactly what he's talking about and isn't confused. He's probably laughing that we're confused.


I must say that for experimentation, I did remove the Sentry Monitor limiter on my Mc250 and I preferred it. It sounded right. The power transistor clipping was less "fizzy" without the Sentry Monitor. Stu Allen heard it and liked it enough to have me do the same to his. I haven't tried this on an Mc2300.


Brad

[mijknahs]

It just seems strange to me that McIntosh would intentionally NOT put the Sentry Monitor on any of their amps after they've been using it for several years on other models and the sole purpose is to protect the amp and make it last longer without failure. Especially industrial or military versions. Seems those would need to be extra rugged. That's all.

I'll have my amp tech buddy help me remove the Sentry Monitor and we'll see how it sounds.

Thanks,
Jim

[Rusty the Scoob]

Do we know how Phil drove his? Will I need to be on a similar quest when I finally go to a preamp/poweramp setup? I'm hoping to get close enough just by using any currently-available good clean poweramp.

[NeebruM]

I'm obviously WAY late into this discussion. Thanks for the clarifications.
I'm still confused, however.
I always thought the sentry guard was for shorts to protect the transistors.

My Mac on their clinic day tested out at 408 watts per. Sorry about the typo.

Actually, the settings I mentioned and named 2300s was BEFORE the wall. At the Fox when I was all over the place. (Some great stories from that night but that's another day)

I could have SWORN the dust up with the government was over 2500's. It doesn't matter a whole lot to me. Here is some info from the McIntosh site linked on the prior page:

MC2300 Navy version with 6, 25, 30, 64, 120, 182, & 256 ohm output

MC2300 modified for Sanders Associates

MC2300E modified for Bolt, Beranek & Newman with 50, 100, 200, & 400 ohm outputs

MC2500 Navy

MC2500 modified for Westinghouse

MC2500 modified for Western Research (Sept. 1986)

MC2600 modified for Western Research

I was first turned on to the idea of using McIntosh in a band setting (I had coveted Mac from Best Sound on Brentwood for a couple of years prior) from Owsley when they played at the Washington U quad in 69. He was using Mac for the PA. (another long story) lol

Anyway, peace to you all on your quests.

I personally liked the wall of tie dyed Fenders of 68 and 69 better as well as the SG. When you think about it the Tiger, et al was a combined SG/Strat best of both worlds kind of thing. I'm sure someone here has stats on if they had a strat kind of neck or not.

I still have 2 McIntosh ML4's - 8 12's, etc....
Somebody want to start their OWN wall of sound? lol

It all depends on when you started, I guess. Your reference starts when you first heard Garcia play to you....

Take care....
John

[SarnoMusicSolutions]

Hey John,

are you here in St. Louis like me? You said "Best Sound on Brentwood" and mentioned the Dead at Wash U...

I think there were numerous battles over mac gear with the military. There were early tug of wars over, I think, some Mc3500 tube power amps, and then again at some point with the Navy who used the Mc2300's for their address and claxon systems in submarines and maybe other vessels too. Waldo has more detail on that.

408 watts is high compared to the 343watts they show in their service manual from one particular test, but doesn't surprise me. That's in the ballpark.

The Sentry Monitor is designed as a protection circuit. In engineering circles it's actually a VERY common and standard circuit found in nearly any transistor power amp after the '60s and is known as a VI Limiter. It's not a unique Mac thing, maybe it was in '67 though. The primary function of that circuit is to protect the output transistors in the case that the speaker wires get shorted and "ask" the transistors to dump amounts of current that could cook themselves or stress the power supply. The Sentry Monitor/VI Limiter circuit does this by detecting excessive current at the output and then sending that voltage information back to the driver stage telling it to instantly mute or attenuate the audio signal, thus turning off the output transistors enough to prevent the high current flow. The thing about this circuit is that you can also get it to trigger by asking the amp to operate beyond its power limits as the transistors begin to distort. In that condition the current flow will also be excessively high and at some point the same high current flow will activate the limiter circuit. It's not a clean or pretty audio peak limiter, but in this distortion territory, it can become active and impart its character to the audio as well as put a limit on the output level. I'm sure the level at which it becomes active varies from brand to brand, but being a protection limiter it's likely to kick in soon after the output transistors are asked to operate beyond their power (current) limits.

So knowing that Jerry clearly operated his Mc2300 channel into the realm of a little bit of clipping and distortion on his louder pick attacks and strikes, this whole Sentry Monitor (VI Limiter) theory came into play, assuming that the circuit was in place. From everything we can gather from Mac, this circuit was installed in every Mc2300 and they had no good reason not to include the protection. What we believe (without 100% knowing) is that DH either had some oddball Mc2300's without this circuit, or he removed this circuit, or it actually was there all along and the confusion was all about his comment that the "limiter" not being in there was referring to the wholly other circuit called the Power Guard limiter. The reason this topic is so interesting is that this is the very realm of Jerry's tone that makes it extremely special and musical. It's what let him get away with such a clean tone but without the spikey, linear, sterile response that one gets with a purely clean rig. This peak clipping, whether it's purely transistors clipping, or if it's transistors clipping along with a bit of this Sentry Monitor activity contributing to the peak smoothing and extra "grit" in the sound, is how Jerry, with his "clean" tone, could lay into his rig and get a bit of rounding and sustain and that very unique squirty, sparkly, crystalline character as he picked hard, assuming the preamp is running clean without significant tube clipping. It may sound trivial, but this is very central to his whole sound. Many of us who have tried a clean preamp into a purely clean amp find right away that it's sterile, has no added sustain, and the ice-picky pick transients are simply brutal on the eardrums. Seems obsessive, but this whole peak clipping factor is major for anyone trying to understand it and achieve a similar result for ourselves. DH has stated at times that the preamp ran completely clean and at other times he's stated that it may have had a bit of clipping. But he always states that the Mac was definitely clipped and also the JBL's being hit with such high power also had a big part in this peak distortion we're talking about. He even said that the power levels were so high for the JBL's that Jerry would literally de-gauss the magnets in them in a fairly short period of time. And we should not discount the role of the iron and coil in the Mac's output "autoformer" as a contributor to the tonal character of this clipping.

Best,
Brad

[NeebruM]

I'm from STL but now live in the country... Cuba to be close to exact though I'm on a 400 acre farm outside of it.

I even recorded at Technasonic next door to best in the day... lol

I understand the variable mysteries and it IS interesting. I think maybe a clue is to go back to the beginning with the Fender Twin REVERB...

Do not discount the tube driven SPRING reverb in the mix as well. I know Jerry liked a bit of reverb in it all at least early. I'm sure this has been discussed but part of the simple answer is he loved the twin reverb sound and used it as a pre amp into the Mac just for more oomph, at first anyway.

I'm talking pre midi's and all the effects he got carried away with IMO, though he did come back to his Jerry sound towards the end for which I will be always grateful.

And yea those locomotive transformers are a part... Imagine lugging all those Mac's coast to coast!

BTW, I have a friend that HAS one of Garcia's silver face twins. He got it from Bob Heil in Morissa, IL YEARS ago after they came through. Thom and I played together for years. Alvin Pivil was one of the names. They opened for the GD at Wash. U.
It was pre 2300 though, I think, but still interesting and a hell of a keepsake.

I know Jerry would go through speakers like crazy... Pick ups too. he just over awed them all! I think maybe he was magnetic!

Maybe some day we can talk dead stories and pre amps. I get up to STL once in awhile. Next time will be to see the Schwag with Tracy Lowe on the 23rd and do a speaker deal.

see ya' around

[SarnoMusicSolutions]

Great stuff John.

Thanks for sharing. I think the tone many of us have our minds on is the '80s tone, pre midi, real simple setup where most of the time he didn't have pedals enabled and it was literally the guitar to the Fender preamp to the Mac to the JBL's. Real pure and simple and had that "thing". That setup would get this effect we're talking about. And true, the spring reverb was definitely a part of his sound for a long time, but eventually for a period he incorporated outboard digital reverb too, yet still retained the basic preamp/mac/jbl setup.

I hope to see you sometime to geek out even more on the topic.

Brad

[RiverRat]

.

[NeebruM]

Great stuff John.

Thanks for sharing. I think the tone many of us have our minds on is the '80s tone, pre midi, real simple setup where most of the time he didn't have pedals enabled and it was literally the guitar to the Fender preamp to the Mac to the JBL's. Real pure and simple and had that "thing". That setup would get this effect we're talking about. And true, the spring reverb was definitely a part of his sound for a long time, but eventually for a period he incorporated outboard digital reverb too, yet still retained the basic preamp/mac/jbl setup.

I hope to see you sometime to geek out even more on the topic.

Brad

I would enjoy that...
also,
Don't forget the other world of TUBES in the mix as well. NOS, JJ's, Russian and on and on. I know on MY Fenders the reverb WARMS it up some too, which was a part of it all.
I'm obviously no engineer, just an old musician that intensly studied the sound since 1968, with a few breaks for growing up that were quickly abandoned...

The closest thing I've heard to the ORIGINAL Fender sounds Garcia had is Derek Trucks sound using the SG and humbuckers through his Fender. Though there are recordings of OLD GD, they just couldn't capture the sound of being there.

later

[SarnoMusicSolutions]

Yeah man.

That tone Derek got with just driving the Fender Pro [edited: I mean Super Reverb] amp with his '61 reissue SG, I love it. He'd just drive the amp till it was nice and fat and dirty. That's very late '60's Jerry. Just do it with 4 or so Twins with JBL speakers in the cab's, and you're really there.

Brad

[waldo041]

Save yours ears, tame the volume, attenuate with the autoformer...

always good advice! but jerry used the 2 ohm tap of the autoformer which is where the coupling for the unity coupled transformer the mc2300 uses. you can attenuate by using the 4 or 8 ohm tap, but that will produce a different tone. healy stated that he did discover that the thing ran much better at lower impedances. the whole goal of this discussion is to figure out what is happening and how, so we can create a lower wattage version for the average player for all the reasons you stated.

as for the topic raised about the twin reverb. brad and i discussed this last night. the preamp taps we have all been using are NOT what jerry had. i myself have tried all the taps that are out there. both the alembicization and fontaine tap at dozin are off the mark. thus why i have removed my preamp mod page. jerry's vibrato was disengaged, as it has been noted, but it wasn't done by lifting the leg or removing the vibrato roach. healy's way which has recently come to light actually removes the entire vibrato circuit, out of the circuit completely. this includes the 50k vibrato intensity pot which is a load to ground that all the other taps have, even with the leg lifted or a completely removed vibrato roach. if you leave that roach intact, and tap it the way we have all been doing it, the vibrato still works tapped and is padded or loaded down by the 50k intensity pot. healy's way does not, because it isolates the vibrato preamp completely but still shunts the power section. brads sms classic does this because he never put the vibrato circuit in his preamp, and the result had guys asking for a pad pot because they were blowing out there input stages of the less robust amplifiers being used out there. i did the same thing with it when i had it here for a test run. those transient peaks and sharp voltage spikes were padded down the alembic or dozin way, but healy's way will produce voltages of 30v+ transient peaks and higher voltages in general. your gonna blow out the input section of the amplifier and not be able to get to the power level jerry got to, to achieve the sustain and grit we are after. this is where the mc2300 shines as its volume pot is at the begining of the input stage, so it can be attenuated and thus why jerry could be at full power with the mc2300's volume lower then you would think( around 11 oclock). he cooked the output trannies and did that as well as de-gaussed his speakers. but to me cooking the output trannys, or as healy stated, he frapayed them, is a major factor in this sentry monitor dispute. fact is with the sentry monitor, you would be less likely to cook them as healy described OR slam those meters and de gauss speakers. healy stated jerry changed speakers every 4 - 6 shows because of the stress he put on them. it's also been stated that he personally stated he could blow the speaker whenever he chose to do so, but he hated doing it. those e 120's have some serious spec's of there own and by all standards should be able to take the beating especially with a VI limiter in the amp, but jerry de gaussed those AND frapayed output transistors. while that is no way proof that the sentry monitor is there or not, it does factor into the equation, because with the VI current limiter in place, that would happen less frequently as it has been stated it did. even though mcintosh will tell you it's only there for shorts, at pushed levels it is current limiting things. just my opinion.

i'll get that preamp page up as soon as possible so you all can hear the difference. but here is the schematic of the healy tap,per dan healy.




peace,
waldo

[tigerstrat]

I love Derek's playing (assuming you are talking about Trucks) but don't follow him real closely- however, isn't he known for using Super Reverbs? (a la Lowell George perhaps)

[SarnoMusicSolutions]

I meant to say Super Reverb and not Pro Reverb. 4-10"s indeed. Yes, Derek Trucks.

Mild brain fart.


Brad