Mark3Boogie
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I always wondered where the Rhythm 2 mode take the additional gain to the Clean channel, is a early cascading gain stage?
Where the Rhythm 2 take the gain in Mark III
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chipaudette
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Based on my reading of the Mark III schematics on the web, the R2 channel does not cascade any additional gain stages relative to the R1 channel. Instead, it utilizes more of the gain that is inherently available in the R1 circuit, but which is simply not used in the R1 setup.
For example, the R1 pre-amp circuit is based on the classic Fender blackface amps from the 60s...in particular the R1 circuit is patterned on the "Reverb" channel of these Fender amps. In this channel, the guitar input goes directly to the first gain stage...then to the tone controls, which causes a the signal to lose a lot of strength, and then to a 2nd gain stage to make up for the losses. After the 2nd gain stage, the signal is sent to the reverb unit. The output of the reverb unit is very weak. So, inside the amp, there is a 3rd gain stage to amplify the reverb output. At this amplifier, it also mixes in a highly-attenuated version of the dry (ie, not reverb'd) signal, so that both signals get amplified together. The "Reverb" knob on the amp controls how much of the reverb signal gets mixed with the highly-attenuated dry signal at this 3rd gain stage.
In the Mark II and later Boogie amps, Mesa moved the reverb unit to be much later in the circuit. They kept, however, the configuration of these 3 gain stages, including the big 3.3M signal attenuator between the 2nd and 3rd gain stage. That's kinda weird, but whatever.
OK, now back to the Mark III...when Mesa designed the Mark III, they created the new R2 channel by partially defeating the big attenuator between the 2nd and 3rd gain stage. This means that less of the amp's gain is "thrown away" by the attenuator. As a result, the channel has more gain overall without actually adding any additional gain stages.
In the Mark III, there are other changes between R1 and R2, but it is my opinion that the partial bypassing of the 3.3M signal attenuator is the most important change with respect to gain. I could be wrong...I'd love to hear other people's opinions.
Hope this helps.
Chip
For example, the R1 pre-amp circuit is based on the classic Fender blackface amps from the 60s...in particular the R1 circuit is patterned on the "Reverb" channel of these Fender amps. In this channel, the guitar input goes directly to the first gain stage...then to the tone controls, which causes a the signal to lose a lot of strength, and then to a 2nd gain stage to make up for the losses. After the 2nd gain stage, the signal is sent to the reverb unit. The output of the reverb unit is very weak. So, inside the amp, there is a 3rd gain stage to amplify the reverb output. At this amplifier, it also mixes in a highly-attenuated version of the dry (ie, not reverb'd) signal, so that both signals get amplified together. The "Reverb" knob on the amp controls how much of the reverb signal gets mixed with the highly-attenuated dry signal at this 3rd gain stage.
In the Mark II and later Boogie amps, Mesa moved the reverb unit to be much later in the circuit. They kept, however, the configuration of these 3 gain stages, including the big 3.3M signal attenuator between the 2nd and 3rd gain stage. That's kinda weird, but whatever.
OK, now back to the Mark III...when Mesa designed the Mark III, they created the new R2 channel by partially defeating the big attenuator between the 2nd and 3rd gain stage. This means that less of the amp's gain is "thrown away" by the attenuator. As a result, the channel has more gain overall without actually adding any additional gain stages.
In the Mark III, there are other changes between R1 and R2, but it is my opinion that the partial bypassing of the 3.3M signal attenuator is the most important change with respect to gain. I could be wrong...I'd love to hear other people's opinions.
Hope this helps.
Chip
Mark3Boogie
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Hi, thanks for the reply, I'm curious to know even what are the other change between the two channel that is not concern the gain. 
chipaudette
Well-known member
I think that the other changes have to more to do with the "voicing" of the R2 channel, meaning how the low and high frequencies are shaped by the circuitry.
The voicing of the R2 channel is one of the areas that was tweaked between the different versions (aka, "stripes") of the Mark III. There are not schematics on the web for the different stripes, so I'm really not sure what was done for most of the Mark IIIs. I'd love for someone with knowledge of the circuitry of the different stripes to post a detailed definition of the circuit changes between the stripes. Through such a desicrption, we'd also better see the differences between R1 and R2.
Sorry I can't be more help.
Chip
The voicing of the R2 channel is one of the areas that was tweaked between the different versions (aka, "stripes") of the Mark III. There are not schematics on the web for the different stripes, so I'm really not sure what was done for most of the Mark IIIs. I'd love for someone with knowledge of the circuitry of the different stripes to post a detailed definition of the circuit changes between the stripes. Through such a desicrption, we'd also better see the differences between R1 and R2.
Sorry I can't be more help.
Chip
That mostly depends on the stripe of the concerned mkIII.
What Chipaudette wrote is right for the black dot. In fact, in R1 mode, the signal at the output of the "attenuator" (a voltage divider) between those 2 gain stages is approx. 15% of the input signal, so it's a big attenuation. In R2 mode, the output is approx. 30% of the input signal, so it is less attenuated but it is still attenuated a lot. (The reality is a bit more complex, because only a portion of the frequency spectrum is less attenuated due to the presence of a cap in the added circuitry which forms a frequency filter, but you get the general idea.)
That is why R2 channel is not that hot on the black dot. It is hotter on the red, blue and green stripes.
On those stripes, it combines this "attenuated attenuator" and other tricks. In R2 mode, it adds a decoupling cap on the cathode of the third gain stage, resulting in more gain on the medium and high frequencies. It seems to me that this trick results in more gain than the attenuated voltage divider.
R2 mode also activate a frequency shift on the tonestack, which is the same function as the pull shift function on the treble pot for the lead channel.
The purple stripe is a different story. It doesn't attenuate the voltage divider, but instead it lifts a little bit the tonestack from ground resulting in more gain. In the vein of the gain boost function of the Mark I and IIA/IIB.
What Chipaudette wrote is right for the black dot. In fact, in R1 mode, the signal at the output of the "attenuator" (a voltage divider) between those 2 gain stages is approx. 15% of the input signal, so it's a big attenuation. In R2 mode, the output is approx. 30% of the input signal, so it is less attenuated but it is still attenuated a lot. (The reality is a bit more complex, because only a portion of the frequency spectrum is less attenuated due to the presence of a cap in the added circuitry which forms a frequency filter, but you get the general idea.)
That is why R2 channel is not that hot on the black dot. It is hotter on the red, blue and green stripes.
On those stripes, it combines this "attenuated attenuator"
and other tricks. In R2 mode, it adds a decoupling cap on the cathode of the third gain stage, resulting in more gain on the medium and high frequencies. It seems to me that this trick results in more gain than the attenuated voltage divider.R2 mode also activate a frequency shift on the tonestack, which is the same function as the pull shift function on the treble pot for the lead channel.
The purple stripe is a different story. It doesn't attenuate the voltage divider, but instead it lifts a little bit the tonestack from ground resulting in more gain. In the vein of the gain boost function of the Mark I and IIA/IIB.
chipaudette
Well-known member
Ah, I agree that adding the decoupling cap to 3rd gain stage will result in a fairly substantial boost in gain. I've modded my IIC(+) to do this. It's kinda fun sometimes, but I don't really like the tone. I've futzed with the value of the decoupling cap a bit (what do they use in the Mark III?), but I couldn't find a value that gives me the kind of sound that I'm going for...the distortion that resulted felt too "spongey", not "crunchy".
Also, as you said, enabling the Treble Shift on the tone stack can have a big effect on the overall gain (depending upon your treble setting). If some of the stripes incorporate the Treb Shift as part of R2, it could definitely be an important contributor to the gain boost between R1 and R2. I've not done this mod to my IIC(+) as it would require a second LDR, or it would require de-soldering the one that's in there right now. That's risky. I can imagine, however, its importance to getting more thrust from R2.
Together, I'd agree that these changes are more important to the boost in gain of R2 than simply reducing the attenuation of the 3.3M voltage divider.
Thanks for the extra info!
Chip
Also, as you said, enabling the Treble Shift on the tone stack can have a big effect on the overall gain (depending upon your treble setting). If some of the stripes incorporate the Treb Shift as part of R2, it could definitely be an important contributor to the gain boost between R1 and R2. I've not done this mod to my IIC(+) as it would require a second LDR, or it would require de-soldering the one that's in there right now. That's risky. I can imagine, however, its importance to getting more thrust from R2.
Together, I'd agree that these changes are more important to the boost in gain of R2 than simply reducing the attenuation of the 3.3M voltage divider.
Thanks for the extra info!
Chip
Mark3Boogie
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Thanks guys, very exhaustive explainations.
At this point i can ask you how the lead channel is changed technically speaking trough the various stripes?
At this point i can ask you how the lead channel is changed technically speaking trough the various stripes?
Always a pleasure to discuss this stuff.
On the III I've seen, that cap is 470n. I've also enabled this cap in lead mode, just like you did on your C+, Chip. I felt that the sound is crunchy but too much in the high mids region, on both R2 and lead channels.
So I've changed the value to 1µF which feels just right and natural for me. I've tested 2.2µ and 4.7µ, but it sounded too spongy to me, just like you mentionned.
Mark3Boogie, I'll try to explain later some of the changes on the lead channel between the black, red and blue stripes. I'm not at home, so i have not my notes.
I've just seen pics of a purple stripe III and I couldn't see some cap's values. So I can't say definitively what are the changes, but it seemed close to the red stripe, at least on the lead part of the preamp.
On the III I've seen, that cap is 470n. I've also enabled this cap in lead mode, just like you did on your C+, Chip. I felt that the sound is crunchy but too much in the high mids region, on both R2 and lead channels.
So I've changed the value to 1µF which feels just right and natural for me. I've tested 2.2µ and 4.7µ, but it sounded too spongy to me, just like you mentionned.
Mark3Boogie, I'll try to explain later some of the changes on the lead channel between the black, red and blue stripes. I'm not at home, so i have not my notes.
I've just seen pics of a purple stripe III and I couldn't see some cap's values. So I can't say definitively what are the changes, but it seemed close to the red stripe, at least on the lead part of the preamp.
chipaudette
Well-known member
Regarding the value of the cathode bypass cap, I find it interesting that the Mark IV uses a big one (16 uF) for its R2. That's quite different than the skinny one (0.47 uF) that's in the Mark III that you've seen. I've never played R2 on the Mark IV or Mark III...does the Mark IV R2 feel a lot different than R2 on the Mark III?
Continuing to look at the Mark IV schematic, I see that it it does have an option for a 2.2 uF cap at this location when using the lead channel. Usually, the lead channel has no cap here...but, if you enable the "Mid Gain", a 2.2 uF cap comes in. This looks really familiar to me know. When doing my mod, I tried a big fat cap here (like the 16 uF) and really didn't like it. Then, I tried the smaller one (probably a 2.2 uF) because of its inclusion in the Mark IV (and, if you're into trivia, its used in the Triaxis "Green" LD2 channel as well).
Since I'm not a big fan of the sound of the 2.2uF in my amp (on either the rhythm or lead channel), maybe I should abandon it and try the 1uF cap that you suggest.
Chip
Continuing to look at the Mark IV schematic, I see that it it does have an option for a 2.2 uF cap at this location when using the lead channel. Usually, the lead channel has no cap here...but, if you enable the "Mid Gain", a 2.2 uF cap comes in. This looks really familiar to me know. When doing my mod, I tried a big fat cap here (like the 16 uF) and really didn't like it. Then, I tried the smaller one (probably a 2.2 uF) because of its inclusion in the Mark IV (and, if you're into trivia, its used in the Triaxis "Green" LD2 channel as well).
Since I'm not a big fan of the sound of the 2.2uF in my amp (on either the rhythm or lead channel), maybe I should abandon it and try the 1uF cap that you suggest.
Chip
I also had a look at the IV's schematic as a reference. But 16µ was way too much in my opinion, for both R2 and lead channels.
I haven't played any IV (revision a or b) so I can't compare its R2 channel to the III. But according to the schematic found on the net (which is for the revision a), and supposing it has no intentional error, R2 channel is different on the IV from any III stripe. It combines the cathode bypass cap and a full shunt of the voltage divider between the 2nd and 3rd gain stage. So it should feel a bit different.
I tested this full shunt on my III, but R2 was squealing a lot, so I reversed it.
I haven't played any IV (revision a or b) so I can't compare its R2 channel to the III. But according to the schematic found on the net (which is for the revision a), and supposing it has no intentional error, R2 channel is different on the IV from any III stripe. It combines the cathode bypass cap and a full shunt of the voltage divider between the 2nd and 3rd gain stage. So it should feel a bit different.
I tested this full shunt on my III, but R2 was squealing a lot, so I reversed it.
Promethean
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I am really interested in this thread.
I have a MK III Purple Stripe. I've modded it with the R2 level control and some changes to deepen the reverb mix. I'm interested in any insight into how to brighten the R2 channel against R1 as it can sound just a little dark unless I run the input gain and treble high, which can overload R1 with hot pickups. I generally get round it using a boost pedal but would like a bright option on R2
My schematic is very blurred and I can't read the values or use it to understand the pcb. Are there some capacitors tuning the R2 EQ that I can change? Do you know their numbers on the PCB?
Any thoughts would be most appreciated.
I have a MK III Purple Stripe. I've modded it with the R2 level control and some changes to deepen the reverb mix. I'm interested in any insight into how to brighten the R2 channel against R1 as it can sound just a little dark unless I run the input gain and treble high, which can overload R1 with hot pickups. I generally get round it using a boost pedal but would like a bright option on R2
My schematic is very blurred and I can't read the values or use it to understand the pcb. Are there some capacitors tuning the R2 EQ that I can change? Do you know their numbers on the PCB?
Any thoughts would be most appreciated.
Third Age Amps
Well-known member
As you've figured out, the larger the bypass cap, the more low-end gets introduced to the signal. I like to limit the low-end in the first two stages by using .47uf caps there, then increasing to .68uf for the next, and 1uf for the remainder.
It really focuses the mids and tightens the lows because the lows aren't being amplified and distorted as much as the mids and highs. The only time I ever use anything larger than a 1uf is on clean channels where you really NEED that extra low-end.
Another way to play with the tone is to change the coupling caps coming off the plates. Changing a .02uf to a .002u or .0047u is a great way to get rid of mushy lows.
It really focuses the mids and tightens the lows because the lows aren't being amplified and distorted as much as the mids and highs. The only time I ever use anything larger than a 1uf is on clean channels where you really NEED that extra low-end.
Another way to play with the tone is to change the coupling caps coming off the plates. Changing a .02uf to a .002u or .0047u is a great way to get rid of mushy lows.
