I'm really curious, when is the treble shift on the mark iv activated? Meaning, schematically, the 750p cap just above the 250p tr cap/treble pot. I've found that the pull bright on the lead drive knob activates the 4th stage cathode resistor bypass cap, and that's the only "shift" type of pull function on the amp for the lead ch besides pull fat and mid/harm. Does the pull bright actually do both functions?
Mark IV treble shift?
- Thread starter eet fuk
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dodger916
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Doubtful. The lead channel has another pull function: presence.eet fuk said:
Yeah, thanks, I forgot about that one, but that has to do with the negative feedback of the power amp, nothing to do with the tone stack in the front of the amp. Unless, of course, that knob engages both functions at the same time. Doubt it.
Not a dealbreaker or a cause to stop playing guitar, I was just super curious, going through schematics, and comparing Mk IV and III, and just thinking. It was like trying to play some kind of map game, trying to see what does what, I found the other switches, but that one is a headscratcher.
The quad pre schem blatantly says "treble shift" right next to it, and the mkIII has the associated knob. Nothing on the IV. Except pull bright. But that's 4th stage cathode cap. When we play card games about useless facts on mesa amps, this knowledge will come in handy. :mrgreen:
Not a dealbreaker or a cause to stop playing guitar, I was just super curious, going through schematics, and comparing Mk IV and III, and just thinking. It was like trying to play some kind of map game, trying to see what does what, I found the other switches, but that one is a headscratcher.
The quad pre schem blatantly says "treble shift" right next to it, and the mkIII has the associated knob. Nothing on the IV. Except pull bright. But that's 4th stage cathode cap. When we play card games about useless facts on mesa amps, this knowledge will come in handy. :mrgreen:
Restless Rocks
Well-known member
Pulling the Gain knob on the MkIV lead channel which says "pull fat" is the same thing as the "pull shift" on the MkIII engaging the 750pf cap on the tone stack.
Thanks, but I think that's incorrect. I've tried the differnent values in the duncan tone stack calculator, and the 750p+250p makes things brighter, not fatter. I believe the mark IV pull fat engages the 2.2u cap in parallel with the .47u in the first stage cathode. Is this incorrect?
Mark III tone affecting pull functions:
1. pull bright - vol knob - 100pf "bright cap"
2. treble shift - trbl knob - 750pf treble cap
3. bass shift - bass knob - 15u cathode cap
4. pull deep - mstr knob - 15u loop b cathode
5. pull bright - ld mst knob - .22u cathode
Mark IV:
1. pull bright - only on clean and rhythm
2. treble shift - ?
3. bass shift - ld gain knob - same as 3 above
4. pull deep - permanently in circuit (fuk yeah)
(poor mark V
)
5. pull bright - ld drv knob - same as 5 above
Mark III tone affecting pull functions:
1. pull bright - vol knob - 100pf "bright cap"
2. treble shift - trbl knob - 750pf treble cap
3. bass shift - bass knob - 15u cathode cap
4. pull deep - mstr knob - 15u loop b cathode
5. pull bright - ld mst knob - .22u cathode
Mark IV:
1. pull bright - only on clean and rhythm
2. treble shift - ?
3. bass shift - ld gain knob - same as 3 above
4. pull deep - permanently in circuit (fuk yeah)
(poor mark V
)5. pull bright - ld drv knob - same as 5 above
chipaudette
Well-known member
Restless Rocks has it right. Pulling the gain knob on the lead channel is the one that adds the 750p cap to the treble. Here's the quote from the manual:
"This LEAD GAIN control incorporates a PULL FAT for thickening the sound and saturating the signal further. This pull shift lowers the frequency of the Treble".
If you go back to your Duncan tone stack calculator, you'll see that adding the 750p (ie, increasing 250p to 1000p) does indeed lower the frequency at which the Treble knob does its job. Those added mid frequencies really make the saturation of the lead channel be more thick, or "fat" if you''re the guy at Mesa who wrote the "pull fat" phrase on the Mark IV front panel.
The schematic at the Mark IV clearly shows that the 750p cap is on a switch (well, an LDR actually). By pulling the Mark IV gain knob for the lead channel, you activate this LDR, which puts the 750p into the circuit. This is also what happens on the R2 channel when you pull its gain knob.
So, to complete your table for the Mark IV...
1. Pull Bright - 120p cap across the volume knob. On the Mark IV, it's only available on the clean (R1), but not R2 nor Lead. On the Mark IV, pull the R1 gain knob to engage. Note that on the Mark II and III, the "pull bright" affects all the channels.
2. Treble Shift - 750p cap added to 250p Treble cap. Only on Mark IV R2 and Lead. On the Mark IV, pull the R2 gain knob or the Lead gain knob to engage.
3. Bass Shift - Used to be available on Mark II and Mark III...added a 15u cap on cathode of V1A. This functionality is not available on Mark IV. on the Mark IV, the R1 and Lead channels only have the default 0.47u cap at this location. On the Mark IV, R2 does engage a 2.2u cap at this location to provide some mid thrust, but it is not user controllable. It engages for R2 automatically.
4. Pull Deep - This used to be available on the Mark II and III. It added a 15u cap on cathode of the gain stage after the effects loop. The Mark IV is permanently wired with the 15u cap engaged. The Mark IV has other options to affect the tonal balance in a similar way...see "Lead Voicing" below
5. Lead Bright - Adds 2.2u Cap to a cathode in the lead circuit. This exactly mimics the same function as in the lead channel of the Mark II and III. As with those amps, the Lead Bright only affects the lead channel of the Mark IV. On the Mark IV, pull the Lead Drive drive to engage.
And the Mark IV has a new option not on the Mark II or Mark III...
6. Lead Voicing -- A switch on the back of the Mark IV allows the user to toggle between "Harmonics" vs "Mid Gain". This switch affects the cathode of V2A, which is the the gain stage where the lead circuit rejoins the main signal path. When set to "Harmonics", the V2A cathode has no cap. This is similar to the Mark II and Mark III. When set to "Mid Gain", it adds a 2.2u cap, which boosts the treble and mids. This is not available on the Mark II or III. Note that this cap in the Mark IV is only engaged when in the lead channel. The amp automatically disengages this cap when switching to the clean channel. Interestingly, the R2 channel automatically engages a 15u cap here, which provides maximum gain for R2 across all frequencies. It is not user selectable for R2, it just happens. In R1, all caps are automatically switched out of the circuit, providing maximum clean headroom.
So, there's the story on the mark IV switches as I understand them. I'm certainly not offended by disagreements with my analysis and opinions.
Chip
"This LEAD GAIN control incorporates a PULL FAT for thickening the sound and saturating the signal further. This pull shift lowers the frequency of the Treble".
If you go back to your Duncan tone stack calculator, you'll see that adding the 750p (ie, increasing 250p to 1000p) does indeed lower the frequency at which the Treble knob does its job. Those added mid frequencies really make the saturation of the lead channel be more thick, or "fat" if you''re the guy at Mesa who wrote the "pull fat" phrase on the Mark IV front panel.
The schematic at the Mark IV clearly shows that the 750p cap is on a switch (well, an LDR actually). By pulling the Mark IV gain knob for the lead channel, you activate this LDR, which puts the 750p into the circuit. This is also what happens on the R2 channel when you pull its gain knob.
So, to complete your table for the Mark IV...
1. Pull Bright - 120p cap across the volume knob. On the Mark IV, it's only available on the clean (R1), but not R2 nor Lead. On the Mark IV, pull the R1 gain knob to engage. Note that on the Mark II and III, the "pull bright" affects all the channels.
2. Treble Shift - 750p cap added to 250p Treble cap. Only on Mark IV R2 and Lead. On the Mark IV, pull the R2 gain knob or the Lead gain knob to engage.
3. Bass Shift - Used to be available on Mark II and Mark III...added a 15u cap on cathode of V1A. This functionality is not available on Mark IV. on the Mark IV, the R1 and Lead channels only have the default 0.47u cap at this location. On the Mark IV, R2 does engage a 2.2u cap at this location to provide some mid thrust, but it is not user controllable. It engages for R2 automatically.
4. Pull Deep - This used to be available on the Mark II and III. It added a 15u cap on cathode of the gain stage after the effects loop. The Mark IV is permanently wired with the 15u cap engaged. The Mark IV has other options to affect the tonal balance in a similar way...see "Lead Voicing" below
5. Lead Bright - Adds 2.2u Cap to a cathode in the lead circuit. This exactly mimics the same function as in the lead channel of the Mark II and III. As with those amps, the Lead Bright only affects the lead channel of the Mark IV. On the Mark IV, pull the Lead Drive drive to engage.
And the Mark IV has a new option not on the Mark II or Mark III...
6. Lead Voicing -- A switch on the back of the Mark IV allows the user to toggle between "Harmonics" vs "Mid Gain". This switch affects the cathode of V2A, which is the the gain stage where the lead circuit rejoins the main signal path. When set to "Harmonics", the V2A cathode has no cap. This is similar to the Mark II and Mark III. When set to "Mid Gain", it adds a 2.2u cap, which boosts the treble and mids. This is not available on the Mark II or III. Note that this cap in the Mark IV is only engaged when in the lead channel. The amp automatically disengages this cap when switching to the clean channel. Interestingly, the R2 channel automatically engages a 15u cap here, which provides maximum gain for R2 across all frequencies. It is not user selectable for R2, it just happens. In R1, all caps are automatically switched out of the circuit, providing maximum clean headroom.
So, there's the story on the mark IV switches as I understand them. I'm certainly not offended by disagreements with my analysis and opinions.
Chip
chipaudette
Well-known member
Oh, and eet fuk, you do have it right with regard to the pull shift on the presence knobs. Those pull functions (for R2 and Lead) only affect the response of the presence circuit. As you said, the presence circuit is not part of the pre-amp, but only affects the frequency response of the power amp. You've got it right.
Oh, HELL YEAH!
Thanks, Chipaudette!!!! And thanks RestlessRocks, sorry for not believing you, you were right. Looking at the Duncan calculator, I can see now about the whole 100Hz-4k area boosted with the bulk of it being the low mids. and I totally missed the "R2" on the bass shift for the stage one cathode, how could I miss that? :roll: Anyways, man, that really makes me want to get a Mk III again side by side with my IV. This has been one of those weird headscratchers for me where I just have to know, so really, thanks for all the tons of info!
Thanks, Chipaudette!!!! And thanks RestlessRocks, sorry for not believing you, you were right. Looking at the Duncan calculator, I can see now about the whole 100Hz-4k area boosted with the bulk of it being the low mids. and I totally missed the "R2" on the bass shift for the stage one cathode, how could I miss that? :roll: Anyways, man, that really makes me want to get a Mk III again side by side with my IV. This has been one of those weird headscratchers for me where I just have to know, so really, thanks for all the tons of info!
