Mark V 90W combo speaker change

[MarkV User]

I am not sure I am reading this correctly. Sorry.
C38 is a bypass cap across the 69k resistor which is part of a voltage divider circuit to the control grid of V4B. Bridging that would effectively short out the signal to ground. C39 on the other hand couples the grid to the cathode of that same circuit, this will tune out some midrange and top end before the signal gets its non-linear amplification. You cannot bridge that component, it has to be removed to determine its affect. The same would apply to C38.

Oh no. C38 is one of those designators that is reused with the same name in two places. C38 also designates one of the caps in the feedback loop that only applies to Mark IV mode, on the Loop/Driver page in the upper right. I wanted to bridge C60 also but had trouble accessing it. I was believing that because the problem didn’t go away in mode 8 that it wasn’t caused by the lack of lower frequency feedback. But I noticed now that I made a mistake and C63 will be in the way as well. For thoroughness I’ll bridge that one too and make a better go at C60. But the fact that turning presence up all the way seems to make the problem worse… the resistance is up around 300K at that point between the 250K pot and 56K at R103. Feedback voltage at C58 would be reduced by 75% just from the resistive divider, then more through the two shorting caps.

On my schematic C39 is the bypass cap a

As for the JFETs (J175 P-channel), they are in full conduction mode when there is no voltage applied to the gate (open circuit, or pulled to 0 VDC). Since they are mostly used in the mute circuit, they will shunt the control grid signals to ground as long as the strobe mute transistor (NPN Darlington) is turned on [this will bring the voltage on the gates of the mute JFETS to 0VDC]. This should only be a momentary function. The collector side of the circuit is connected to the 24V supply but will also have some capacitors connected to ground. So, when the NPN transistor turns off, there will be a charge effect on the circuit that feeds the JFETs so they do not turn off instantly but gradually. (The JFETS must be turned off with 6V to 12V on the gates to stop them from shorting the signal.) The Mute for the reverb circuit has a larger capacitor so its charging rate will be slower than the other JFET mute circuits. As the voltage on the gate rises with the charging of the caps in the circuit, the channel will get pinched off slowly. In short form it begins to act like a variable resistor controlled by the rise in the gate voltage until it reaches the 6V which is where the pinch off occurs. It may still be conducting some current but will no longer be acting like a short. Once it gets to 12V, the JFET channel is turned off and the signal is permitted to pass onto the control grid of the next stage. The charge rate is quick but not immediate.

Well, that is not quite as safe as I thought things were, but It still seems the signal path shouldn’t be able to affect the switch except during switching, which only happens when the control panel switches change?

The other factor to consider is the amps you have compared the Mark V90 too. I would assume they are either full class A or are Class A/B. I doubt they are a combination of both as that would infringe on the patents for the Simul-Class method used by the Mark V90 and its predecessors like the IIC+ DRG, Mark III DRG, Mark IVA or Mark IVB. The extended class A sockets of the Mark amps is what defines the overall tone of the output. More so with the IV or V than the III or IIC since they ran with a 15W class A for a total power of 75W, the IV class A was pushed a bit to 30W triode, and a bit more in pentode with a total power of 85W. Mark V90 also has the triode/pentode switch to deliver 45W in class A to 90W total power. I am unsure if the intent was to retain or recreate some of that EL34 Class A sound with the use of 6L6GC tubes so some boxy tuning may have been done that was to simulate that effect. I feel they missed the Mark with the IV and V.

I looked at the Mark III and IIC+ schematics for this and also my favorite touchstone amps.

To me boxiness means the thing that happens audibly when the presence frequencies and high treble and to a lesser extent the bass frequencies are attenuated, leaving a sharply defined midrange like the speaker is shut up in a box. Is that also what you mean? It could be caused by an impedance mismatch to the OT created by all the simul-shenanigans. If instead it is about an unnatural phase shift in the upper midrange… the presence feedback caps and phase splitter output caps can do that maybe?

By the way, I’ve switched back to two 5881 tubes on the inside and two 6L6GC tubes on the outer pair. Now that I’m not hurting my ears I prefer that tone to the EL34s I was using. I do want to add a bias pot and see if I can improve on what I’ve got.

Another change for later is to take out the extra capacitors so the phase splitter and driver looks more like a Mark IIC+. I feel it is likely to improve the tone. And I’m going to try using 45W pentode mode only for a while, with the HotPlate power attenuator to help get lower volume and earlier breakup. I feel it might be easiest to optimize the amp for the best sound in just one mode for now.

I would love to convert this amp to straight class AB running somewhere in the 30-50W range. It could use a pair of 5881 tubes and the best available upgrade OT for a Fender Bassman. I shouldn’t get into a big project in this crowded chassis though, and maybe I will want to leave things close to the original so I can still sell the combo some day.

The JP2C on the other hand does not have that tuning. Does not sound shrill or boxy. But then again it is based on the IIC+ HGR model. 100W/60W class A/B amp. Not odd overtones or shrill artifacts in the signal at all. Power tube saturation is there but different.

Mark VII is still a Simul Class (DRG) but Mesa must have decided to ditch the boxy tones and ice pick flair for something more useful. It is very close to the character of the JP2C but yet different in its delivery and tone density. I much prefer the Full Class A/B of the JP2C at 100W but that is my preference. Seems that my favorite amps are all Class A/B. the Mark VII is an exception, so it has gained more respect and admiration. As for the power capacity, it is on par with the JP2C or the Badlander 100.

I think I’m with you on that. I’ve never had an amp with four output tubes except for V90 though, so I’m still liking the two tube AB amps best.

 

[rarebitusa]

The one thing that makes me think this only happens on some amps is that no one would like the Mark V if they were all like this.

Yea I'd have to agree with this. Certainly there are many users who dig the V... me being one of them. I do think there are some who are lead to believe it can be the ultimate chameleon amp, oh it can be a plexi, or a JCM 800 or a Fender, it can make you breakfast too. Then they are disappointed.

The V:90 does have a different overall tone, more compressed, less organic then my other Mesas. I can understand the "boxy" feel comments by some, with the right speaker/cab it doesn't really sound that way to me. However there does seem to be some of them that have some underlying issues with ice-pick and biting frequencies that can't be dialed out. Maybe you can get to the bottom of that.

 

[bandit2013]

The strobe mute circuit only responds to the channel bus change. Each channel has a set of different relays that are controlled channel selector. When the channel changes, there is a short pulse to trigger the strobe mute, in turn shorts out the gates. Since it is based on an RC time constant to restore the gate voltage back to 12V, the transients from the relays making changes to the circuits does not transfer through to the speaker. There are some relays tied to the power supply that change the plate voltages for the preamp tubes associated with each channel bus. The same would be for the assignment of the tube rectification, triode/pentode and overall power setting defined by each channel. Making changes on an active channel will not stop the pops, since it is not the channel bus that is changing state but the associated controls that may change relay states without any pulse to the strobe mute circuit. Why does the amp make pops after power up? I have no clue. It could be that the channel selector is not yet defined since all of the LEDs turn on, the default is CH1. The Roadster and MWDR do this too.

As for having two reference designators on the same board layout is poor practice. Funny that they have the same value for capacitance. Sort of an indication they do not use any form of PCB layout programs since they usually run DRC before importing the net list into the PCB tool. Sure you can use a label or text that will not change if they suppressed the reference designators. For what I know, it is still probably laid out on film considering the shape of the traces, it reminds me of tape. The reference designators also look like they are hand written. No wonder why it takes years to make a change. I can only imagine how large that film is assuming it is not done with any CAD.

As for the boxy and nasal tone of the Mark V90, I would not doubt it has something to do with many aspects of the design. There are differences in the feedback circuit on the lead drive triode pair, several attenuators following V3A and such. I should try slaving into the Mark V and see what happens. The Mark IVB was not that much different with its primary tone, boxy and sort of had that nasal sound but not ice pick. I had the Mark III blue stripe combo at the same time I had the Mark IVB and compared the two. Mark III had the EVM12L black shadow speaker and the Mark IVB had the MC90. I did try running the IV into the EV speaker, (had a small extension cab with an EV) it was boxy and terrible, much like the Mark V90 when pushing the EV classic. Since it was a combo it was not all that epic. The Mark IVB sounded better through a 412 cab. It was more than obvious the amps were tuned differently so I would not doubt it has something to do with the phase inverter, feedback, and other areas. Also the Mark III Simul-class only used a 15W extended Class A primarily to be used with the EL34/6CA7 tubes. It worked with a full set of 6L6GC tubes but the issue with the 6L6 in the Class A sockets would not last long due to the bias voltage difference on the class A circuit. The voltage divider used resulted in a bias voltage more suitable for EL34 tube. The Mark IVB was capable or running all 6L6GC or a mixed quad of the EL34 (in class A) and 6L6GC in the A/B sockets. Not sure if the Mark V90 is suitable for that combination.

Getting a better understanding of the amp would be a good thing. A study of the older designs starting at the IIC+ and Mark III would be a good start (assuming you can find the DRG schematics). The Mark IVA circuit can be found but not the Mark IVB. I was able to get that before Mesa shut down the files on "theTubeSore" website. I deleted them when I sold the amp, also included the schematics on print with the manual. I do recall they are not the same animal as there were changes made to some areas in the design. Never played through a Mark IVA so it is hard to say how it compared to the widebody Mark IVB. I can vouch for the character of the IVB, it was much better than the Mark V90 as it lacked the ice pick character. That could be a negative feedback issue but I doubt it, slaving into the Roadster, the Mark V preamp was still an ice pick.

 

[MarkV User]

Rarebitusa:

The one thing that makes me think this only happens on some amps is that no one would like the Mark V if they were all like this.

Yea I'd have to agree with this. Certainly there are many users who dig the V... me being one of them. I do think there are some who are lead to believe it can be the ultimate chameleon amp, oh it can be a plexi, or a JCM 800 or a Fender, it can make you breakfast too. Then they are disappointed.

The V:90 does have a different overall tone, more compressed, less organic then my other Mesas. I can understand the "boxy" feel comments by some, with the right speaker/cab it doesn't really sound that way to me. However there does seem to be some of them that have some underlying issues with ice-pick and biting frequencies that can't be dialed out. Maybe you can get to the bottom of that.

Thanks! My case was a little like that. The way I interact with things is to deeply learn them and I often make changes even when they start out good. If the Mark V wasn’t going to be perfect out of the box as I expected it to be then I should have used a worse is better platform to learn on—a couple of Peavey Windsors so I could A/B changes, and a good 4x12—something like that. But of course I didn’t expect to make deep changes to the Boogie. Any time I heard one of their amps in a guitar store for the previous 20 years they always sounded better than anything else. So better is better was my reasoning then.

Anyway I will get to the bottom of where the unwanted characteristics come from. From playing just the preamp out the effects loop I feel like this amp can be a lot of what I always wanted it to be. There are mysteries left to solve though.

 

[MarkV User]

The strobe mute circuit only responds to the channel bus change. Each channel has a set of different relays that are controlled channel selector. When the channel changes, there is a short pulse to trigger the strobe mute, in turn shorts out the gates. Since it is based on an RC time constant to restore the gate voltage back to 12V, the transients from the relays making changes to the circuits does not transfer through to the speaker. There are some relays tied to the power supply that change the plate voltages for the preamp tubes associated with each channel bus. The same would be for the assignment of the tube rectification, triode/pentode and overall power setting defined by each channel. Making changes on an active channel will not stop the pops, since it is not the channel bus that is changing state but the associated controls that may change relay states without any pulse to the strobe mute circuit. Why does the amp make pops after power up? I have no clue. It could be that the channel selector is not yet defined since all of the LEDs turn on, the default is CH1. The Roadster and MWDR do this too.

The pops and relay flutter on startup only happen for me before the channel is defined when in Footswitch mode. And presumably only when the footswitch is actually connected. You can sort of imagine what they got wrong there.

As for having two reference designators on the same board layout is poor practice. Funny that they have the same value for capacitance. Sort of an indication they do not use any form of PCB layout programs since they usually run DRC before importing the net list into the PCB tool. Sure you can use a label or text that will not change if they suppressed the reference designators. For what I know, it is still probably laid out on film considering the shape of the traces, it reminds me of tape. The reference designators also look like they are hand written. No wonder why it takes years to make a change. I can only imagine how large that film is assuming it is not done with any CAD.

Yes that is nuts from my perspective too. It is fascinating because these guys are at the top of their field. So they are doing other things right not related to the PCB design and that is interesting.

As for the boxy and nasal tone of the Mark V90, I would not doubt it has something to do with many aspects of the design. There are differences in the feedback circuit on the lead drive triode pair, several attenuators following V3A and such. I should try slaving into the Mark V and see what happens. The Mark IVB was not that much different with its primary tone, boxy and sort of had that nasal sound but not ice pick. I had the Mark III blue stripe combo at the same time I had the Mark IVB and compared the two. Mark III had the EVM12L black shadow speaker and the Mark IVB had the MC90. I did try running the IV into the EV speaker, (had a small extension cab with an EV) it was boxy and terrible, much like the Mark V90 when pushing the EV classic. Since it was a combo it was not all that epic. The Mark IVB sounded better through a 412 cab. It was more than obvious the amps were tuned differently so I would not doubt it has something to do with the phase inverter, feedback, and other areas. Also the Mark III Simul-class only used a 15W extended Class A primarily to be used with the EL34/6CA7 tubes. It worked with a full set of 6L6GC tubes but the issue with the 6L6 in the Class A sockets would not last long due to the bias voltage difference on the class A circuit. The voltage divider used resulted in a bias voltage more suitable for EL34 tube. The Mark IVB was capable or running all 6L6GC or a mixed quad of the EL34 (in class A) and 6L6GC in the A/B sockets. Not sure if the Mark V90 is suitable for that combination.

I tried and my Mark V preamp sounds great. I play the power amp from an external signal and I hear and see all the problems. I tried a cab and the speaker in the combo to see if that made a difference. Well, it is worse with the MC90 than into my warmer V30 2x12. The speaker matters some but if the presence is all the way up then feedback is virtually off and the problem is at its worst. So the problem is not meaningfully related to the speaker or the backwards half of the feedback loop. Feedback from a faster and more trebly speaker may just produce sharper edges at the input to the phase splitter, which tickles the problem. I may be wrong but I think at the time of the test that cab was 8 ohms and so similar to the MC90.

I‘ll look tonight at the circuit and I’ll reference the Mark IV schematic which I haven’t been using.

One thing I don’t understand is how simul-class is partly class A in 90W mode. That is not my reading of the schematic and I suspect you understand it better. I see the outer pair driving a higher inductance with more transformer gain and less signal amplitude, so I guess more transformer coloration and less tube distortion? But it still looks like AB. And then there is the 7.5W mode and I see how that is class A, albeit I haven‘t understood the circuit clearly.

I think there should be two adjustable bias supplies on my Mark V, one for inner and one for outer tubes. Then it would be easy to run different tubes. The signal attenuation resistors should be adjusted for EL34s. The screen resistors are different in the IIC+ vs Mark V but 1K is fine for all the power tubes we might want.

I don‘t know what input impedance Mesa used for the output transformer. I feel like it might be kind of low because the Mark V sounds crisp like a Hiwatt not soft and fuzzy like a JMP Marshall. That impedance is a hard gate on what sounds are possible from the power section.

Getting a better understanding of the amp would be a good thing. A study of the older designs starting at the IIC+ and Mark III would be a good start (assuming you can find the DRG schematics). The Mark IVA circuit can be found but not the Mark IVB. I was able to get that before Mesa shut down the files on "theTubeSore" website. I deleted them when I sold the amp, also included the schematics on print with the manual. I do recall they are not the same animal as there were changes made to some areas in the design. Never played through a Mark IVA so it is hard to say how it compared to the widebody Mark IVB. I can vouch for the character of the IVB, it was much better than the Mark V90 as it lacked the ice pick character. That could be a negative feedback issue but I doubt it, slaving into the Roadster, the Mark V preamp was still an ice pick.

I’m still testing with the GEQ disabled because I know I’ve seen excessive highs just from that. But otherwise we had opposite experiences when testing the amp and preamp separately. It might matter which amp and preamp we each paired the Mark V with. To see the power amp most clearly I use a square wave from a function generator. All signal sources can do it if there is clipping but it is the fast transition that seems to tickle the power amp.

Ive been using the Mark IIC+ and Mark III schematics and at your prompting got the Mark IVB schematic (attached). It looks a lot more like the Mark V. I haven’t spent time with the Mark I either.

More soon.

 

[bandit2013]

So to understand the Extended class A operation, you have to give yourself a lobotomy first (not recommended). What is Extended Class A, it is also referenced as Push-Pull so where does the Class A/B come in? That is also Push-Pull but may have cross over distortion once the power tubes are forced into clipping. Sure the negative feedback aids to reduce the higher order harmonics caused by the clipping. It is also required for the extended class A to work properly. I had thought that the Class A sockets remain in conduction at all times like a Class A circuit but since there is a phase inversion or negative transition, I would assume the opposing phase acts more like a ballast while the other phase is in conduction. Well that is what they did with the 10W power mode. Two tubes on the one phase will be operating in pure class A with a cathode bias wiring. The one with the voltage divider on the grid is active most of the time where as the other one without the voltage divider and just a grid stopper resistor adds in signal once the signal reaches a specific voltage. The two tubes have different load lines. Now for the third tube, the other class A tube will retain its bias voltage on the control grid but will not have any signal from the phase inverter, it will be acting as a ballast. the Simul-class operates much like the Class A/B but the outer pair of tubes are not always active, depends on signal strength. So, with the Mark V90, the outer pair will have cut-off points but the inner pair will not. Note that the Mark IV has the class A tubes in the outside and the Class A/B in the center (this is the same arrangement from the IIC+ DRG, and Mark III DRG). Mark V90 swapped the positions and placed the Class A/B sockets on the outside and Class A sockets on the inside. The Mark VII also has this topology but does not run a pure class A power mode at 10W, instead they switch from pentode to triode with the extended Class A tubes. In other words, the Extended Class A is not Class A, it is more or less a push-pull much like Class A/B but minimizes the cross-over distortion.

This link is a bit easier to understand than trying to figure out how it works based on the schematics alone. It does not fully explain it but close enough. I was unaware that there was an original design by someone else. However, Mesa improved on the concept and was able to secure a patent on the innovation.

https://www.ampbooks.com/mobile/cla...,four 807 beam power tetrodes in the circuit.

 

[MarkV User]

I have an update.

Reducing the filter capacitance after the phase splitter does not affect the feedback the way I hypothesized, and removing it does not help. And although The feedback network through the presence knob is not solely responsible for the problem, it’s balance with the feedback-cutting C57 and R104 turn out to be the problem.

Also the Mark IVB and Mark V are remarkably similar in the output driver section.

The incremental changes I've made to the feedback may have reduced the severity of the problem after all. I thInk bypassing that cap in the feedback helped. Channel 8 no longer has the problem after that. So I should take out the second feedback pass cap and test again.

To review, when I send a square wave to the power amp through the effects return and with the presence knob at 12 (clean channel presence for example) and I see a strong overshoot after each positive and negative edge. That overshoot is where the higher frequency negative feedback is being shunted to ground. I can turn the presence down to make the overshoot go away and up to make it bigger. The spike seems to contain the original harmonics but boosted way too high.

This spike is caused by the feedback loop shunt network. C57 and R104 are cutting negative feedback too aggressively. Either R104 needs to be increased or the feedback needs to be increased and the response balances so the frequency response of the power amp at neutral presence is more sane and I personally think it should be flat. I’m going to try a few different approaches and see which one works best.

A small value cap across the presence knob (1nF, 2nF, 220pF, 440pF, and 660pF were tried) pretty well solves the problem but makes the presence control less useful or disables it entirely for the larger values. I need to check all the resistor values to make sure they are right.

More in a few days.

 

[bandit2013]

The Simul-Class is not much different than the Class AB. Both are Push-Pull amplifiers. We know what class A is, single ended gain stage that conducts 100% of the input signal. This can be done with a single tube or having two tubes in parallel. Class B, is a push-pull such that each tube will conduct 50% of the signal but will have some cut-off areas as the tube stage is biased at cut-off. Class AB is similar to class B but it is biased to be in conduction for more than 50% of the signal. Still a push-pull but far more efficient than Class A, or Class B as it does not create cross-over distortion due to the overlap of each phase. So if it had 4 tubes, each pair would be operating at the same load line. Now for the Simul-class, all that but the operating point on each tube is different. This is to achieve more power tube distortion or clipping in the center pair of tubes vs the outer pair as they are just power adders.
Class AB power tube distortion I feel sounds much different and more organic in nature. Actually, I like it better than that of the Simul-Class character. To be honest, I never heard a DRG IIC+. I did own a DRG Mark III. So the closest thing I have to the IIC+ HRG is the JP2C. I believe with the Mark IV and Mark V90, they backed off on the bias on the class A sockets to get more power out of that section. It may as well just be a class A/B but it has more tube saturation on the inner pair that are operating at a different bias point than the outer pair of tubes operating in Class AB. Or is it more of a Class B since those tubes do not contribute more than 50%. ? Since Class B is also a Push-pull method. The center pair we call the class A pair are more of a Class AB section with a hot bias so they conduct more than 50% and probably up to 100% and the class AB pair is probably more of a Class B operation. Forget-about-it. seems to come to mind.

 

[MarkV User]

The Simul-Class is not much different than the Class AB. Both are Push-Pull amplifiers. We know what class A is, single ended gain stage that conducts 100% of the input signal. This can be done with a single tube or having two tubes in parallel. Class B, is a push-pull such that each tube will conduct 50% of the signal but will have some cut-off areas as the tube stage is biased at cut-off. Class AB is similar to class B but it is biased to be in conduction for more than 50% of the signal. Still a push-pull but far more efficient than Class A, or Class B as it does not create cross-over distortion due to the overlap of each phase. So if it had 4 tubes, each pair would be operating at the same load line. Now for the Simul-class, all that but the operating point on each tube is different. This is to achieve more power tube distortion or clipping in the center pair of tubes vs the outer pair as they are just power adders.
Class AB power tube distortion I feel sounds much different and more organic in nature. Actually, I like it better than that of the Simul-Class character. To be honest, I never heard a DRG IIC+. I did own a DRG Mark III. So the closest thing I have to the IIC+ HRG is the JP2C. I believe with the Mark IV and Mark V90, they backed off on the bias on the class A sockets to get more power out of that section. It may as well just be a class A/B but it has more tube saturation on the inner pair that are operating at a different bias point than the outer pair of tubes operating in Class AB. Or is it more of a Class B since those tubes do not contribute more than 50%. ? Since Class B is also a Push-pull method. The center pair we call the class A pair are more of a Class AB section with a hot bias so they conduct more than 50% and probably up to 100% and the class AB pair is probably more of a Class B operation. Forget-about-it. seems to come to mind.

Thanks for the pointers to the simul-class info in the previous post. I did find that useful and also found the article you linked enlightening.

On the class A-ness, that is what I thought I was seeing. It is a little surprising they were able to get a patent issued for that aspect of the design. It works well with their fixed bias approach because the same power stage with two bias pots probably wouldn’t be covered by that patent. I eventually want to add two bias pots and external test points though.

I haven't gotten into it, but I have been wondering if crossover distortion could be contributing to or creating the excessive brightness problems. Crossover distortion is at its worst with a square wave input and that is what we see.

The excessive density and the way the Mark V is constructed has been annoying me in debugging. I’m going to get faster at this over the next few iterations, but I seem to spend most of my time searching for components. It is making me wonder how much work it would be to make a layout diagram showing a photo in the middle and lines pointing to each component with its designator, value, and basic bom info.

 

[bandit2013]

Good luck with that.
If the amp is in good working order, why not just sell it? If the Mark VII or JP2C is more to your liking, get one of those instead. Sometimes you can find a good deal on a used one. With the Mark VII or JP2C, it is more about the power tubes used in the amps. The JP2C works great with the STR415, STR448, STR445 and the STR440. What Mesa has been using lately is the STR443, they are ok. Mark VII is is more to deal with the bias color of the power tubes. The STR445 are probably the best for the amp. but in a yellow bias color. Mine came with the green bias tubes, They were a little rich so at gig level you lost some note definition. I change to yellows and that made a world of difference in sound quality.

The Mark V90 on the other hand, well, it was long overdue for obsolescence. I have one of the bad one's as I have said many times. There are some good one's out there as well. However, that being said, I could still hear the common tone and nuances of the amp I disliked. I will never buy another one, even if it sounds good. Would I buy another Mark VII or JP2C, I have been considering it. It is a hard decision which one to get though. At full price, probably the Mark VII. At a discount, the JP2C would be more to my liking. It may only have three flavors of tones but it is not endless and limited with just that. If I wanted to manage the 4 amp setup and still use the Badlanders, then a second Mark VII would make sense as the crunch and IIB will blend with the crunch channel on the BAD and be in phase and the other modes will match up with the rest of the Mark VII modes. I can control two Mark VII with one footswitch controller or two JP2C doing the same daisy chain trick but I cannot make use of the Mark VII and the JP2C due to conflict in the midi commands. Sure I could go full midi with a main controller to manage everything so no need to add another MK7 or JP. I am just a mere amateur so why go down that path?

 

[MarkV User]

I might sell the Mark V but I wouldn't replace it with another Mesa amp. It still has some good qualities, and I've been learning a lot from looking at its problems. It is similar in some ways to but probably better than the amazing studio caliber preamp of the past. I could sell the Mark V and buy a studio caliber, but that seems kind of dumb because those are old now and capacitor aging eventually mandates an overhaul which might be tedious. The resale values of my amp and that preamp are similar.

It would make sense to pull out the power tubes, mount the chassis on a rack shelf, and use the Mark V as a preamp. I could figure out a way to foot switch it into the Bogner Shiva's effects loop like another channel or get something like the Marshall 20/20 with a stereo delay in the middle. With the big transformers it would be a very "heavy" preamp as the kids like to say, but it would be fine for me. So, keeping in mind that I have that as a fallback plan I don't think I need to go that far. I'm just going to keep working with it to make it sound its best and leave it in the combo.

I will come up with a mod to feedback/presence and implement this dual adjustable bias thing. If it were easier for me to get around inside the amp then I would do more, but it is getting easier.

After a lot of thought I chose a Celestion BN12-300S to try in the combo. It is a very flat response type and very pretty sounding for cleans, a smooth speaker rather than a gritty type, but also handles light and heavy distortion well. If it has a flaw it is that it doesn't reproduce the high presence very well. But it is no different from the Redback that way. It lacks the mid-boost of a Redback, so it has less "character" and won't produce the singing high mids. It also lacks the clarity and presence of an EVM 12L and has a different character than that even though it is a very similar kind of approach. I feel like it might be a good match for this amp. It is rated for 300W, and I got two of them in good used condition. When they arrive I'll put one with a V30 in the Bogner cab and the other in the Mark V and I think it will be a good match. If it doesn't work in the Mark V then I'll put both BN12-300S speakers in the Bogner cab to use alone or in a full stack with the 16 Ohm 1960A 4x12. Either way that stack will be 8 ohms and work with any of the amps.

 

[MarkV User]

[edit: I removed some intermediate speculations about the presence fix, which were incomplete and misleading]

One funny side note is that the guy who designed the Mark V was (If I understood correctly) Metallica's old guitar tech. And their stuff is beautiful but the "metal up your ass" tone is so different from what I'm trying to do. I can fix this amp for me, but I don't think I can competently evaluate what my changes will do for people who care about a great metal tone.

 

[MarkV User]

I use a closed back 2/12 deep cab with 1 shadow and one EM Texas Heat. That did it for me. Tight with a huge thump. Perfect for the stage, but that's just me. Good luck.

Are you in France? I may have found some of your playing on YouTube, and if that is yours then your amp sounds really good. If I can get a sound similar to that I'll be happy.

You are one of several people who told me that what worked for them was an oversized 2x12 with one smooth and fairly flat response speaker (the shadow or EVM 12L and one speaker with a pronounced midrange).

I'm going to try an oversized Bogner 2x12 with a V30 (because I have it already) and a Celestion BN12-300S bass speaker because it has a flat response, works good into the lower presence and rolls off the upper presence. That speaker sounds good for guitar clean and dirty and heavily distorted. The V30 is also not too bright but will carry the presence better. So maybe they will be a good match.

In the combo I will use the BN12-300 by itself. I think that is going to sound a little dry, which might be the tone to go for with this amp. If I think I can get away with it later I'm also interested in trying the EVM 12L... when I'm sure I've made the presence circuit work.

 

[MarkV User]

I did a bunch of experiments, bridged and removed caps at various places in the presence loop, checked the resistor values, and don't have any single component change that completely fixes things. My best guess is that at various points in the history of the Mark V different capacitors with different properties were sourced. I've seen it makes a big difference to replace some of the ceramic caps with plastic caps of the same value. The amount of work to fix all the channel specific behaviors separately is seeming kind of large. But maybe there were changes to the circuit that are different from the schematics I have. I have learned enough now and I need to give up on the goal of finding the root cause of this issue. It would be easy if I had one of the good Mark V amps here to compare with but I don't.

I am going to make the feedback and presence work a little differently than it does currently and get rid of all the channel differences. First this is a review of the feedback/presence circuits on the Boogies:

The Mark I has a normal resisttive feedback loop with the presence knob arranged to reduce feedback at the high frequencies. This is the same as the Marshalls and Bogners. On the output of the differential driver we have a really big 250pF filter cap whereas the Bogner and Shivas have 47pF

Mark IIC+ (which I now know is the Metallica amp) and Mark III have a nearly identical "modern design" where the feedback loop is resistive but with a 4.7nF bypass cap on the resistor, and the presence knob is in the feedback path. Then there is a 47nF feedback shunt cap in parallel with a 3.3K resistor.

There is no filter cap on the differential drivers for the Mark IIB, Mark IIC+ and Mark III.

Mark IV and Mark V have complicated mode-dependent presence circuits and don't have a DC negative feedback path.

Those both have mode-dependent filter caps on the differential drivers.

Some of the other Mesa amps do interesting/crazy things. The DC2 drives one phase as an open loop and derives the other phase from output transformer feedback. That feedback is a resistive path and the presence boosts that feedback through a 5nF capacitor. I am really curious how this amp sounds now and will try to listen to some recordings.

People really love the tone of rhe Mark I, and I possibly like that one best... But I feel it is not in the spirit of this MarkV amp to make it sound like a Mark I.

People are not really excited about the Mark IV and V tone to the same degree, even though they have their fans.

But I have now talked to a lot of people here who love the Mark IIC+ and Mark III style amps. The presence and feedback are almost identical in those two amps, and they are simpler. So I've come to the conclusion that my Mark V presence and feedback circuit should be made to look like the Mark III:

It is possible that this change won't solve the high frequency problem by itself, but I'm guessing it will improve the tone of the amp, and it simplifies the circuit so it should be easier to debug and make improvements.

 

[Inthenoise]

I own a Mark V combo and seven other Mesas including a DRGX MkIIC+ (modded) head and Mk VII head. I like the gain section of the V but the clean and crunch tones just don't stick it for me. Sadly, same for the VII. Good but not great. After much Petaluma tone chasing, my ideal Mesa would be a Filmore 50 clean and crunch with an actual MkIIC+ gain section. I'm tired of trying to get the Marks (V90, V25 & VII) to deliver rounded clean and crunch tones that are punchy, pleasing and work well together. Those are just there with the Filmore. But that's me.

 

[MarkV User]

I have good news! The modification of the Mark V feedback/presence circuit to match the Mark III is a big success, and I celebrated by making some preamp tube swaps.

The amp is even brighter than before, but in a pleasant way now. I'll be making further changes to globally reduce the upper presence without losing all the brightness. Turning the presence down to 9 o'clock on all channels gives a pretty flat response from the power amp, and that is promising.

I saw lots of interesting things:

• Many little generic-looking 1kV ceramic caps were removed as part of the change. I don't know what kind of ceramic caps they were, but I labeled them and set them aside. It might be possible to characterize them at some point. My expectation is that these caps were the source of bad tone and irrational-sounding presence.
• C117 was 1uF instead of 0.1uF, 10 times the value in the schematic. My reading is that this probably helped the tone instead of hurt it.
• When the change was made the difference in the sound of the amp seems very significant. I'll characterize it better later, but it feels more real. I didn't make before and after recordings, and it is too late now, but I got really excited when I heard it.
• All three channels have benefitted a lot. The clean and crunch channels are so much crisper sounding now, and in a good way.
• After making the change I swapped the V7 tube (phase splitter) for a balanced 12AU7. Now... I know this is an extreme move, but I think it helped a lot. One thing that was particularly sad to see on the scope is how so many different overdrive/gain scenarios resulted in the same waveform. There are so many gain stages in this amp, and each one should be able to distort a little bit. It is totally useless to have all the knobs and extra tubes if the result is going to be boring clipping over a wide range of control settings. Lowering the phase splitter gain by such a large amount gives me more range to turn the gain, volume, and master knobs (and the tone and EQ knobs) before the volume gets too loud. The change to a 12AU7 might mean that I can't get a full 90W from the amp anymore, but I suspect that I still can, and the channel gains and volumes are useful for a larger part of their range now.
• Having had success with V7 I changed V4 to a 12AT7 on rarebitusa and others' recommendation. Channel 3 got more useful and had a better tone right away. More of the gain range was usable. This made me want the same improvement for channels 1 and 2, so...
• I also changed V1 to a 12AT7. This dramatically improved the tone of all three channels.
• I think there is room to lower the gain a little more. All of these gain reductions are creating more places where I can get a little bit more useful distortion out of the amp.
• My Mark V still lacks the chimey overtones and percussive dynamics of my other tube amps, and I don't think these later day Boogie amps are really designed to sound that way. Lowering the gain by changing tubes has helped bring some slightly chimey tones to channels 1 and 2, which is a big improvement. It is a very percussive feeling amp even though it feels like it doesn't droop at alll. What is going on there?

Now that I've had these nice successes I have some follow-on steps. I'll probably only do some of these.

• Important: set up independent bias supplies, two bias pot accessible from the back of the amp, and test external points for reading the bias voltage and current for each tube.
• Important: add some low-pass RC filters to the amp driver (across the plates, across the screens, and mid-loop shunt maybe) to kill ultrasonic frequencies and roll off some excessive presence. Three stages with a -3dB point around 6-7KHz might be about right
• Is there any other tube position that should have its gain lowered? Can I get a further improvement to the amp tone by replacing more preamp tubes?
• Try making all four tubes have the same value screen resistors, and possibly drive only the higher impedance winding with all active tubes?

 

[MarkV User]

I own a Mark V combo and seven other Mesas including a DRGX MkIIC+ (modded) head and Mk VII head. I like the gain section of the V but the clean and crunch tones just don't stick it for me. Sadly, same for the VII. Good but not great. After much Petaluma tone chasing, my ideal Mesa would be a Filmore 50 clean and crunch with an actual MkIIC+ gain section. I'm tired of trying to get the Marks (V90, V25 & VII) to deliver rounded clean and crunch tones that are punchy, pleasing and work well together. Those are just there with the Filmore. But that's me.

Wow, that is a cool collection. I haven't played a Fillmore 50 but it sounds really in the demo I just listened to. I don't hear much power supply sag. There is no bass bloom. There must already be strong feedback. It sure sounds good and it sure sounds like a Mesa Boogie amp.

Do you mean you want the preamp of a Filmore 50 with the power amp of a MkIIC+? The Fillmore 50 looks like it is not super hard to modify. Since it is already a tight sounding power amp I think you could do the same kind of thing of changing the feedback loop to match the Mark IIC+ and get that kind of sound. In fact the Fillmore sounds like that kind of power amp already to me. If it were me I might just play that Fillmore amp as it (because it sounds beautiful) and mod the Mark V instead.

The clean and crunch channels of my Mark V got so much better after the changes I made today. I mean I think it is a huge improvement. I'll feel a little more confident about it once I've finished the changes and am playing it the normal way. But I feel like this is a whole new and much better amp now.

The Mark amps have their incredible crispness! It is like dry leaves crackling, sticks breaking. That is what is so great about them. The power response is instant, so you can get super percussive sounds that way of a different kind from the Bassman-derived amps. It feels like the reasonance of the amp is spread instead of concentrated in a midrange peak. And the flat EVM type speakers seem to be a good match for that reason. And I've come to realize that for all their beauty other amps can't even get close to what the Mark V does or vice-versa. It is like they are in two different universes.

I've had this amp since 2014 and only realized that I should work with it so it can reach its potential.

 

[bandit2013]

The Mark V90 has its own thing going on with its tuning. That sort of separated it from the pack. As for chime in character, it depends on power tubes used. STR440 (red, yellow or green bias colors) provided a pristine clean sound that had plenty of chime to it.

Modern amps with that characteristic: JP2C, Mark VII, Badlander, TC100, TC50, MWDR, Roadster. Most of the time it is power tube dependent. JP2C with the STR443 will not provide that characteristic. However with the STR448 or even the STR415 it is there in spades. Mark VII, depends on the bias color of the STR445. I found the yellow bias color sounded detailed. For more low end and chime, a pair of the STR448 in the outer sockets in red or yellow bias color was the key. No change in preamp tubes required. Badlander, epic sounds if you blend in green and blue color STR447. TC100 and TC50 seem to favor the STR447 but after you get some burn in time. MWDR the clean channel has plenty of chime to it. Roadster, that needs some preamp tube swaps to get there. JAN 5751 in V1 did the trick but I went a different route as I was after a different character for the Roadster.

Three of the Marks I have owned that have that 3D characteristic: Mark III with its original tubes STR415 and the STR416 (6CA7). Mark IVB with the original coke bottle shaped STR420. Then again it was more sinister with a mixed quad of SED =C=6L6GC and TAD 6L6GC-STR tubes (black plates).
The Mark V90 with the SED=C=6L6GC tubes. The other candidate with that 3D characteristic is the RA100 with the SED =C=EL34. That 3D character will fade when the tubes begin to wear out due to change in the complex harmonics.

Also, since the available schematic for the Mark V is for the 2009 original release. The design has had some upgrades in 2010 that improved the excessive brightness of the amp. From what I recall, the original release was brittle sounding. I would expect some variations in component values even after production has been in progress for the past decade. Changes or revisions for product improvement are common. BOM changes and other aspects are tracked by serial numbers more so than a board revision. Then there are sourcing changes such as transformers and other components that may occur during the production life cycle.

It was good you got some resolve to satisfy your needs.

 

[MarkV User]

Thank you and happy New Year! I would not have bean able to eventually get this were it not for help from many people here and most of all from you.

Here is an important question. Do you know any of the electrical characteristics of the 100W simulclass transformer? The replacement part is the same transformer on several of the amps.

More for fun, do you know the factors in the circuit design that create the 3D feeling you’re talking about? I am currently using 5881 tubes in the hot inner pair to reduce some of the chime I had before, and when I put them in I felt they were a big improvement. The 6L6GC tubes were always noticeably microphonic and has a little too much chime for my taste. The 5881s are the current Tung Sol brand and just happened to bias about right. I know one of the most important differences between tubes is how much gain they have. And I’m not seriously considering it for the Mark V because the inside chassis is so crowded... But an awesome improvement would be to add trim pots at the most important triodes to adjust the gain. In a new amp the pots could be panel mounted next to each socket or in the back adjacent to it.

That thing about matching the gain stages… I know at Bogner they spend hours swapping tubes to get the gain staging right, just before they ship each amp, because he talked about it in an interview. And the whole Soldano SLO innovation was to match the gain stages by design. My Mark V was initially doing all the clipping at one stage, or that is how it looked on the scope. The lower gain tubes helped with that. Makes me think they could have designed all the stages with lower gain and gotten much more intense distortion.

I know those big orange film caps are pretty microphonic. Does that improve the amp sound in any way you know? I mean it very well could and I don’t know. And I know from other experiences that the X7R ceramics often sound bad in high impedance circuits—due I guess to their various nonlinearities—plus they are microphonic with much higher mechanical resonant frequencies than the film caps, and piezoelectric. But C0G caps ought to be pretty ideal at least for audio circuits. I think it has been less than 20 years since all the wonderful C0G caps became available and it is possible that they used X7R caps in the signal path of the Mark V. That would have been “traditional” but sound far worse. When I substituted same-value film caps for ceramics in the Mark V over the past few days it was as if the capacitance went up. X7R caps can act like that because they have some nonlinearities, and I don’t think I would have seen that with a C0G cap.

if you loved the Mark III… it is easy to make the rest of the Mark III changes to the Mark V. I want to change the screen resistors to all be 470 Ohms and simulclass grid signal dividers to match anyway. The Mark 3 runs its outer pair of power tubes in triode mode always with no screen resistors. I don’t know if it will make a difference but the resistor can be moved to the switched side so it is only connected in pentode mode. I may do this a different year though. I am nearly finished with my Mark V fun for this season.

I know that the supply filtering and the size of the supply caps are also hugely important for amp tone, I haven’t looked at whether it would make sense to alter it. I wonder if tuning the per stage bypass caps is how they get the stuttering distortion you hear in some amps, like the SLO and even more in the Rectifiers. You know what I mean? You would set the RC time constant of the triode bypass filter on successive stages to be the same, e.g. set them for 30Hz. And over multiple stages you would get that frying sound naturally.

The bass bloom (phase shift through the output transformer, greatest at low frequencies) can be increased by increasing the size of the resistors in the feedback loop and shunt. I don’t want to do too much of that and completely lose the dead character of the amp, because that is what makes it special. It would also decrease the effective range of the presence knob. But the amp could stand to be a *little* less dead. Maybe I should add trim pots for the feedback and shunt resistance. Normal tube bias adjustment will also make it less sterile.

More RC low pass sections (after the phase splitter, between the screens, and between the knobs and 56K resistor in the loop) is a way to make the amp even more dead, more ear safe, and possibly sound better if the cutoffs are set just right.

 

[rarebitusa]

FWIW

I find the feedback circuit changes very interesting. Kudos for sticking with it, experimenting and sharing here. It will certainly allow options for those willing to modify. I concur the presence control reacts much differently on my IIC+ vs the V. The V presence feels more like a tone control vs the IIC+ which has that 3D (pull to the front) feel.

My limited hands on experience with the Stiletto Depth mod just demonstrated that how important and powerful the feedback can be to the overall feel of an amp. That amp was not only bright, it was difficult to give it a fuller lower end, a GEQ in the loop helped but the addition of enhanced feedback control was a game changer.

The one thing the V:90 has in it's favor is it does seem to respond well to tube changes. I am a limited tube sniffer often swapping a tube doesn't seem to make a major difference to my burnt ears, however the V4 swap to the AT7 was big and the power tube combo of STR-440 YELs STR-441s GRYs was also a very noticeable improvement especially in 90W mode.

Now my other observation is the positive comments WRT the Fillmore. If you like that voicing you may like the LSS too. This is probably the best review of the amp I've come across.

LoneStar Special