2C Mode Coupling Cap Mod.

[bandit2013]

Just to cap off what was stated... Since the 5BEQ is positioned before the loop, the attenuation of the signal occurs in the 5BEQ circuit for the FX loop or is bypassed to the PI if the loop is turned off. Mind you the send level will by taken out of the signal path in hard bypass. Even with the 5BEQ turned off, the signal still passes through the emitter follower transistor circuit, summing amp and final output transistor which is a PNP driven by the summing amplifier (or differential circuit). There is no hard bypass around the 5BEQ. This circuit will effect the tone of the amp to some degree and usually becomes boosted with the 5BEQ turned on as the differential amp is effected by the tuned circuits for the EQ. I doubt that this circuit is any reason for concern as it does the same thing the others do just with different components.

Many other factors dictate the overall tone of each amp, most of which is in the power supply and the voltages applied to each preamp tube. Perhaps one way to determine any differences is to slave the FX loop send to a return of another amp. I may actually try this just to hear what differences the preamp of each compares to each other. I have done this with the Mark V slaved into the Roadster already since I was trying to figure out if it was the bias on the power tubes or the preamp causing the ice pick tone. It was the preamp circuit as the Roadster definitely had the ice when slaved by the Mark V. (yeah the output level may be a bit too high for an instrument level loop of the Roadster but I was able to rule out the power section of the V causing the ICE).

 

[IronSean]

Since you have both the JP-2C and the Mark V, you definitely should experiment with slaving them into each other. It would be an interesting exploration of the power section vs preamp section on the perceived differences between them.

Also, because of what we observed above relating to the loops position compared to the GEQ, you'll find that slaving the JP-2C into the Mark V will bypass both GEQs, while slaving the Mark V into the JP-2C you'll have both GEQs in the signal chain.

 

[bandit2013]

Actually my plan was to slave JP or Mark V into the Roadster for the Preamp comparison. I may need to get a line level shift (was planning on this anyway) to slave out with the Mark V as that runs at line level where as the other amps are basically closer to instrument level. Not sure why Mesa went that route with the V since most effects in the market are not line level compatible.

Also since I have both JP-2C and Mark V, I do not have that urge to try to make the Mark V into the JP (well I did for a short while, even experimented with bypass caps on the cathode resistor and anode resistors in a few triode circuits. Nothing major and was easy to solder a cap on the resistor leads. Trial and error approach was not ideal and found the original circuits to sound the best, still prefer the C39 removal on V4B circuit. Found I prefer the 12AT7 in V4 even with the C39 removed.

As for the JP-2C, I am uncertain if the two 5BEQ fall before or after the FX loop. It is assumed that the rebirth of the IIC+ is identical but is it? I have not seen any schematic on the JP-2C so where the FX loop is located relative to the post gain EQ is unknown.

My only point to bring that forward as to indicate the attenuation for the FX loop is managed by other means with the previous models and the Mark V is handled by transistors. I have not fully looked at the gain stages and voltage dividers between each stage (voltage dividers serve two purposes, one is to reduce the signal level to drive the next stage, and the second it acts as a grid stopper and grid leak function). Signal level may just be suitable to drive the FX loop direct from V6 (CH3 output).

 

[IronSean]

I didn't mean to slave one into the other to try and make one the other, but when people talk about the feel being better on the JP-2C than the Mark V it would be interesting to do the comparison to see if the source of that is in the preamp, power amp, or a bit of both.

Also, the GEQ in the JP-2C is in the traditional order after the Effects Loop (which was returned to it's classic location), we know for sure because in another thread a user was attempting to run both the Mark V and JP-2C amps into Left and Right on his AxeFx to easily switch between the two preamps when recording. The final configuration was sending both preamps into the JP-2C's power section because that was the only way to still use the Graphic EQs.

 

[bandit2013]

Good to know on the JP.

I was curious as to why the Mark V has more of an ice pick issue on several voices, CH1 Tweed, CH2 edge, and all of CH3 but not as bad on IIC+. CH3 can be cured with the 12AT7 in V4 but as for tweed and edge that requires a different type of 12AX7 as some will work against the ice and some will enhance it. Since the speaker I am using with the Mark V has more roll off on the top end frequencies it does not seem to bother me as it does with the V30 Speaker. However the preamp tube choice does make a difference in most cases. Also that is not something I found to be common with the newer versions of the Mark V which makes me wonder what if any changes were made or if none then why is mine an ice pick. This ice pick is not just the top end, it is odd order harmonics causing a shot noise effect common with transistors where the signal is clipping and cut off symmetrically. This is mostly related to the 5BEQ I will have to put stock tubes in it to confirm with and without 5BEQ active. Based on circuit topography there is no bypass around the active transistor circuits, eq off only disconnects the ground on the tuned filter network and has no effect on the emitter follower NPN transistor. Sure there is a considerable cut in the signal level with the EQ off since there is no boost effect from the filter network. Odd harmonics are more associated with solid state amplifier circuits than would be associated with tube or triode circuits as those are non linear. Odd order harmonics are generally not associated with tubes due to the non-linear effect and doubling or halving of frequencies as a result from distortion. It will always be even order harmonics. Could it be the EQ circuit responsible for the ice pick is the question here.... only way to find out is to bypass the entire circuit with a jumper wire which may be more difficult to actually do. I never had ice pick tone with the Mark III or Mark IV. Both of which have the EQ following the FX loop so the signal level was attenuated before the front end of the EQ circuit, even if it was returned to the proper level it most likely was not overdriving the input buffer of the EQ. Of course this is just an assumption based on the design of the Mark V.

 

[Bornhard]

You mean the markV is not all tube signal path? There are solid state devices in the signal path?

 

[bandit2013]

That is correct. The solid state devices, aka transistors are used for the 5Band EQ circuit. The transistors are used for boost/cut of the frequencies and is also set up as a differential amplifier for the EQ. This was also apparent when the 5Band EQ came into the first Mark Series amp. Sure they could have used tubes to do this, but it would require two more 12AX7 tubes and one that does not exist unless there is a tube out there than can do the same task as a PNP transistor. Also early on after the solid state devices became popular, it has been common to mix both technologies as it may be difficult to perform the same task with all tubes. Since the 5Band EQ requires linear gain devices and that is not possible with a vacuum triode. At least the 5Band EQ is still analog. It could have been done with an op-amp and digital comb filter using DSP chip and software which would introduce digital artifacts (one reason I do not care much for modeling amps).

Also, the preamp gain characteristics (distortion, signal levels, and fundamental tone) are purely tube driven.

I do not see any reason for concern about that. If you are a purist, you can turn off the EQ, or buy an amp that does not have the multi band EQ like a Rectifier amp, Triple Crown, LoneStar, Electra Dyne, Royal Atlantic and many others that do not have the 5Band EQ. This also applies to the JP-2C, Mark IIC+, Mark III, Mark IV and I would assume the Express series too.

Also, once you put anything in the FX loop, you have added solid state in the signal path. Even if that device has a tube in it, the rest of what makes up that device will be solid state, Digital dealy (think about that one) , Analog delay (generally is a bucket brigade chip which is solid state, unless you can find a 1970's tape delay unit, may have tubes in it but also transistors of sorts).

 

[Bornhard]

You wrote before: Based on circuit topography there is no bypass around the active transistor circuits
So if there is no bypass, turning OFF the EQ is not a solution.

 

[bandit2013]

You wrote before: Based on circuit topography there is no bypass around the active transistor circuits
So if there is no bypass, turning OFF the EQ is not a solution.

Yes, that is correct. The Mark V uses two transistors to attenuate the signal level down to line level for the FX loop that will be boosted back up to drive the phase inverter using V6 tube. The Mark V used all tube circuitry to create the signal, clean or distorted. the question is with the Mark V to what extend does the 5-band EQ have on signal integrity. I have not noticed any additional distortion or other issue with the 5Band EQ in use or not but there is a notable boost when it is active (result of the settings).

In regards to other amps, the Mark V does not use transistors (BJT, FET or MOSFETS), or OP-AMPS, or any other solid state devices to generate the distortion or fundamental tone. For questions regarding why there are solid state devices in the signal chain, you can ask Randall Smith or anyone else at Mesa Engineering for that. I am just a consumer, like you. I have no part in the design of any products manufactured by Mesa Boogie or it's affiliates.

My digging into this amp in particular is in my nature as I am an Electrical Engineer. Trying to get a good sense of this amp in its character has been an interesting experience. Some of us have had issues in the past, too bright, and I do not mean just bright, ear piercing bright, brittle and ice pick to no avail on the CH3 (main reason why one would want it). Most of that issue is in the tone stack and too many high frequency bypass caps. At least after 2010 the change for the tone stack took place. Then there is the brittle tone of mine. 2012. I have played a few others from 2015 and a more recent 2016 when I found them in a store in the North East when I was on vacation. Those sounded way better than the one I have.

Then you have those that want the JP-2C or an actual IIC+ without spending the coin to get it or perhaps one amp was all that was affordable. Mark V does have a IIC+ voice on CH3. Since I do not own a IIC+ to compare it too, I have no idea if it sound the same or not. I do have the JP-2C and so far the Mark V is no where close to that. Still it is not a bad amp by any means and it does have its merits. It may or may not be for all who have tried it. Sure I play metal and the JP-2C takes care of that need. I also spend a lot of time with Classic Rock too so the Roadster, TC-50, JP-2C, RA100 and the Mark V can perform quite well. Each having a different fundamental tone. I could have traded my Mark V in for another one as I felt the newer versions sounded better than what I have. Since I have other amps, that desire or need for perfection is no longer an issue for me.

 

[IronSean]

You should note as well that for most audio applications Solid State is more than adequate, and in many cases better than the tubes they replaced. They are exceptionally efficient, linear, low distortion. Some of the best clean tone amps in the world are entirely Solid State, like the Roland Jazz Chorus series, because they are exceptionally good at cleanly and clearly amplifying things.

The biggest downside, and the reason Guitar amps are the main holdout against them, is that most don't behave as nicely when pushed into clipping. But in the Mark V GEQ circuit the clipping, distortion, and tone generation has already occurred. So they just need to cleanly and accurately help shape the final sound. The other downside is that they can sound a little too good, a little too clinical, to the point that they might be too sterile. But the Mark V has plenty of tubes in the signal around it to make that a non-issue as well.

The Mark IIC+ rose to extreme renoun with a Solid State Graphic EQ so it obviously didn't cause an issue for anyone using those amps.