I am liking what I am hearing so far with the Mark V after going back to the MC90 speaker. I did not like that speaker for some reason or another with the Mark V combo. However a few things have changed in the amp that made the bond of the MC90 more pleasing. Removal of C39 from the V4B circuit. Sure the cap is there to prevent any unwanted oscillations due to the design of the gain stage having the highest gain ratio in the amp. There is a similar cap on V5A but that component has historic presence so I left it in place. Since the V4B gain stage follows the low gain overdrive stage, the coupled grid to cathode through the C39 capacitor also contributed to some midrange and moderate treble loss. Since this circuit has an extremely high gain for higher frequencies, what gets filtered out in the treble gets put back in due to the non-linear doubling effect of harmonics. This also acts to cut midrange to some extent which makes CH3 sound a bit thin and brittle. Removal of C39 reduces the upper midrange filtering which seems to balance out the tone of the amp. From what I have seen in older designs of the Mark Series amps, this particular gain stages does not have this grid to cathode capacitor but the remaining components are identical, cathode resistor, plate resistor, cathode bypass cap. The only difference may be in the plate voltage and input impedance to the next gain stage (which looks about the same as the older designs). I thing what sets the Mark V apart from its heritage may be in the tone stack for CH3. Any mods to that can wait for now. Another lesson learned, if you make a mod, only do one. I took the liberty to restore the bias back to its original state (or close enough). This definitely resulted in more power delivered to the speaker so when I installed the MC90 I almost blew it out as it did not sound like it would survive more than a few minutes of use. I did not realize at the time but when I hooked up the Rivera Rockcrusher to the amp I noticed the input was pegging the peak meter. Not sure how accurate it is but that would mean the amp was pushing 150Wrms. With the bias mod I made, it would only peak around 110Wrms. I did the bias mod in hopes to prevent instant tube red-plating. I had issues with this amp early on and for some reason Mesa tubes kept failing. I would not recommend any mods to the bias circuit in your amp. If you are not burning out tubes in a short time, leave the circuit alone. If you are having power tube issues, send it out for repair by Mesa or Mesa Certified Tech. So far my amp has remained quite stable after the fix. Before I made any change to the bias of the amp, the only tubes that would survive the stock bias setting in the amp were SED =C= 6L6GC. Since those are now scarce I needed another avenue to continue using the amp so bias reduction by a small amount seemed to work out and since then I was able to use the Mesa tubes for extended periods without red plating the center pair. I did check all component values for all of the power tubes and everything appeared to be in order. I do not have a bias probe to check currents and plate voltages on the tubes. However a gamble to change an 82.5k resistor to a 91k resistor paid off. It measured resistance is actually 89k. To get the gain closer to stock, I used a parallel resistor placed onto the 91k resistor and the measured result brought it to 83k. I did not red plate any tubes and use the amp for a few days this way, but in the process I damaged the MC90 speaker, dust cap broke free from its glue. Now that I have it back to where I fixed it with the 91k resistor, I figured I may as well give the MC90 another run in the Mark V combo as I never had a chance to try it with the C39 removed from V4B circuit. I can say I have more respect for the MC90 speaker now than before.
Played for quite a while last night and had some fun for a change. The dog did not like it as he took his favorite toy with him the second I reached for the power strip switch. This time I had the Mark V stacked on top of the Mesa wide body 112 open back cab and ran it parallel with the combo speaker. Having two MC90 stirring up the air was quite rewarding for a change. The preamp section is not quite stock although having the stock tubes in the amp was not all that bad. What works well for me is the Beijing Chinese 12AX7 in V1, V5, V6, Mesa 12AX7 tubes in V2, V3, V7 and the JAN./Phillips 12AT7 in V4. A quad of Mesa STR440 Greens and the good old EHx 5U4GB (same tube Mesa uses).
The Chinese tubes do tailor the tone just a bit but with the Mesa tubes in V2 and V3, CH2 remains about the same with plenty of rich overtones. Mark I mode is definitely deep in character but the midrange is still dominantly pleasant, jump onto crunch and it still has the soul of the Mark I but with a bit more edge and less bass dominance. Edge mode also sound OK, actually better than it ever did but that is one voice I do not use very often if at all. CH3 still has its reborn tone with thanks to the 12AT7 reducing the top frequency gain (low end gain remains the same as the 12AX7). That is what makes triode tubes interesting, since they are non linear devices, the gain characteristic is also non-linear over the band of frequencies. It is still a puzzle why this amp in particular is so different in its voice compared to the JP-2C, Mark IIC+, Mark IV, and Mark III which basically share the same driver circuit topography with the exception of where the GEQ is placed.
So why the pull on the OTR speaker? It has great qualities but with a driven source the top end chime gets eroded away. Still the speaker is bright enough to rip out some interesting harmonics, pinch offs, and the like. Its enhanced midrange coupled with the high end roll off filtered out some of the character of the amp that I was missing. It was quite more evident with an amp that is not overly bright to start with such as the JP-2C. That amp has more balance across the frequency range of the guitar that sounded like a blanket was placed over the cab when driving the OTR speaker with it. I am not done with this speaker as I have plans to put the two OTR in the OS Recto 412 that is sporting the EVM12L black label speakers. The added midrange will make things a bit more interesting. The only setback with the MC90 speaker is that it does not sound as good as the OTR on the clean channel when using a Piezo pickup. For a clean amp, the OTR makes more sense to have. So far it was the best performer and I would rate that above the Jensen Blackbird Alnico speaker (at the time I thought that was the pinnacle of perfection for a clean channel speaker).
I would say the Mark V is definitely ready for resale since it sounds exceptionally well at the moment. By no means does this indicate I will sell it but it has crossed my mind on several occasions. As I have found new enjoyment with the tired Mark V it would be difficult just to sell it as I still have a purpose to keep it. Recently I have been blending amps together as this adds more layers to my sound. It is sort of freakish how well this concept sounds. I decided to give the Mark V another go with the JP-2C. As it seems the CH3 IIC+ and the Mark IV voices blended extremely well with the JP-2C set to CH2. I found it interesting how well the JP-2C cleaned up with a reduction in guitar volume but the Mark V remained about the same. Definitely there is a difference in its touch sensitivity in signal response. Definitely a worth while endeavor to experience. Also, the Mark V blends extremely well with the TC-50 which is just as rewarding as blending it with the JP-2C. There still remains a lot of potential for the Mark V, just figuring out how to use it is the key to any rig.