Who's gonna be the first?

So who's gonna be the first to change that iic+ mode cap out with the big one that should have went there. I understand that IV mode has a larger cap but it would still be cool to try. When mine runs out of warranty I want to do it. Maybe even find a way to adjust the preset level controls also.

I am no amp tech but this I wanna see and learn too..

I have tried this. Didn't notice hardly any difference if any at all unfortunately. However, had 2 month old baby upstairs at the time so was confined to very very low volume. Sure I would have noticed more if I could have gotten the volume up. Had a gig the next night though so I took it straight back out incase it might make something go bang. Still very interested in trying again though. Hopefully someone does try it and has success as I probably won't get the chance to really give it the time it needs for a while what with work and family commitments.

So what is the reference of the component in question? In other words, what cap are you referring too and what tube position are you referencing?

I have no idea where it is on the board or it's value but I've read about it in the manual.

That gave me an idea to look though all the resources to compare IIC+ up though the Mark V on the lead drive circuit for each amp.

MkIV and MKV has the same circuit with the exception of C39 grid to cathode cap on the secondary triode circuit. The differences seem apparent on the first stage of the drive circuit relating to the cathode bypass capacitor. Cathode resistor and anode resistor is basically the same. MkV and MKIV have the same capacitor value of 2.2uF. The IIC+ has what looked like a 15uF which is also the same used on the MkIII circuit.

Perhaps it would be possible to add an additional capacitor across the cathode resistor to increase the value. A 10uF may be close enough but it will be large in size (the two caps in parallel will equate to 12.2uF as capacitance is additive in parallel. I personally would not jump to such a large value off the bat.

One also would have to consider that there are two additional gain stages that follow the overdrive circuit of the Mark V that is not apparent in the IIC+. Change in the bypass capacitor on the first drive stage would probably make things a bit muddy. It may be possible to shift the midrange characteristic by modifying the secondary drive circuit bypass cap as this will also raise the base response but this would have to be a small change in capacitance as it would also contribute to a rise in lower frequencies. It is almost tempting to mess around with this but not sure I am up to it.

I say go for it! You may discover some magic hidden in there. You sound like you know what your talking about.

I sort of do but most would be trial and error and one balloon I do not want to follow at the moment. Chasing tubes and speakers is enough.

If I had a choice which triode circuit to mod it would be the primary gain stage for CH3. Perhaps adding an addition 2.2uF to the cathode circuit would shift the gain response but may lead to mud. I thought about adjusting the secondary gain stage but that may require coupling cap change from 0.047 to 0,02 or smaller as I would not want to mod the next stage which is common to all channels V3. I am satisfied with the one change I already made but may have to restore it if I were to perform this task. This is actually an easy mod as it would be a capacitor soldered to the cathode resistor (assuming it is not under the large organ Mesa logoed Mallory capacitors. I believe the resistor is accessible. At least the voltage dividers on each stage provide grid stopper and grid leak functions to prevent diode clamping in the even the grid goes positive. (usually what leads to muddy tone). So much to learn about tube amps compared to solid state components (some things are similar but some are not even close). I am not a tube amp tweaker but it is so tempting to try out a few things on a temporary basis. I like my JP-2C and I do not mind that the Mark V does not replicate its tone.

I think what would be a challenge, figure out what to do with the unused half of V2.... yeah right.... However edge mode needs something as it stands I cannot use that voice on CH2, well I can now that I threw in an expensive tube in V1 (Northern Electric 12AX7, almost looks like a Chinese recreation to an RFT tube as the plate structure is very similar to the 12AT7 RFT I have in V4 at the moment (wanted to try something different than the JAN/Phillips 12AT7.

Sometimes things are best left alone. Sure I would like a bit more tight bass than flub but some of that may be related to the V3 tube and the larger coupling capacitor between V4B and V3A. Before anything is done, need to calculate load lines, based on current design and to determine the frequency gain response in order not to end up in the wrong direction.

If this assumed capacitance is what I think it is, Ok then, I am the first to try it and the first go back to original. Mesa uses a 2.2uF tantalum capacitor to bypass the cathode resistor. I have seen schematics of the IIC+ and that particular first stage of the driver circuit has a 15uF cap. I first tried a 2.2uF electrolytic 50V cap soldered across the cathode resistor. Only a slight change was noticed in the midrange but did not seem to change much at all. Tried a 10uF 50V bipolar cap and that changed a few things. Sure the bottom end was there much more apparent. Top end did not change much and the overall tone was muddy as I thought it would get. Not worth the effort but my curiosity was peaked so why not try it. There is a lot more with the Mark V than what one would think as it has more gain stages involved. A change in one or two coupler caps and probably other tweaks would be needed to improve the bass response to match that of the JP-2C or the IIC+. Also change in bypass cap on V5 will not be pleasant with the 12AT7 in V4. (actually the best mod to make as it requires no solder iron is to change the 12AX7 to a 12AT7 in V4. You can even go one step farther and swap V6 with a 12AT7... Reference to the "Saturation mod" incase you have not seen this yet.

Besides, I would not recommend removal of the tantalum caps (yellow bullets) as they cost more than the cheap aluminum electrolytic capacitors as well as they retain there capacitance over a longer period of time and temperature.

Hi Bandit,

I believe the cap that BarrySwanson is referring too is the coupling cap from the 5 band EQ. The manual explains how Doug preferred the smaller cap of the non EQ models so that's what they installed for iiC+ mode and the larger cap of the EQ equipped models is present in iV mode. Some were disappointed with this as although the smaller cap gives the fast attack which doug prefers, the iiC+ mode has been criticised as not having the balls of the original EQ equipped models. Even though you can have that if you select iV mode.

This is C99 a .22uf on the schematic, it is only in the signal path when M7 ( iiC+ mode on the schematics switching matrix ) is selected. All other modes use C49 which is the original EQ equipped value of 10uf. I have tried this before with negligible results, but probably because of the volume constraints I was stuck with at the time. Will try again tonight if I get a chance, also gonna try your C39 mod too.

Went to try the coupling cap mod, got the soldering iron hot and ready and then couldn't find my box of Capacitors and bits so no go tonight. Bummer, will have a look in town tomorrow to see if i can source some. Nevermind. Will keep you posted if i find some.

Keep things simple and only do one thing at a time, that way you know if what was changed had what effect if anything. C39 is just a mild low pass filter but does help bring out some mids that seem squished as well as add some brightness a bit. It is not critical. Before clipping or cutting anything, use a multi meter that can measure continuity to confirm you are addressing the correct part and not something else. Designs do change over time and may not be the same as what is on an old schematic. At least the 2012 appears to have most of what is on the schem.

I love hearing about this stuff! You guys are pioneers when it comes to tampering with Mark V. Yes it was the coupling cap on the five band EQ I was referring too. I'm also curious if there was a way to mod the preset drive and volume? values. I'm not sure if they are the correct terms but basically the missing knobs on the V that the original IIC+ has.

I see. I am fine with the cap as is on the EQ. Unfortunately this amp is a bit complex and dense in is layout which leaves little room for mods or changes. Simple changes like adding an additional capacitor on the Cathode resistor can be done as long as you do not create any solder bridges between the leads of the cathode resistor and anode resistor as they are right next to each other.
I would not call this pioneering but tinkering... The pioneer here was Randall Smith and company. After all it was his vision, passion, innovation and pioneering skills that lead to the founding of the company in the first place.

as for the coupling cap mod on the EQ, if you change the capacitor value you may have to adjust the shunt resistance value to compensate.

Adding pull pots and or other controls would not be practical as it may require removal of circuit traces to redirect signal path or bias points. That would be a bit much for little or no effect. The same thing would apply to a 2004 Mustang Cobra, why go to long tube headers vs the stock iron headers? You only get a small amount of Hp and torgue but you will not get the effect of exhaust scavenging due to the supercharger and the effect of back pressure helps to prevent the loss of unburned fuel mixture from getting pushed out. Also the cam overlap will also hinder that as well. It may sound better but that is all but not much different than just open resonators on the stock pipes. I could say very much the same thing on the C39 mod, it may elevate the midrange a small amount as the low pass filter effect is removed (note that the pole from the RC time constant (effective input impedance) will have little effect on overall performance. However, with a guitar amp, a small change does go a long way. I think there would be a better way to reduce the high frequency and yet retain the full guitar signal by filtering on the anode resistor vs on the grid. A small capacitor suitable for the frequency above the guitar frequency would be a good idea without causing an input burden. So far I have not demodulated any radio signals. Perhaps my favorite broadcast station is off line. I was learning some new Portuguese words for a while but may be the room the amp is in now.