Mesa Mark IV B changes

[M3dicine]

I am looking at getting a Mk IV. It is well documented the differences between Rev A vs Rev B. However, there seems to be some differences in Rev Bs. Most easily noticed is the difference between an attached power cable and a power plug with a disconnect-able power cable. Is this an early vs late Rev B change? Are there any other differences within the Reb B of which anyone is aware?

 

[timothymartin220]

Wow...not one answer in all this time...well...I dont know all the changes but I have a widebody IVB combo and it has the stereo loop (which is the B revision) and the toggle for the mid boost/harmonics on the front (I prefer this on the front)...also the eq switch (on, switchable/R2 always on) is on the faceplate as well (not as helpful...I wish the tube select was up here, maybe the Pentode triode switch here)(my plug disconnects, I had to buy a replacement for it...also I believe the reverb tube is 12ax7 on the B revision and 12at7 on the A, footswitchable on the B and TOTALLY USELESS ON EITHER(I dont know if it is the case or what...but I wish it had the reverb from the Studio Pre...simple 3 short springs floating on a spring harness...it worked and sounded glorious!!!!)..NONE of their encased reverbs sound good at all (it is like they took the floating 3 spring reverb from the studio pre...doubled the length by adding another section and resistors...then floated it in a little case of its own on springs..and for some reason...it sounds TERRIBLE)(I own a Mark V 90 watt..horseshit sounding reverb...also the IVB...horseshit sounding reverb...used to own a studio pre....GLORIOUS sounding reverb)

 

[bandit2013]

Love the reverb descriptions here. I can say for one thing, the reverb on the California tweed does not sound like pony poo. Mark VII is also not bad either. RA100 is better and the TC is the stubby 3 spring pony poop sounding unit. JP2C, it could be better. The Badlander has the best spring reverb of them all that I cannot complain about, it has no reverb so if that is required it is in the FX loop.

Other than the chassis change from small to medium wide body, I never played through a Mark IVA to start with so I cannot comment on that. I did have a Mark IVB combo. Hated the MC90 speaker it came with. Had I known the Mark IVA had the EV black shadow I would have sought after one of those instead. I tried an EV in the Mark IVB, it was not a good fit, similar to the Mark V90, sounded boxy and terrible. Sorry for not much content. I no longer have the Mark IVB amp.

 

[kmanick]

I 've owned an "A" a later "B" and currently own a very early, what's called a "transitional B"
the "A" and my current "B" have attached chords.
the R2 on the "A" to me was useless, not only because it didn't have much gain, it was just a flubby mess.
The lead channel on the "A" was a bit less compressed sounding to me than the lead channel on both of my "B"'s
closer to a Mark III sound I guess.
The R2 on my current early B has been the best of the 3, very gainy and much more useful as a crunch channel.
the cleans on all 3 sounded very similar to me, (nothing jumped out at me, that one was better than the others).
The later "B" I owned had the mid gain/ EQ toggles in the front, my current one does not.
the lead channel on my Current early "B" sounds closer to the "A" lead I had than the later "B" but is still a bit darker than the "A" I had.
Honestly besides the difference in R2 I don't think it really matters, I'm almost tempted go track down an "A" just so I can compare it to the one I currently own and seeif I can tell the difference on the lead channels.

 

[FedWar]

to those that say that R2 is very different between rev A and rev B.
I've checked the schematics available online and the only prea amp differences I could find are in the lead circuit, where the rev A is more similar to the IIC+.
I did not find any difference in the circuit used by the R2.

 

[kmanick]

to those that say that R2 is very different between rev A and rev B.
I've checked the schematics available online and the only prea amp differences I could find are in the lead circuit, where the rev A is more similar to the IIC+.
I did not find any difference in the circuit used by the R2.

I can hear a big difference on R2 on the ones' I've owned

 

[GJgo]

to those that say that R2 is very different between rev A and rev B.
I've checked the schematics available online and the only prea amp differences I could find are in the lead circuit, where the rev A is more similar to the IIC+.
I did not find any difference in the circuit used by the R2.

I suspect what people are hearing here is simply tolerance differences on aging amps.

 

[edge59AlnicoIV]

I modified my IVB Lead channel with the IIC+ values,made a world of difference, I also swapped out the output tranny that I purchased that came out of a earlier Simul-Class stereo power amp that used the IIC+ style output transformers. Also in the power supply string there are 2 1k 2watt? resistors on the early IIC+ some have a 5.6k, I swapped the 5.6k for another 1k, it allows more voltage on the 12ax7 plates, giving each stage more headroom and singing sustain, they may have started using 5.6k so the 12ax7's would last longer, it also gives them a slightly darker browner tone, with slightly less sustain with the 5.6k. A friend of mine has a 1984 IIC+ and I have taken pics and notes to the differences, a great sounding IIC+. It also has the 105? power transformer seeing almost 500 volts!

 

[Henz]

I modified my IVB Lead channel with the IIC+ values,made a world of difference, I also swapped out the output tranny that I purchased that came out of a earlier Simul-Class stereo power amp that used the IIC+ style output transformers. Also in the power supply string there are 2 1k 2watt? resistors on the early IIC+ some have a 5.6k, I swapped the 5.6k for another 1k, it allows more voltage on the 12ax7 plates, giving each stage more headroom and singing sustain, they may have started using 5.6k so the 12ax7's would last longer, it also gives them a slightly darker browner tone, with slightly less sustain with the 5.6k. A friend of mine has a 1984 IIC+ and I have taken pics and notes to the differences, a great sounding IIC+. It also has the 105? power transformer seeing almost 500 volts!

I have a C+ and I'm very familiar with the circuitry. The MKIV lead stage is nearly identical except for two or three component values. The 500pf cap in parallel with the 270k R, the pull bright resistor is a 22k. And , did you put that 120pf cap in? It was removed in later C+'s . Are these the components that you changed and what was the difference that you liked? I believe I'm referring to MKIV A , not sure.

 

[edge59AlnicoIV]

I have a C+ and I'm very familiar with the circuitry. The MKIV lead stage is nearly identical except for two or three component values. The 500pf cap in parallel with the 270k R, the pull bright resistor is a 22k. And , did you put that 120pf cap in? It was removed in later C+'s . Are these the components that you changed and what was the difference that you liked? I believe I'm referring to MKIV A , not sure.

Yes, I did the 120pf. The IVB was a little different in the lead channel, I have a friend with a mint IIC+ and the power supply has the 1k and 5.6k dropping resistors, my friends IIC+ has a 1k in both positions, putting more voltage on the 12ax7 plates, it sings more than the 5.6k which was more of a browner/darker sound like a variac effect, there was more distortion but not as articulate and singing as the 1k. That was my experience anyway

 

[edge59AlnicoIV]

Yes, I did the 120pf. The IVB was a little different in the lead channel, I have a friend with a mint IIC+ and the power supply has the 1k and 5.6k dropping resistors, my friends IIC+ has a 1k in both positions, putting more voltage on the 12ax7 plates, it sings more than the 5.6k which was more of a browner/darker sound like a variac effect, there was more distortion but not as articulate and singing as the 1k. That was my experience anyway

I meant my Mark III and IVB Had the 1k, and 5.6k, his IIC+ had the 1k in both positions. I think I'll put the 500pf in parallel with the 270k, .001uf seems a bit much to my ears. Also the IIC+ had the 680k to ground at the grid after the 3.3meg (10 or 20pf cap) mixing resistor? On my III it had a 470k resistor there, both versions have a 47pf from the grid to ground from what I have seen. I think the 470k helps get a cleaner headroom on the clean channel and slightly scoops the mids, when I tried the 680k on my III, it had more mids and a bit more gain. I did swap out the cathode bypass caps on the 12ax7's to the values seen on the IIC+ schematics, which helped get more of that voice/gain but you have to be careful as too much will get muddy, so one might turn the bass down on the lead channel with higher gain settings, but it sounded more like the IIC+ to my ears, I had an older IIC+ output transformer, but the stock power transformer.

 

[edge59AlnicoIV]

I meant my Mark III and IVB Had the 1k, and 5.6k, his IIC+ had the 1k in both positions. I think I'll put the 500pf in parallel with the 270k, .001uf seems a bit much to my ears. Also the IIC+ had the 680k to ground at the grid after the 3.3meg (10 or 20pf cap) mixing resistor? On my III it had a 470k resistor there, both versions have a 47pf from the grid to ground from what I have seen. I think the 470k helps get a cleaner headroom on the clean channel and slightly scoops the mids, when I tried the 680k on my III, it had more mids and a bit more gain. I did swap out the cathode bypass caps on the 12ax7's to the values seen on the IIC+ schematics, which helped get more of that voice/gain but you have to be careful as too much will get muddy, so one might turn the bass down on the lead channel with higher gain settings, but it sounded more like the IIC+ to my ears, I had an older IIC+ output transformer, but the stock power transformer.

Oh and yes the IVB lead channel was pretty much the same as the IIC+, but I was told by a friend that worked there, the had crosstalk issues between the reverb and one of the lead channel 12ax7s, as they shared the same 12ax7, one half to the reverb, the other half or 12ax7 stage to the drive section, so they changed the layout on the IVB, also the was an overheating problem in the Poweramp section, I think they changed the bias feed resistors? 220k to 150k on the class A sockets to bias then cooler, I think they did that in the V as well and they recommend 6l6 only in the V is what I remember reading on the Mesa site.

 

[edge59AlnicoIV]

Oh and yes the IVB lead channel was pretty much the same as the IIC+, but I was told by a friend that worked there, the had crosstalk issues between the reverb and one of the lead channel 12ax7s, as they shared the same 12ax7, one half to the reverb, the other half or 12ax7 stage to the drive section, so they changed the layout on the IVB, also the was an overheating problem in the Poweramp section, I think they changed the bias feed resistors? 220k to 150k on the class A sockets to bias then cooler, I think they did that in the V as well and they recommend 6l6 only in the V is what I remember reading on the Mesa site.

Sorry, I needed to proof read I'm not seeing clearly, the IVA WAS more like the IIC+, the IVB lead channel was different maybe more compressed sounding, when I changed all the parts, resistors caps etc. It sounded closer to a IIC+, sorry about that

 

[FedWar]

MKIV Rev A lead channel

Vs MKIV Rev B Lead channel

 

[edge59AlnicoIV]

MKIV Rev A lead channel

View attachment 3486
Vs MKIV Rev B Lead channel
View attachment 3487

Yep, those were the differences, I prefer the IVA/IIC+ values