yep this looks dead on
red does best c+ scooped
blue is tuned and can step on any marshall voicing
green is similar to a blue on steroids and tuned even more aggro but w the perfect Mark amp hair
I think I'm still not grasping how the "R2 CHANNEL RELAY" works, plus It's just the simulation for a piece of the circuit, so definitely don't take it as gospel. I'll come up with a more sophisticated analysis asap.
I don't own a real Mk3, but a Quad, which should be somewhat similar in its L2 channel (but with lots of weird values and redundant resistors possibly related to how they implemented channel switching), so It's the only "reality check" I have at hand.
If I decode the Mk3 schematics correctly, when the relay is active (which mode is which?), R133 and C28 should be grounded, as well as R105 and C47, but it's not what I'm doing here.
This would be also different to what I found in my quad, that is R133 + C28 in parallel with C27 and R106 and R105 // C47 to ground. This would be the classic arrangement that we usually see in modded IIC+ or in the III. Maybe someone more expert than me can chime in.
Now, I'm trying to experiment with many different models using the "classic arrangement" above, and I noticed some funny things from simulating them:
- C47//R105 play an important role on taming high frequencies and interacts highly with the other two components, especially in lead mode when they are in parallel with the lead EQ shelving circuit.
- My 1990 Quad has NO R105 and C47 is 47pF, so it's more akin to a red Mk3. However it's soaring, beautiful and uncontainable, in a very different way compared to the IIC+ on channel 1. So, by looking at the values it is a Red stripe with a different lead output EQing, (that makes it loose a lot of volume) but it's also nothing like a IIC+.
Weird thing is that if I simulate it, I get a crazy ramp up of ultrasonic frequencies, so it could oscillate wildly. May be a problem with SPICE, but I would put a resistor there anyway. They seem to have put a global patch by putting a feedback loop between the tone stack and the input.
- Most importantly, in the BLUE STRIPE, I get that the with C47 at 82pF just eats out most high frequency content, maybe too much. This may be due to the 2.2M resistor swap, but It's late and I don't feel like doing the math now. So, the question mark on the schematics may be more relevant than you think.
I tried fixing it by putting there the classic 680k/47p (like in the purple or IIC+) and I get the most "1KHzish-ear-piercing" frequency response of all the II(I/C+/C++), which can be consistent on how everyone is describing it.
...but then I get a Green Stripe which is annoyingly close to a IIC+.
Any chance you can add purple to the simulation? I can get you any component values.
If you can get any component values than you could double-check the schematics that jrb posted, it would be much more useful for everyone to have everything in a single place. However, yes, I can do it as soon as I have some spare time. However, being the primary difference in how the clean and lead frequencies get mixed, my bet is that I wouldn't get a graph very different from that of a IIC+. After all as this stage is concerned it's just a Green stripe (in triode mode) with the Lead output EQing of a IIC+, so it may stay in that ballpark. After all, in these simulations you can't actually see the combination of clean and distorted signals anyway, which is what may be the real difference.
So, this is what I'm getting now. Again, take it with some grains of salt since there's a lot going on in this circuit and may be entirely wrong.
He'll be able to advise on whatever you need and do the work including the R2 mod/service/anything else. You can also usually just ship the chassis to save on shipping costs rather than the whole amp.
