Mesa Mark IIB. Anybody have info on the Loop Mod?

[Marcus]

I purchased this amp with the idea of turning it into a IIc+. I should have done some research and found out that would require a total rebuild with a different PCB. I did hear that the loop mod gets the IIb very close to the IIc+. Mike B from Mesa calls this the poor mans IIc+. I can't seem to find info anywhere. I would like to do the mod myself to avoid shipping charges from Georgia to Cali. Any help would be greatly appreciated.

[HotBluePlates]

I know very little about Mesa amps, so disclaimer up front.

I found schematics for both the IIB and IIC+, and found significant differences between them. There are a few items which are cursory changes, and could be easily implemented without swapping the board.

However, I assume the "loop mod" refers to changes in how the effects loop is tapped and returned to the lead circuit. This happens at a very different point between each version. The loop is the IIB is driven by a cathode follower, while the IIC+ converts the same tube section to a conventional plate-loaded stage, and taps signal off for the loop from a different stage. Additionally, the reverb circuit taps of the dry signal in a different place in the IIC+.

Looking at it, and knowing both amps are pc board amps, you would enough skill and bravery to not need to ask the question to successfully pull it off. I say that not meaning any slight towards you, but rather because you you need to be willing/able to cut existing traces and replace them with wiring to match the IIC+'s signal path.

There are also some parts value changes here and there, which are less problematic. There may be additional changes (I took a fairly cursory look at mainly the lead channel and the effects loop) that I haven't mentioned. Also, my copy of the IIB has part of the switching for the lead channel cut off in the scan, so I can't be sure there aren't other changes I wasn't able to see.

Shipping is a bear... But if you could ship just the chassis with no tubes, it should be easier. In my opinion, $250 for a new populated board installed is a pretty good deal, shipping aside. If the IIC+ is your dream amp, it may be worth the cost. I don't know what the cost might be to have Mesa simply send you the new board, and have a local tech install it. It would probably be more cost-effective for you, but they may not go for it.

[Marcus]

Mesa no longer makes the IIc+ PCB so obviously they do not do the IIc+ mod. I haven't compared the two schematics yet. I only have the schematic for my IIb. I don't feel confident enough yet to take on a IIc+ conversion yet. I'm getting a little more tempted to try it though. I got the IIb for real cheap because it was not working and I fixed it so......

[HotBluePlates]

Scroll down to the post by Reeko81 in this thread.

[Marcus]

Cool. Thanks for that link.

[JayB]

I'm wondering what the effect of those capacitors from grid to cathode would be?

[HotBluePlates]

They reduce treble.

[JayB]

They reduce treble.

I figured they would reduce treble to some extent, but was thinking maybe some positive feedback? It's kind of bootstrapping at those frequencies.

[HotBluePlates]

I figured they would reduce treble to some extent, but was thinking maybe some positive feedback?

So what you're thinking is a cathode-follower-like output from the cathode being fed to the grid input, which would then amount to positive feedback.

Let's take your thinking in another direction. Look at the formula for capacitive reactance. It's 1 over a bunch of other stuff. If any of that stuff on the bottom gets smaller (capacitance smaller, frequency lower), the reactance is bigger (which then means it represents a larger impedance).

Look again at the stages with those caps from grid to cathode. From cathode to ground, there is another, larger cap. The smaller cap looks like a larger impedance than those bigger caps. So from the grid, the small cap can be a path for treble to bleed towards the cathode. At the cathode, the bigger cap to ground is a lower impedance path than the small cap; therefore, the signal present at the cathode takes the path of "least resistance" and is shunted to ground.

As a result, any signal that might tend to travel from the cathode through the small cap to the grid is really diverted down through the bypass cap to ground. So no positive feedback in this case.

[JayB]

I figured they would reduce treble to some extent, but was thinking maybe some positive feedback?

So what you're thinking is a cathode-follower-like output from the cathode being fed to the grid input, which would then amount to positive feedback.

Let's take your thinking in another direction. Look at the formula for capacitive reactance. It's 1 over a bunch of other stuff. If any of that stuff on the bottom gets smaller (capacitance smaller, frequency lower), the reactance is bigger (which then means it represents a larger impedance).

Look again at the stages with those caps from grid to cathode. From cathode to ground, there is another, larger cap. The smaller cap looks like a larger impedance than those bigger caps. So from the grid, the small cap can be a path for treble to bleed towards the cathode. At the cathode, the bigger cap to ground is a lower impedance path than the small cap; therefore, the signal present at the cathode takes the path of "least resistance" and is shunted to ground.

As a result, any signal that might tend to travel from the cathode through the small cap to the grid is really diverted down through the bypass cap to ground. So no positive feedback in this case.

I forgot about the cap bypassing the cathode resistor. 

[HotBluePlates]

I wouldn't have caught the significance, except I read up about some VTVMs I own.

The voltage function has an fairly small upper limit on the allowable input voltage, before you run the risk of pegging the meter. So, there is a voltage divider for the various ranges. That's great, except practical resistors also have some degree of capacitance and inductance, which may throw off the reading slightly when measuring high-frequency AC.

So the VTVM maker used a resistor voltage divider, but then also had caps across each resistor. If the resistor was a large-value, the cap was a small-value. I found out that the caps were there to act as a second voltage divider to ensure the division of a.c. was the same as the division of d.c. But, the small-value cap is the larger impedance, and has more of the voltage dropped across it. I had to re-orient my thinking, and now I notice that effect in other circuits.

I didn't do it when I started learning guitar amp stuff, but sometimes learning electronic circuits that are not amp-related help you understand on that is.