Trouble with Viewport Scale Control

I've been drawing a model which I plan to plot in different scales, 1:50, 1:100, and 1:200. I've drawn in 1:1 in model space and use the Viewport scale control to set the scale in each VP. I've checked the "Scale lineweight" box but it doesn't seem to do what I want. The lineweights look perfect in 1:50 but the lines get way to thick in the larger scales. Isn't that what the "Scale lineweights" box is for...?

I believe that the scaling is what's causing your thick lines -- your lineweights are already set to plot at a specific width. Also, you do have to option to 'display lineweights', so you can play with this without actually plotting. You can change the display in Options>User Preferences>Lineweight Settings

Your lineweights are already set to plot at a specific width.

My .ctb is set to "Use object lineweight". Would that be correct? The layer itself is set to 0.18, and the line's properties are all set to ByLayer. Also, is it so that if my layout had several viewports, each one would require its own .ctb? That's not the case here, I'm just wondering.

Can you post a screenshot of the good and bad plot previews?

I got it wrong. Here's from the help file:

You can scale lineweights proportionately in a layout with the plot scale.

Typically, lineweights specify the line width of plotted objects and are

plotted with the line width size regardless of the plot scale. Most often, you

use the default plot scale of 1:1 when plotting a layout. However, if you

want to plot an E-size layout that is scaled to fit on an A-size sheet of

paper, for example, you can specify lineweights to be scaled in proportion to

the new plot scale.

After looking into it further, it seems to me that your setup is correct, and that you may want a secondary .ctb to specify a thinner lineweight for your small scale plot.

Can you post a screenshot of the good and bad plot previews?

Here you go

1 to 50.dwf

1 to 200.dwf

Also, has anyone had the viewport scale change when using vpmax?

I believe your problem is that you have the "Scale Lineweights" box checked. I've never used that option but it is my understanding that the lineweight will change in different scales with this box checked.