Remove ground lift network? JCM900 dual reverb

[minim]

I've been tinkering with this amp 2-3 years now. I removed the preamp pcb and wired up a ptp cascaded preamp, and tweaked the PI to more traditional Marshall values. Results not too bad, but I'm looking for other areas where the signal might be suffering:

A: the "ground lift network", and

B: the Line Outs.

First of all, the line outs. A tap is taken off the signal path just prior to the PI and taken to the line outs. Do you think all this extra gubbins (i.e. R16, 13, 15, etc) could be "bleeding off" some signal?

As for this so-called ground lift network (circled), I'm considering just jumpering C16 and removing R36 and D6,7. Can you foresee any issues with this? I'm concerned mainly about the bias network, as its ground is about 22 ohms above chassis ground, although I would have thought 22 ohms is bugger all.

Thanks a lot.

PS the line out circuits are at the bottom left, the resolution of the pic is obviously not ideal.

[Enzo]

If you think the line out and emulator out are loading, just lift R16 and see. I tend to think it isn't having much of an effect.

As to the ground thing, first, if you are going to jumper a part, why bother removing the other parts? They will have zero effect, and will be there when you want to restore the grounding. The thing is intended to help prevent ground loops with other gear.

I don't understand your concern over the bias circuit. Yes, its ground is 22 ohms above chassis, just as everything else's ground is 22 ohms over chassis. Look at your schematic. In the middle of your circle, more or less, is a point called star earth, at the center of the "X" formed by the lines. It is that point which is referred to chassis by the network. Note the line going down to the left from the star, it joins the ground buss across the drawing. All your circuits are joined to that. So no circuit ground currents flow through the chassis. note insulated input jacks for example.

Now if in building your point wired preamp circuits you grounded things to chassis instead of the ground bus, then you have already destroyed the chassis ground situation and might as well eliminate the network. it may work out or you may have hum issues from the now unisolated circuit common/ground. If the power supply and power amp are isolated from chassis and the preamp is not, then the potential for ground loops is present.

[minim]

Okay well thanks for that I guess it pretty much sums it up.

I will remember in future to not bother removing parts if I'm going to jumper parts.

I wired the preamp grounds to star ground.

I will remove R16 tomorrow when I get to the amp and see if I notice a difference.