Understanding/improving JTM60 FX LOOP

[torcamaniac]

Hi guys.
Can someone help me understand and analyse the fx-loop of jtm60/jcm600 and improving it. I think the loop is passive. Would like to make it more pedal friendly, if possible maybe switchable from +4dB to -10dB?
The amp has a series and parallel loop. I am concerned only about series loop. I use parallel as a Fat-Boost with its mix pot ( design flaw turning out to be a cool feature).
Is there a way to adjust the series loop circuit to get it buffered?
Here is a schematic:

[Chuck H]

Unfortunately the situation is compounded by:

The provided schematic does not show the effects return (which is more important than the send actually)
The loop is probably in a less than ideal location to achieve what you want (noted by Juan in the link)
In a new location you would probably need to add an amplifier for the necessary recovery gain (noted by Juan in the link)
It would be a somewhat complicated circuit to make it switchable adding tender signal leads to an already high gain situation.

http://music-electronics-forum.com/t41834/

It seems the initial problem was already visited some eight months ago. The link provides better circuit clarity (if you can call it that with a Marshall schematic )

Padding the output of the effects loop and making that pad switchable is easy. Boosting the recovery isn't. If you reduce signal in one place you must raise it back up to achieve unity. And (noted by Juan in the link) the headroom of the return circuit is simply too low. At least that's what I interpret from Juan's assessment in the link.

SS circuits aren't in my arena anyhow. Interpreting Marshall schematics is in no ones. Juan was extremely vigilant with his efforts in April. I thought I'd provide the link as a heads up for anyone else viewing the thread.

[torcamaniac]

Will try to redraw the original schematic further to make it easier to read. This is not complete. In red is modded stuff. The two 100k's are not drawn well. These are connected as voltage dividers. Have to fix that too.
This is not complete but covers most of the preamp.

[Chuck H]

A real problem with what you want to do is that the series loop feeds the parallel loop, both loops feed the reverb and then the series loop, the parallel loop and the reverb are summed and re amplified at the same amplifier circuit right behind the PI. If you mess with the level of the series loop (send and/or recovery) you mess with the level of the parallel loop and the reverb as well. Just like Juan said in the older post, in order to accomplish what you propose you must add another amplifier stage (possibly two). Don't even ask me what component values and transistors to use, I don't know. The redraw below shows where you'll need to affect the changes.

[torcamaniac]

Thanks Chuck. Sounds complicated 
Guess I'll leave it for now the way it is...
Still experimenting with other mods...
It was confirmed a while back that there are at least 3 different schematics for JTM 60.
Everything is almost the same, except that some resistor values differ... And the biggest difference was the clipping diodes... On some JTM's such as mine the clipping diodes at V102 (which I removed anyway) don't go to ground but feed back to V101B stage (some form of local feedback is my guess), where as on others they feed to ground.
Dunno what effects those two different circuits have...