Understanding an unusual op amp configuration

[McCamish]

I've been examining different schematics to help understand how and why they sound the way they do. I think i understand op amps pretty well, but I am having a hard time understanding a piece of the Marshall 9004 preamp. Ic1a, and b in the attached schematic are in a configuration that I have never seen. I have done some research and have some possible ideas, but still very confused. Anyone have any idea what is happening here?  It looks like it uses positive feedback made up of the gain pot and a small cap. The positive and negative of the other side are tied together with a resistor.  IC3 and 4 are both M5201s, which are switchable op amps. IC1 (which is the one in question) is unlabled on the schematic, but appears to be a standard op amp like TL072, or 4558 type, as far as I can tell. I haven't been able to find that info either.

[McCamish]

I am guessing that IC1A has something to do with current, or possibly adding some saturation?  I really don't understand positive feedback in this situation. Possibly mis-labled?

[Rixen]

yes, according to the datasheet, pin 6 is actually an inverting input.

[Rob Strand]

Off-hand that opamp was special:
It has a control signal which effectively lets you select the output of the first or the second opamp.  As you can see there is only one output pin.   There is another pin that does the selecting.
You wire the both opamps however you like then the control selects between the two.

The only side-effect is the selected opamp will drive current into the feedback circuits of the other.  Which won't effect the selected channel, and usually will have minimal effect on the other channel.

[reddesert]

Speculation:
IC1 is a switched op-amp like the others.
If the boost channel is selected, IC1b is selected and is wired as a typical inverting amplifier, given the +/- drawing error that Rixen pointed out.
If the boost channel is not selected, IC1a is a fancy way of holding the input to the boost channel at 0 v.

If IC1 were a normal op-amp like a 4558, the input pins are 2,3 and 5,6; not 6,7 as drawn.

[anotherjim]

Better drawing of it here...
https://elektrotanya.com/PREVIEWS/63463243/23432455/marshall/marshall_jcm600_sch.pdf_1.png
...IC3 in area F4 is selecting channel 1 or 2 reverb returns. Even drawn like that, it's puzzling until you find the chip data.

[McCamish]

Awesome guys!  Thanks for the input. It does appear that IC1A must be a M5201 like the others. I would then assume that you could reproduce the boost section only using the "b" op amps, as marked in the schematic, substituting a more common op amp, and forget about the other half. Or vice versa for the normal channel.