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Can you make a note of the resistance measured between either of the outer tags and the inner tags?

I've personally not seen a 4-tag pot before.

What is connected to the fourth tag and where does it go? It may just be part of a grounding system. Is this pot a switch as well?

A fourth lug probably means it's a tapped pot, which has the usual two legs and a wiper, plus a tap off the resistive strip at a set place. Your drawing looks consistent with this. One place I've seen them that comes to mind is in the tone circuits of some brown Fender amps from the early '60s. Oh, also the bias pot in many '70s Fender amps.

In your Sunn schematic, the only pot that looks like it might be 4-lug is the Channel A midrange control (100KL), though it is drawn a bit differently than I've seen before, with the tap pointing to the wiper, instead of across the resistive "zigzag", which doesn't really make sense. Now that I'm thinking, this extra arrow is probably just part of the labelling, as it appears in Channel B also, where it doesn't look like it's a connection to ground, but rather just a way of pointing out the value of the pot.

I might have missed it, but I don't see anything else resembling a tapped pot in the schematic. Are all four lugs wired to something? Can you see how it's wired into the circuit?

-Tom

Looks like a normal pot in the schematic to me. Are you sure the extra "lug" isn't just a solderable mount to fix it to the PCB?

To test, remove the pot and measure the resistance of this lug against the rest.

Rodabod wrote:Looks like a normal pot in the schematic to me. Are you sure the extra "lug" isn't just a solderable mount to fix it to the PCB?

That was my guess as well.

thirded.

The treble control in the schematic looks like a regular pot, assuming this schematic accurately represents your amp. I'd check it (out of circuit) as Rodabod suggested, to see if that fourth point has any measurable resistance to the rest of the pot's lugs.

-Tom