New Amp - SA112 Combo with 5 Preamp tubes

[JammyDodger]

Hey All,

I guess the title pretty much sums it up. I recently bought a '78 SA-112. It sounds great, quiet and responsive.
Being the curious sort that I am, I decided to pull the chassis out and have a look at the insides. As I was looking
at the back of the amp, I realized that there are 5 preamp tubes!

I've yet to figure out how to pull the chassis. I seems to be mounted on a board that is (maybe) slid into the cabinet.

Could this be an amp that someone have modded? Any thoughts?

Cheers, Mike

[Bossco]

Good photos will be suitable evidence for us to determine originality.

[Dr.HI-TONE]

yes, slide that board out.
it definitely has been modded.
open it up.

[JammyDodger]

Wow, this may take some time to sort out. I've attached some pictures. Let me know if you need any other views. I'll leave the chassis out for a few days.

I should say that the amp sounds great. It is dead quiet. It has some nice 'crunch' on either channel at 7 and above.

Cheers, Mike

[mhuss]

Wow, it looks like there isn't a stage that whomever didn't modify!

[JammyDodger]

I've tried to draw out the circuit as I have traced it so far. There is some funny business going on elsewhere but not as extreme. One funny thing is that they put the P.I. in V4 and the last gain stage in V5

03/18/21 - Minor changes made in red

[JammyDodger]

Here is the rest of the preamp portion. Note red highlights on drawing.