finding the NFB Resistor in Marshall Origin 20?

[Quixote]

Hey everyone! Lately I've been poking around my Origin 20 from Marshall to tweak some things around to taste. I know it's not an amazing amp, but I'm going to keep this brief and not talk about my opinions on it.

My question is regarding the negative feedback resistor, or rather the fact that I can't find it. For some reason, it seems that when everything is unbuttoned, all secondaries of the output transformer go straight to chassis. Why that is, I'm not entirely sure. This is making my search significantly harder. I've poked the presence control lugs and searched for continuity on any of the resistors on the output board but I can't seem to find any. I'm convinced that it's not on the output board, but that wouldn't make sense considering the output taps terminate on this specific isolated board. I'm going to attach a picture of just the output board. Perhaps I'm just overthinking things and it's right here? I just need someone to shed a bit of light. I want to say it's R208 since the bottom lead near C201 connects to ground, but 22k is a little low, don't you think?

[acheld]

Could you provide a schematic please?

[Quixote]

Heh. If I had one, I wouldn't even be asking! As usual for all Marshall amps built in the last ~15 years, they seem to be the only ones with it and they don't seem too keen on disclosing it any time soon.

[vampwizzard]

Id trace it off the presence control myself. You may have fun depending on how they did the NFB loop and if the Origin used solid state phase inversion.

[Quixote]

Gotcha. I'm 90% certain that its tube driven though. It has the same compliment as a JTM45 minus the rec tube. Looks like I'll keep doing the test-lead jig around the board then. 

This is odd though. So far its been almost 1:1 preamp-wise with a late JTM. The ones with the 33k slope resistor and ~250pF treble cap in the tonestack. I haven't poked around too extensively though, just revoicing the front a bit.

[PRR]

> all secondaries of the output transformer go straight to chassis. Why that is, I'm not entirely sure.

One end of secondary is grounded. The DC resistance of the windings is less than 1 Ohm (so it hardly impedes a 4/8/16 Ohm speaker). Most ohm meters won't read <1 Ohm well.

[Quixote]

Oh, that's really good to know. I don't know why I didn't think of that. I just checked and yeah there's one ohm of resistance from output to chassis. Gotcha.

I went ahead and checked the resistance between the ungrounded end of the primary and the presence pot and I'm reading ~150k. I don't know what is going on, but this sure isn't a JTM. I think I might just be chasing dragons at this point.

[sluckey]

I went ahead and checked the resistance between the ungrounded end of the primary and the presence pot and I'm reading ~150k. I don't know what is going on, but this sure isn't a JTM. I think I might just be chasing dragons at this point.

No telling what that 150k reading is. But doesn't matter. The presence pot will be connected to the secondary, not the primary.

[Quixote]

Oops, I meant secondary. Sorry for the mistype.

[vampwizzard]

Here are some gut shots of the head. The presence control should be the knob on the far right in one of the last pictures (chassis looks upside down) https://imgur.com/gallery/KfxSbpK

Not sure how they did the layout.. but a presence control is usually just a pot+cap in parallel combo. That extra resistor right there is interesting. then it jumps over to the main board. Keep tracing the circuit out and you'll have to run into the NFB resistor between this control, the grid sides of the PI, and OT secondary.