Marshall NegFback

[samwheat]

I am almost finished with a jcm clone however my board uses a 1987 NFback circuit which is different than the jcm

would could i expect if i leave it alone?

and why is it different between the 1987 and jcm?

i also have a ppimv if that matters

[JJman]

Which JCM model# are you building? A 1987 is a JCM. One of many model numbers.

[samwheat]

a 1987 is a 4 holer input ......

i'm building a jcm800 50w

[jlummaa]

a 1987 is a 4 holer input ......

i'm building a jcm800 50w

There are several 50W JCM800 guitar amp models made:

2204 Master volume
2205 Split channel
also
1987 Non master volume

[John Phillips]

The only real difference in the NFB circuit on any of the older Marshalls is that the earlier ones were DC-coupled, using the track of the Presence pot itself as the final part of the PI tail to ground, with only the cap bypassing it; and the later ones, including all JCM800s, were AC-coupled using a fixed resistor with the pot and cap bypassing it. The circuit was changed some time in the JMP period, I can't remember exactly when.

The reason for the change is most likely that in the earlier version, because the pot has a DC current passing through it (the bias current for the PI), it's always scratchy when you turn it. Cleaning the pot or replacing it makes no difference (though many people try, and a lot of old amps lost their original pots), it's a fundamental characteristic of a pot with DC on it.

Personally I think that the earlier version sounds better though, I'm not exactly sure why! Or maybe it doesn't really and it's just a perception because the old amps sound a bit better anyway :).

The PPIMV won't affect which is better, but remember that with any PPIMV, reducing the volume also makes the Presence control less effective as the MV is within the NFB loop, so the NFB is progressively defeated along with the main signal as the MV is turned down. (This is actually a good thing IMO, since it makes the amp sound a bit more lively at low volumes.)