Which side of the OT do I tap for NFB?

[ted01]

How do I tell which side of the output tranny should be used as the tap for the negative feedback loop? The tranny I'm using is a Hammond 1750N. It has a common wire and three other wires, one for 4/8/16 ohm operation respectively. Do I use the common wire or "8 ohm" wire (as I am using an 8 ohm cab). I know that getting this backwards will cause massive motorboating, as I have messed this up before. Is there a "scientific" way to figure this out, or is it just trial and error?

Thanks!

[Firestorm]

It's definitely not the common wire; that is just a ground connection. As to which of the taps to use, it sort of depends on the feedback resistor and load resistor values you have chosen, but you can use ANY of them. Marshall was all over the place over the years, tapping the feedback loop from the 16, the 8 and the 4 without always changing the R values (sometimes they did that, too). Each will sound a little different (not much). It also depends on what load you're using, but also not much -- the feedback loop is purely resistive, but the load is an impedance so it's equivalent resistance varies with frequency.

[Alexo]

Tap the hot lead, or one of 'em.... if it squeals, swap the primary wires and you'll be fine. I think the scientific approach is to wire your primaries in such a way that it won't be a royal pain in the butt to swap them if things go awry.

[ted01]

Tap the hot lead, or one of 'em.... if it squeals, swap the primary wires and you'll be fine. I think the scientific approach is to wire your primaries in such a way that it won't be a royal pain in the butt to swap them if things go awry.

Thanks ya'll. I like your idea of "science", Alexo!!

[briane]

Actually not so difficult, and yes there is method that prevents squealing.

2 separate questions.

Which tap to NFB and resistor determines amount of NFB. Thats simple.

OT supply:
Right half of PI to righmost power tube, left to left. Brown to left, and blue to right, looking in from the back side. Thats the science. No reason at all to screw around with trial and error.

[KT66]

Ted01 wrote :

I think the scientific approach is to wire your primaries in such a way that it won't be a royal pain in the butt to swap them if things go awry.

I think that it is a good idea to leave both OT primary wires long enough to swap them if you get the positive feedback squeal thing. I have built an amp where the brown/blue wire rule Briane mentioned did not apply. Also, I just find it easier to switch the PI feeds to the power tubes rather than unsolder and resolder OT wires that often have an easy to melt jacket.

[KT66]

Ted01 wrote :

I think the scientific approach is to wire your primaries in such a way that it won't be a royal pain in the butt to swap them if things go awry.

I think that it is a good idea to leave both OT primary wires long enough to swap them if you get the positive feedback squeal thing. I have built an amp where the brown/blue wire rule Briane mentioned did not apply. Also, I just find it easier to switch the PI feeds to the power tubes rather than unsolder and resolder OT wires that often have an easy to melt jacket.