Connecting NFB Right first time

[David Root]

Example: Bassman OT, the dwg shows matching polarity dots on the brown primary lead and black secondary lead (common).

So if I connect the NFB from the green lead of the secondary (non-dot end) to the inverting side of the PI (normal connection) and connect the blue primary lead (non dot end) to the power tube that is the inverting side of the power tube pair? Or should it be blue lead to the non-inverting power tube?

Then, assuming I get that right, if I connect the amp to a pair of old JBL s that are wired +ve to black terminal (not to red), am I in phase with the amp output?

[martin manning]

I'm pretty sure that it goes like this, where (in) and (out) shows the phase of the signal leaving each element wrt the preamp output:

Signal: Preamp output (in) -> Inverting side of PI (out) -> Inverting side Pwr Tube (in) -> Blue Pri lead of OT -> Green Sec lead (in)

FB: Green sec lead of OT (in) -> Non-inverting side of PI (out) -> Non-inverting side Pwr Tube (in) -> Brown Pri lead of OT -> Green sec lead (out)

Note the above is consistent with your diagram that shows the brown pri lead and the black (common) sec lead as being in-phase.

Green sec lead goes to whichever speaker terminal produces fwd motion of the cone with positive voltage applied (9V battery test), assuming an even number of inverting gain stages in the preamp.

The FB signal is positive since it comes off the non-grounded side of the OT, but then it gets applied to the non-inverting side of the PI and so becomes NFB.

MPM

[Bob-I]

The way it works is simple.. connect it the way you think it right, cover your ears and power it up, wait until your ears stop ringing from thesqueel, reverse the secondaries and wham, it's right.

Reverse psycology doesn't work, if you connect it backwards from how you think is right, it'll be automatically wrong.

[Sonny ReVerb]

What Bob said...

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