PRRI: Where to put the NFB loop when having two different OT taps

[shapeshifter]

So I exchanged the original OT of my PRRI with a Hammond 1760E which has three different taps (4,8 & 16 Ohm) and connected the 4 and 8 Ohm wires to each of the Princetons two speaker outs (originally the were wired in parallel). The idea was to have a seperate 4 Ohm output so I can use two 8 Ohm speakers (the internal speaker plus an ext. 1x12 cab) in parallel with the correct impedance.

I connected the NFB loop to the 8 Ohm speaker out, since I figured, it will have the same effect as the original wiring.
But now I am asking myself:
Is there a difference if I wired it to the 4 Ohm out?
Or is there another way to do this?

Thanks!

[RedRock]

You did the right thing.

[shapeshifter]

Thanks RedRock!

[Blue Strat]

Agreed, but in the spirit of adventure, you might want to try the other taps just to see how it sounds.

[shapeshifter]

I might just try that...or I put a switch on it.

[teemuk]

From a practical application of Ohm's law you can determine that to generate equal output power to higher impedance loads the signal's magnitude (in volts) must increase. Therefore taking feedback from higher load impedance taps of the OT will increase magnitude the feedback signal and therefore also magnitude of negative feedback applied. Conversely taking feedback from lower impedance taps will decrease magnitude of the feedback.

There is a difference, the difference that higher or lower magnitude feedback makes. Generally, negative feedback decreases output impedance and reduces distortion - it does both at the cost of overall gain. What is gained is flatter response to inductive loads and cleaner amp with the expense of clipping distortion becoming more sudden. As a rough generalisation, a more HiFi-ish response. If you use less NFB these effects will decrease. You get amp with more gain, weaker damping and overall greater amount of distortion, even on "clean" tones. As a rough generalisation, an amp that tends to introduce its own "colour" to the overall output.

How much difference taking feedback 4-ohm tap instead of the 8-ohm tap really makes really depends a lot on the amp. It's not a drastic difference to begin with and some amps reflect the effects in lesser magnitudes than others. It really depends on several interacting factors and I'm not going to start to list and explain them here. Yep, put a switch to it to try yourself. You can even make it a three pole to include all the taps so you can go from the "initial setting" to both higher and lower amounts of feedback.

[guitarcapo]

You get more negative feedback using the higher value tap.

[shapeshifter]

Thanks for the input!!!