NFB considerations with Lar/Mar MV

[sickman82]

Got a SL reissue, modded to 68-72 specs with the Lar/Mar MV. I originally put in a Vox style tone cut because I was using the amp at home, so I wanted something useful to tame sharpness.

Now I'm using this amp weekly at rehearsals I'd like to put the presence set up back in, or perhaps a resonance control.

I usually have the MV about 2 o'clock at rehearsal. What does everyone else have as their NFB set up?

[sickman82]

Just to add to my question.. I suppose I'm asking what would be the best way of setting up a variable NFB control.. A pot in series with the existing 47k resistor? Or a lower value resistor, say 33k, to get a good sweep? What value pot?

[Carbia]

try a lower resistor, 1k or something like this with a 100k linar pot in series.

[sickman82]

Isn't 1k a little low? Might that cause motorboating?

[Carbia]

I don't think so... anyway, you can always turn the pot

[darkbluemurder]

I think 1k is too low but you can try it. If you have an oscillation with the pot turned down, turn the pot and leave it where the oscillation stops. Measure the resistance and replace the 1k with a resistor value of what you measured.

Good luck,
Stephan

[Carbia]

guys...

the lower the value of the NFB resistor, more NFB, so... less prone to oscillate.

Funder uses 820ohm and nothing weird happens.

[sickman82]

I'll try the 1k and see where it gets me. I suppose that the PPIMV is limiting the NFB even with the MV at 2 o'clock.. So 1k or less might be a good start.

I'll let you know how I get on with it.

[darkbluemurder]

guys...

the lower the value of the NFB resistor, more NFB, so... less prone to oscillate.

Funder uses 820ohm and nothing weird happens.

Ad 1: it's the other way round - more NFB is more prone to oscillate. It's not a matter of pure gain. The lower oscillation threshold is due to the phase shifts caused by the various R/C networks within the feedback loop.

Ad 2: You have to consider the entire NFB network, which consists of the series resistor (820 ohms in the Fender) and the shunt resistor (100 ohms in the Fender). This means that Fender used a 8 to 1 ratio (approx.). Marshall used a similar ratio where they used 47k into 5k (the presence pot). You have to further consider from which secondary tap the feedback is taken - the 4 ohms tap has less voltage than the 16 ohms tap.

You did not say which shunt resistor to use but in the case of a 1k series resistor into a 5k shunt resistor (say the old presence style with the 5k pot or the new style with the 25k pot in parallel to the 4k7 to ground) this would be a massive load of negative feedback. I would use a higher minimum resistor value and also a higher pot value, say 250k as in some Boogies.

Cheers Stephan

[Carbia]

good explanation.

Anyway, 1k resistor and a 100k series pot.

He could turn the pot till he gets the sound he wanted and he can tell which is the minimum value that sounds good to later use that value as resistor.

[sickman82]

Just letting you guys know that the 100k pot and 1k resistor works like a charm. Yes, there was some oscillation by setting the pot too low for the given MV setting but the pot allows me to dial in the perfect amount of NFB for my purposes. I'll be keeping the pot in place, great stuff.. thanks guys!

[darkbluemurder]

Cool that it works for you.

As I wrote before, In order to have all pot settings accessible you only need to raise the 1k resistor to the point where the oscillation stops. If you don't want to measure, you can start with double the value (closest standard value would be 2k2). If that does not stop it, double again (closest standard value would be 4k7), then 10k and so on until you find the value that stops it.

Cheers Stephan

[Slowhand428]

I'm a little slow on the uptake. Could someone explain the wiring of the cut control? I'm trying to get the best tone out of my JTM45 with the Lar Mar PPIMV at low volumes.

-Erik