Ab763 vs AA964 PI/negative feedback look question

[Toxophilite]

Hello
On the Ab763 vibrolux  there's a 47 ohm going to ground just prior to the PI and I think part of the negative feedback loop. the aa964 vibrolux has a 100 ohm resistor in this position
How does the change affect the amps sound/performance if at all?

[sluckey]

That 47Ω or 100Ω resistor works with the 820Ω resistor to form a voltage divider for the feedback signal applied to the PI.
 
The AB763 has an OT for a single 8Ω speaker. The 47Ω resistor sets up a certain amount of feedback.

The AA964 has an OT for a pair of 8Ω speakers (4Ω total) This OT creates a reduced amount of feedback. Using a 100Ω resistor will provide a larger feedback voltage to be applied to the PI. The net result is about the same amount of overall feedback created in both amps.

The sound/performance should be about the same for both amps.

[Toxophilite]

oooh interesting! Thanks for the reply!


I was asking because I inadvertantly built my latest Ab763 deluxe reverb (powering a 8 ohm JBL D130) with a 100 ohm resistor in that spot (I think I was using my vibrolux build as a template)


I found the sound to be duller and more low mid heavy than my earlier deluxe reverb builds..I thought it was the speaker as that was the only significant difference that I was aware of


My vibrolux build has the same 100 ohm resistor in this spot and is also powering a single JBL D130-F 8 ohms
It's also a little dull and boxy sounding compared to my earlier deluxe builds and I thought it was the speaker


While I was waiting for a reply I realized I could easily alligator clip another 100 ohm resistor in paralell and hear the difference, and voila my amp was noticable louder and brighter


From what you are saying I'm understanding that if I'm using a single 8 ohm speaker with either amp I'm likely better of with 47ohm value in the spot?  It does sound better, more like what I'm used to from that sort of amp.



To confuse issues and out of curiousity, I sometimes like to run an extension speaker (mini leslie 8ohm)in paralell with the built in speaker reducing the speaker load to 4 ohms
On the vibrolux I have a switchable multitap OT so I can switch it to 4ohm
In this situation would the 100 ohm be more appropriate?


With the deluxe I just built I'd like to do the same thing (leslie extension spealer) but I have an OT with just an 8ohm tap that I figure won't die with an 4 ohm load(feel free to tell me I'm horribly wrong...) how would that affect the feedback sent to the PI?

[Toxophilite]

I'll streamline that question
How would a multitap Ot on a vibrolux affect this situation?

[shooter]

How would a multitap Ot

the NFB line stays with the tap, not the speaker......I think

[sluckey]

Negative Feedback can be viewed as a personal taste thing. Some people say their amp sounds better with no NFB. Others like to make it variable. Some even put a cap in the variable circuit and call it presence. There's no right or wrong here. Sure, messing with NFB will change the sound of your amp. But the amp voice is a big part of the electric guitar scene. You have the amp, experiment.

If you mess with Leo's recipe though, your amp probably wont sound quite like he intended it to sound. Is that a bad thing or a good thing? It's your call. If you come up with something you like, change the name to something like Marshall and make some big money! 

[Toxophilite]

Ha!
Well I don't know enough about electronics to be truly innovative
nor am I motivated enough to start building amps as a business and most importantly my French last name whilst a perfectly good one might not have the same blunt appeal as Marshall and Fender. Maybe too many syllables, But such is life
Also I live in an apartment with a very ornery neighbour and even though I don't play loud experimenting with different permutations at any great length is not going to happen.


So assume that the fender design/sound is the constant and that is what I am intending to achieve (albeit with a different speaker setup)


I imagine if I was using a 4ohm load(via the 4 ohm tap) I would achieve roughly the same sort of sound with a 100 ohm resistor as if I was using the 8 ohm load (via the 8 ohm tap) with a 47 ohm resistor.


good to know.  I wonder if it'd be a bad thing to make switchable, too bad I couldn't do it with the speaker tap switch but it's a single throw.


I've installed negative feedback controls on a few amps, but usually I end up not really using them.

[sluckey]

I've installed negative feedback controls on a few amps, but usually I end up not really using them.

Think about that a minute.....




That accomplishes the same thing as changing that 100Ω or 47Ω resistor, or changing which secondary tap you connect the NFB loop to. All three of those things just change the amount of NFB signal sent back to the PI.

[Toxophilite]

I have thought about it
I like the amp how it sounds wi the designed NFB and in the limited time I have to play my amps loud monkeying with an NFB control is just not going to happen..maybe in the studio but even then time is money.

I'm just trying to figure out what i need to do to be able to best use sometimes variable speaker configurations I like(dependant on venue, size of rehearsal space and personal inclination)
I think I understand what's happening with vibrolux


So with my Deluxe using the 47 ohm resistor and single 8 ohm speaker to achieve the designed sound(that's what I want )


Shooter said the nfb goes with the tap and not the speaker load


What's the scenario if I have a single tap Ot (8 0hm) and I want to hook up a remote speaker in parallel giving me a 4 ohm load. I know it affects the power tubes some


Does that mess withe NFB loop at all??

[sluckey]

Maybe. Probably. But will you hear a difference? And if you do hear a difference is it because the NFB changed, or just because of now having two speakers connected, or both?

[Toxophilite]

Gotcha
Thanks!

[jjasilli]

Agreed. In my experience bells and whistles, heck even tone and volume controls on an amp, just don't get used much in a live situation.  The amp may not be easily reached and you just gotta play.

[shooter]

I don't know enough about electronics to be truly innovative

Ah, but this inovation is sound, you're just swapping in "black-boxes" to hear what happens, plug in an AU7, same thing, sound.  I am real sure PRR or HBP will help you with the math if you can't sleep , to be fair, it is kinda fun.

But at the end of the day;

you just gotta play

[HotBluePlates]

Shooter said the nfb goes with the tap and not the speaker load

Normally, yes.

Power = Voltage² / Impedance, and so
Voltage = √(Power * Impedance)

Your amp is designed for a certain power output.  Pick an OT secondary tap, and there will be a certain voltage present at a given power output.  That "certain voltage" is the source of feedback voltage, which is then divided down and injected earlier in the amp.

Regardless of which OT secondary tap you attach a speaker load, when you play through the amp all of the taps are energized with a voltage proportional to the tap's impedance.  Leaving the NFB loop attached to a single tap when changing which tap feeds the speaker keeps a consistent NFB voltage.

What's the scenario if I have a single tap Ot (8 0hm) and I want to hook up a remote speaker in parallel giving me a 4 ohm load. I know it affects the power tubes some

Does that mess withe NFB loop at all??

Indirectly.

If you attach 4Ω (or 16 Ω) of speaker to an 8Ω tap, the reflected primary impedance is different.  Power output will probably drop a little.  That will leave a little less voltage on your 8Ω tap (in accordance with the formula above) than if you'd attached an 8Ω load.  So we should expect NFB will be a little-reduced.

You'd probably get a lot more possible NFB reduction with a variable series resistor in the feedback loop, than in this case of mis-loading with 4Ω on the 8Ω tap.  Sluckey is right that maybe you'll hear a difference, but you'll also definitely hear a sonic difference (even with no NFB) due to the extra speaker and its contribution.

[Toxophilite]

Regardless of which OT secondary tap you attach a speaker load, when you play through the amp all of the taps are energized with a voltage proportional to the tap's impedance.  Leaving the NFB loop attached to a single tap when changing which tap feeds the speaker keeps a consistent NFB voltage.


Excellent,  So on my multitap Ot vibrolux I have a switch on the Ot for different taps. So I can just put the negative feedback on the 8ohm tap, install a 47 ohm resistor (or on the 4ohm tap with a 100 ohm resistor) , instead of having the NFB loop attached to the pin on the switch that goes to the speaker

Interestingly I just checked my vibrolux and I did have it hooked up this way...but to the 8 ohm tap and I had the 100 ohm resistor in the NFB. Time to either swap the resistor or hook it up to the 4 ohm tap


Thanks!