NFB wire to.......?

[samtheman]

Now it goes to 8 ohm tap, what about if I move it to speaker jack? Like old days (Marshall)

NFB resistor is 47k and I´m use 16 ohm cab....

cheers

/sam

[mark norwine]

leave it alone.

The "NFB resistor" is not a stand-alone component. It's part of a voltage divider; the other resistor's value is critical to understanding what's going on.

But the designer selected the 8 ohm tap because that tap produces the voltage which, when fed to the afore-mentioned voltage divider, provides the desired amount of NFB.

Irrespective of what tap you're using to drive your speakers, the NFB is being driven by the 8 ohm tap. Leave it there.

ps....it would help to know what kind of amp this is....

[zzmoore]

Your question has been answered by Mr. Norwine. The speaker jack is under the influence of the impedance selector switch. If left to the 8 ohm tap of the OT your NFB will operate as designed.

[SatelliteAmps]

When exactly did Marshall put the NFB directly on the jack? Maybe I just don't remember those old days.

[samtheman]

When exactly did Marshall put the NFB directly on the jack? Maybe I just don't remember those old days.

68 Plexis?

I had Germino LoVo55 and that amp has nfb wire straight to the speaker jack..

I dont mind which one is right or wrong, just wanna know hows it change tone?

cheers

/sam

[SatelliteAmps]

68 Plexis?


Nope. As far as I know, it's never been directly attached to the output jack on a Marshall. The only reason to ever attach it directly to an output jack would be on a fixed single output impedance.


The NFB is not dependent on what your output impedance is, but where it is attached to the output transformer. Putting it on different impedances will change the ratio and how it works (as will changing some tube types).

[VaughnC]

Interesting......in my homebrew amp I put a 100k detented pot in the FB loop so I could gradually loosen/tighten up the amp when I wanted by changing the level of the feedback signal. It was interesting how it changed the feel of the amp...but, I found I preferred around a 50K feedback resistance value for most things so I replaced the pot with a 47k resistor. The amp got more wooly as the feedback was reduced which was cool for certain things. Interesting experiment though.

[Jade]

Some 60's era Marshalls did in fact have the NFB tapped right off the speaker jack. As for why they did it this way, I have no idea.

Back to the OP's question, Moving the lead to the 16 ohm tap will give more voltage to the NFB circuit, which will result in less gain, and a tighter sound. Conversley, the 4 ohm tap will be just the opposite with more gain, and a looser feel. Depending on the particular amp, while unlikely, it's possible that you could run into stability issues by moving from the stock 8 ohm position.

I'd leave it where it is, but If you like to tinker, what the heck, you're not going to hurt anything.

Jade

[SatelliteAmps]

Any idea which specific 60s Marshall had it tapped off the jack? They were attached directly to the output on amps with only a single output impedance. If there is one with a variable impedance selector with it attached to the jack, I would be very interested to see it. I am really curious now.

Also, if you want to run 6550's, then it should be on the 4Ω tap, not the 8 or 16. The 8Ω is best for the EL34s. Assuming this is a Marshall clone.

[JimmyR]

I believe that some JTM45s had the NFB attached directly to the output jack. But then they were a single output impedance amp so it made no difference. Early JTM45 RIs had 4,8 and 16ohm taps but still connected the NFB straight to the jack. It meant that the amp felt looser or tighter depending on the impedance tap used. I had one of those early RIs!

[hamfist]

Also, if you want to run 6550's, then it should be on the 4Ω tap, not the 8 or 16. The 8Ω is best for the EL34s. Assuming this is a Marshall clone.

I've never heard anyone state such a rule of thumb before. It all just comes down to personal taste really, doesn't it ?

More NFB (increasing the connected ohmage or reducing NFB resistor value) giving a tighter feeling amp with slightly less gain. And obviously the reverse for reducing the NFB.

Attaching the NFB wire to the output jack just selects the connection to the OT that you have selected on your ohm selector, so the NFB effectively becomes slightly adjustable, according to what ohmage output you are using.

THe OT tap connected to by the NFB wire controls the voltage applied to the circuit. 4 ohm gives least. 8 ohm doubles it, and 16 ohm doubles it again. So, if your NFB is connected permanently to the 16 ohm OT tap for example, you always get the greatest voltage (and most NFB) available, whatever the ohmage output you have selected to your cab.

Definately something that can be worth tinkering with ! There is no optimum, just personal taste !

[SatelliteAmps]

The 6550/4Ω tap came straight from Marshall. It can also be found in both Aspen Pitmans & Gerald Weber's books. I am pretty sure it's in a few other spots.

Good to know about the RI JTM 45. Thanks.

[Jade]

I believe that some JTM45s had the NFB attached directly to the output jack. But then they were a single output impedance amp so it made no difference. Early JTM45 RIs had 4,8 and 16ohm taps but still connected the NFB straight to the jack. It meant that the amp felt looser or tighter depending on the impedance tap used. I had one of those early RIs!

That's what it was. Thanks for jogging my memory! I knew I had worked on one, or two before like that, but didn't remember them being the RI's.

I still don't understand why they did it that way, unless they just wanted to sell more cabinets...

"Dude, you gotta get rid of that 16 ohm cab, and get a 4 ohm cab... they have more gain!!" LOL

Jade

[JimmyR]

I bought the RI JTM45 head from a friend who had been playing it through a 16ohm 2x12 cab and it sounded as woolly as anything apparently. This was before I knew anything about NFB! Maybe I wouldn't have gotten it from him so cheaply if I had!

[donnyjaguar]

I like to run my builds off the 16Ohm tap myself. Like Satellite says, its a voltage ratio. The voltage on the 16 Ω tap is not double that of the 8 Ω tap. You have to consider this and also the other resistor to ground (usually via cathode) in the first stage of the power amplifier section.

[donnyjaguar]

I was going to mention that in my builds what I usually do is ground the COM terminal of the output transformer at the same point the cathode resistor of the first PA stage is connected. This is a trick from my days gone by of power supply design. In so doing you get the best regulation. In an audio amplifier you get the most faithful feedback voltages and hence lowest distortion. You also lessen any possibility of NFB induced instabilities. Some amplifiers actually use a tertiary winding on the OPT that keeps the speaker leads floating. You still get your NFB alright, but removing the speaker leads from the equation takes away the residual DC from the speaker line. Granted, this makes little if any difference to tone and is most relevant when speaker leads are particularly long, ie 100ft. Not generally the case with a tube combo! :)