phase nfb question

[C Moore]

A friend of mine bought a Marshall clone that was a total mess. So I gutted the chassis and started all over.
I copied the former builders OT to power tube plate wires...as far as what color goes where.
From the back of the amp....The left tube socket got the White wire...and the right tube socket got the Red wire. That is how he did it, and I copied that.
But now that I am powering the amp, I get that high pitched (and loud) squeal.
I disconnected the NFB wire and the squeal goes away.
I am assuming I somehow flipped phase at the Z Selector-Speaker Jack.

He ran:
Gray 16
Grn 8
Yel 4
Ong 0 to speaker jack sleeve, and that to ground.
NFB to the 16 tap.

I ran:
Gray 16
Grn 8
Yel 4
Ong 0 to speaker jack sleeve, and that to ground.
NFB to to Center Of Z Selector.
Is THAT where I am having trouble, or did I screw up some place else.?
Thank You

[martin manning]

I think if you swap the primary leads you are good to go. Does "center of the Z selector" mean you have the NFB on the 8 ohm tap?

[roberto]

I think he means the actual impedance, so the NFB will depend on the impedance of the cab connected to the amp.

[martin manning]

Thanks Roberto. If "center of the Z selector" means the same lug that goes to the speaker jack, then that's not a good idea. The NFB ratio should remain constant regardless of the selected impedance, so connect the NFB wire to one of the secondary leads, e.g. to Gray if you want it to come from the 16 ohm tap.

[roberto]

Well, some Marshalls have that configuration, but I don't remember when.
I have to ask to a friend of mine who knows pretty every detail of all Marshall amps of the early years.

It's something like that:
[img:900:599]http://fc08.deviantart.net/fs70/f/2010/ ... 32btgw.jpg[/img]

[roberto]

Found the whole article on metroamp!

Marshall Negative Feedback Circuit

Negative feedback is an out of phase signal fed back into to the amp to cancel out certain frequencies, designed to bring the amp out front more in the mix. The negative feedback circuit consists of the presence control potentiometer, feedback resistor and where the feedback resistor is connected. The negative feedback affects not only gain and frequency response but also dampening, a lot of tonal variation can be achieved, simply by changing out the value of this resistor or by changing which output transformer tap it is connected to. The output signal is fed back into the amp before the phase inverter and the presence knob controls the amount of negative feedback.
The negative feedback circuit was changed over the years by Marshall, from a large amount of negative feedback, to less and less, over the years. Also the changes were not constant, but actually varied wildly at times.
The entire JTM series used 27K and 16 ohms. Pre '67 the 27k and 16 ohm combination was pretty universal, but there are several rare occurrences of a small valued bypass cap across the 27k resistor (this would increase the bass, but have less overall gain). I believe this was changed to the 47k and 8 ohm setup for Lead amps only, circa late '67, even though there was still lots of variation from the factory. The NFB lead was normally connected to the 8-ohm tap in SL amps, to the speaker jacks in Bass and PA. In late '68-'70 some of the SuperLeads had the NFB connected to the far side of the impedance selector or directly to the speaker jacks (which made the NFB variable). The 100k resistor seems to begin appearing around '70, again Lead amps only. The Bass, PA, etc., retained the original NFB setup (even though there are exceptions).

Typically this depends on if you have a 50 watter or 100 watter. 50 Watters have a 47K on the 8-ohm tap and 100 watters have the 100K on the 4-ohm tap. They are *supposed* to sound the same but if you A/B them you'll hear a sonic difference. Be aware that there is lots of variation in Marshall’s as they came from the factory, every now and then you'll see a 47K on the 16ohm tap (which is a lot of feedback) or a 100K on the 8ohm tap but those were pretty far and few between. Marshall used to experiment a lot in the 60's.
Using 100K will increase the gain and mids. Finally, The lower resistor value gives a bit more smoother sound, while the higher value gives more of a harder edge at the expense of smoothness. Larger value means less nfb, therefore more gain.
On some older Marshall’s (like my '69) the negative feedback (purple) wire was connected to the speaker jacks, so the feedback varies depending on which speaker impedance is selected. The more negative feedback means less gain in the power section; less nfb means more gain. The 16 ohm tap is gonna have more voltage coming off of it than 8, and 8 will have more than 4, so you can control the overall gain of the amp by choosing which tap you use. Use 16 for the least gain, 4 for the most. Remember, this is only power amp gain, not preamp. Also, the Presence control will make more of a difference when there is more voltage to work with. So, if nfb is on the 16 ohm tap, Presence will seem to do more than if nfb is on 8 or 4.
A little known fact (and all the George Lynch followers may like this since he preferred the '73 hand wired heads) is that the common .1uF cap on the Presence pot was a .68uF cap resulting in a very cool "cut" to the upper mids that was not as harsh as the .1uF. With the 100k feedback resistor, the 0.68uF cap bypassing V2a may be too much. With a 47k feedback resistor, it's just right though. My '72 has the 100k feedback resistor and no .68uF cap on v2a.

[C Moore]

I think if you swap the primary leads you are good to go. Does "center of the Z selector" mean you have the NFB on the 8 ohm tap?

Hey Martin...and everybody.
Yeah...I have the NFB connected to the tip of the speaker jack, which is connected to the center of the Z Selector.
But how does that, in itself, change phase.?

What I am wondering is.....I have the OT wires connected to the same power tubes as the previous builder (although his chassis was Such A Mess, he may not of even had the nfb connected) so why do i get Positive Feedback.?

It would be easier for me to change something at the Out Jack, than to change the OT wires...one of them will be too short at this point.....will have to splice on some length, and they are "kind of" hard to get at now.
Could I swap the wires on the Out Jack instead.?
Thank You

[sluckey]

Could I swap the wires on the Out Jack instead

No. Just swap the red and white primary wires. They are plenty long. Just route them under the board.

[C Moore]

10-4.
The Red and White wire it shall be.....

[TUBEDUDE]

Posted: Wed Jul 09, 2014 2:51 am    Post subject:Found the whole article on metroamp! 

Odd, the version I saw.....

Marshall Negative Feedback Circuit 

Negative feedback is an out of phase signal fed back into to the amp to cancel out certain frequencies, designed to rob the signal of dynamics, ambiance and presence. The negative feedback circuit consists of the presence control potentiometer, feedback resistor and where the feedback resistor is connected. The negative feedback affects not only gain and frequency response but also dampening, a lot of tonal damage can be achieved, simply by changing out the value of this resistor or by changing which output transformer tap it is connected to. The output signal is fed back into the amp before the phase inverter insuring the feedback signal is phase skewed enough to remove the coherence of the higher frequencies, and the presence knob controls the amount of damage done.
The negative feedback circuit was changed over the years by Marshall, from a large amount of negative feedback, to less and less, over the years as they tried to limit the tone sucking, yet still prevent oscillations that were inevitable in their design. So the changes were not constant, but actually varied wildly at times dependant on layout of the particular model.
The entire JTM series used 27K and 16 ohms. Pre '67 the 27k and 16 ohm combination was pretty universal, but there are several rare occurrences of a small valued bypass cap across the 27k resistor (this would increase the bass, but have less overall gain). I believe this was changed to the 47k and 8 ohm setup for Lead amps only, circa late '67, even though there was still lots of variation from the factory. The NFB lead was normally connected to the 8-ohm tap in SL amps, to the speaker jacks in Bass and PA. In late '68-'70 some of the SuperLeads had the NFB connected to the far side of the impedance selector or directly to the speaker jacks (which made the NFB variable). The 100k resistor seems to begin appearing around '70, again Lead amps only. The Bass, PA, etc., retained the original NFB setup (even though there are exceptions for individual amps that were unstable at those values). 

[C Moore]

Ar any rate.....I swapped the Red and White...now all is fine.

I have to tell you, I was sick to my stomach when I hit the SB Switch, and the amp screamed murder...even with the volume off.
I spent so much time putting this all back together...I could not believe I had this oscillation problem.
At first I rejected PFB, because I had the same color wires as before I gutted the amp. I still do not understand where I flipped phase.
Which, again, makes me think that the other builder made a mistake in the NFB connection somewhere. I have a feeling it was not functioning.

So my luck with fixing other peoples amps remains 100%...but when I BUILD something, it is always a nightmare.

Oh Yeah...this was a favor for a friend. I told him 50 bux plus parts.
So I then proceed to break BOTH face-plates.
I bashed my knuckles against a corner of the rear plate, and broke of a small piece. I am hoping he can live with that.
While soldering the pots, I noticed they were all fairly loose (beginners mistake).
The previous builder had used all those locator tabs that come on some pots...they go into that little hole in the chassis, so the pot does not turn when you tighten it.
Well.....those tabs are longer than the thickness of the chassis and face-plate. So when I tightened the pots, they stuck through the chassis,into the face-plate, and cracked it at the Mid and Bass pots.
Cost.? .....50 bux from Ceriatone.

I have a feeling he will be more than happy to give me an extra 50 for gutting and rewiring his amp.....but we shall see.
Live and learn as the saying goes.
I always clip those tabs off of the pots. Just did not even notice they were there.

Per Usual.....Thanks for all the Help/Advice

[cbass]

The phase could have got swapped on the plates or grids of the PI if you rewiresd that.

[C Moore]

That is a good point. I id not pay attention to how the other guy did it.
I followed the schem regarding the 82k vs 100k resistor.
I guess I will never know.....

[pdf64]

If positive feedback and it's a pain to swap the OT leads, then consider swapping the wires to the power tube control grids.
Pete

[C Moore]

If positive feedback and it's a pain to swap the OT leads, then consider swapping the wires to the power tube control grids.
Pete

Its done...but looking back at the pics I took, I think that is where I flipped it.
His grid wires are the only ones I cannot see clearly. They were probably crossed, under the board, and then I pulled them "straight when I spliced more length on.
Thank You