Question regarding NFB

[iknowjohnny]

In a marshall style amp that is. I remember reading somewhere about what causes the effects of NFB to be lessened when turned up. But i cannot recall what it was. the reason i'm asking is because i decided a 250pf across the NFB resistor sounded really good in giving the top end a better more springy top end feel. But next time i tried it i had put the cap on a switch and i couldn't detect any noticeable difference. then i found out why....i had the amp up a tiny bit more. It seems the effect of that treble bleed only makes a big difference at TV listening levels. Long long B4 it gets remotely close to stage volume it disappears. In fact, just a hair louder than where i first listened at TV volumes and the effect is all but gone. Any idea how i can change that? I first had two 47k's in series as the NFB resistor with the cap across one. But i later wanted more and put it across both. so max effect there. But what can be done to retain that effect as the volume is turned up?

[PCollen]

In a marshall style amp that is. I remember reading somewhere about what causes the effects of NFB to be lessened when turned up. But i cannot recall what it was. the reason i'm asking is because i decided a 250pf across the NFB resistor sounded really good in giving the top end a better more springy top end feel. But next time i tried it i had put the cap on a switch and i couldn't detect any noticeable difference. then i found out why....i had the amp up a tiny bit more. It seems the effect of that treble bleed only makes a big difference at TV listening levels. Long long B4 it gets remotely close to stage volume it disappears. In fact, just a hair louder than where i first listened at TV volumes and the effect is all but gone. Any idea how i can change that? I first had two 47k's in series as the NFB resistor with the cap across one. But i later wanted more and put it across both. so max effect there. But what can be done to retain that effect as the volume is turned up?

When a stock Marshall amp using NFB and not having post-PI MV is 'turned up', NFB is not reduced. Your amp is no longer a stock Marshall.

[iknowjohnny]

Can you clarify that? I'm not sure what you mean, but it sounds like you're saying the master affects the NFB. I didn't think it would since it is adjusting the signal level that enters the PI and the NFB is a level that should be past that point and adjusted as well. Anyone ? Is there a way to keep the NFB level from varying like this?

[mdroberts1243]

To take another tack... if you 'turn up' presence that essentially lessens the NFB associated with high frequencies giving more gain and less linearity to the highs... maybe that was what you were thinking of in the original post?

Any master volume (e.g. Post Phase Inverter MV) that is 'inside' the feedback loop will affect the NFB too... except I think it works the opposite... as you turn up the PPIMV it increases the NFB towards 'normal' levels, at low volume levels you get very little NFB. (If I've got it straight myself).

[iknowjohnny]

Thats just it....the master is PRE PI, not post. So nfb i would think would be affected equally with master settings.

[PCollen]

Thats just it....the master is PRE PI, not post. So nfb i would think would be affected equally with master settings.

All other controls remaining constant, meaning you set them and leave them alone, with a PRE-PI MV turning it up will increase signal level into the PI AND on to the power tubes. The amount of NFB back to the PI will increase with increasing OT secondary signal amplitude, just as the amp is designed. The PI gets the right amount of NFB all the time. That is the proper realationship between PI input signal level and NFB level is always maintained.

With the POST-PI MV the PI gets the full signal (just as if the PRE_PI MV was MAXED). But if the Post _PI is set lower than max, the signal to the power tubes is reduced and so is the NFB to the PI and so the PI is not getting the right amount of NFB realtive to the PI input signal level.

[Gaz]

@ the original post: I think what you're referring to is the feedback loop 'breaking down' when there is a lot of gain. I can't explain this phenomena off the top of my head, but I pretty sure that what you were thinking of.

[iknowjohnny]

Are you saying there is too much nfb? I'm using the 8 ohm tap with a 100k resistor, and the rest of the loop is typical. Could it be the cap is allowing too much signal since the frequencies it basses are not attenuated by a resistor? If thats the case maybe i should use a 27k in series with the cap to bypass the 100k. But on the other hand, i can't imagine at the low levels this happens at that it could be that.

[PCollen]

@ the original post: I think what you're referring to is the feedback loop 'breaking down' when there is a lot of gain. I can't explain this phenomena off the top of my head, but I pretty sure that what you were thinking of.

I could understand this happening when the signal into the PI continues to increase and is not compressed/clipped, but the power tubes and resutlant OT secondary signal is clipping. In that condition, there may not be any increase in NFB, and the NFB itself would be clipped