Lets discuss amp tone and how it changes w/volume and why

[daz]

I was wrong about NFB and the single pot...it DOES give a fair amount. I had just forgotten to connect that side of the PI back to the grid 1 DOH ! Anyways, this is the first time i have ever been able to compare nfb vs no nfb because in the past i had never really compared them with the non nfb setup having enough tone control so it was always too bright to use. Now i have that cut implemented, and when i just now tried the single pot with nfb I realized the no nfb sound with cut as high end control sounds much much more complex. There is another benefit of sorts too...i found that if i leave the cut at full brightness and use the tone controls so tame it vs tones up higher and lots of cut, the 2 methods yield very differnt tones. One is that very marshally mid tone i mentioned earlier, and the other is more neutral. Makes it quite versitile because i can not only control the high content amount, but change the character. I think what i will likely do since i will otherwise have a unused hole in the front panel is keep the NFB pot and see if it adds to the tweakability by using much smaller amounts than i just tried.

But the original topic of the thread has been solved, and i will use the single pot method with the split load CF cathode as per chuck. I think i'm done with this, unless of course some of you tempt me with more good ideas. Maybe i should stop reading this thread

[Dave H]

...And I don't evem want to know what happens when you short grids and cathodes together.

I didn't intend to imply that the grids were shorted to the cathodes. Perhaps I should have said - "When the dual pot is CCW the two triodes of the LTP are connected together at the grids and at the cathodes". I can only speak one language and badly at that.

[Dave H]

If you dial down the NFB you also dial down the bootstrapping making the input impedance of the LTP PI lower so you will get more signal bleed through on the master vol at the CCW end but that isn't going to be a problem is it?

[daz]

If you dial down the NFB you also dial down the bootstrapping making the input impedance of the LTP PI lower so you will get more signal bleed through on the master vol at the CCW end but that isn't going to be a problem is it?

I dunno, but i just set it up with NFB pot and presence control and the single pot master and now NFB isn't working. So i checked the pots, tried different ones, even wires a 33k resistor directly from the 10k tail to the 16ohm tap and no NFB ! I can't for the life of me figure out whats going on. Had to get away from it for a while, as my brain is fried after this.

[daz]

Well this is just bizarre. I had the NFB working with the single pot and was getting a good amount of nfb with a 33k resistor and no pot. That was with everything just tacked in haphazardly. So i redid it all permanent and added a presence and nfb pot and i was getting no NFB. I couldn't figure out why so i removed the presence, then the pot. Eventually i just wired a 33k from the tap to the 10k tail and could not get any NFB at all. I gave up and came back to it later trying everything, but i could not get any nfb ! Strangest thing. I looked at the wiring over and over and over and checked things with a meter and i was positive nothing was wrong. Yet i couldn't get any NFB like i did when first i tacked in that single pot. I'm just baffled ! Anyways, screw it. Going back to the dual pot and cut. It works great and the pot turns volume all the way off at zero. And Chuck, i'm using your split load CF cathode idea and i really like it. It totally fixes the master's propensity to get loud quickly and become out of control up high like it's just far too much gain. i've always felt the master is as loud as the amp should be around 10:00 or thereabouts, so that told me theres just too much gain. This makes it much more usable and linear thru the range. I may even go with a 68k/33k to lower gain even more.

By the way, if anyone else knows of a good way to control high end without NFB in the PI/PA like the cut control but something different from that one, i'd love to hear about it. The cut control is ok, but i'd like to try something different.

[Dave H]

There should be NFB with the single pot. I'd check it again. There must be a wiring mistake. Make sure that the (0.1u?) capacitor is connected between the right side triode grid and the junction of the feedback and tail resistors.

[daz]

I've looked it over for hours. I give up. I have checked every pathway, every solder joint both visually and with a meter, and yes, that cap connection is there and solid. The cap and 10k tail are in turrets that are tied together with wire and soldered to the turret like a rock and the other side was soldered to the via wire coming from that turret, no possible mistaking it and the connection was solid.. Theres just nothing i can find. So i put the dual pot back. I may try again eventually, but right now i have spent way too long on something so simple i can't imagine in my wildest dreams what it could be. Theres NOTHING i haven't checked over and over so i'm done for now.

[soundguruman]

I do this all the time- listening to an amp.
But I do it automatically, and don't think about it in the process.

But after I read the post, I realized just how important listening really is...

It's like 50% of the job. And, I can see how crappy repairs would be without making final adjustment based on what I HEAR.

PS ALL my amp mods are done, based on listening, and playing a guitar.
i can't imagine a person, who does not play an electric guitar, modifying an amp. (although many obviously DO)
Amp mods would sound REALLY crappy, without a reference point. (and obviously, many mods done by non-musician techs DO sound crappy)
WHERE is your reference point? YOU don't have one.

There are many competent techs - who fail miserably - because they have no knowledge of music, or guitar playing.
That's a fact, Jack.

Now I would say this is a reference point:
"A Marshall 100 Superlead, EL34s(biased into a state of fission), late 1960s, plexi, thru 2X4X12 cabinets, Les Paul custom, turned up to 7-8..."
You see, there is a sound it makes, but also the way it "makes" you feel, when you hammer a powercord, when you sustain a note, when any slight touch of your finger is amplified (and re-amplified) 20,000 times...the guitar seems alive, and you control this life...this "horsepower."

How do you measure that on a scope?
Or any of the thousand X thousand possible variations?
But that "feeling" and that sound is exactly what musician wants. You are not measuring that with test equipment...

[Chuck H]

I agree. In fact one reason I started working on amps was that the only RELIABLE amp repair shops in my area were non guitar players and my amps always came back "working" but bland or with poor OD character. Later to find out that the scope/crossover notch bias method was responsible. Not a huge faux pas, but definitely a non guitar player maneuver. Still... The amp was always fixed on time, for a fair price and I could reach them during normal business hours. Try THAT with a shop that employs guitarists as techs!?!

I will say that "point of reference" really is just as subjective as tonal preferences. But your point is a good one. A "functioning" amp isn't the same thing as a properly playable amp to a guitar player. At least not always. And a tech familiar with the tone and feel of a good amp knows the difference.

[daz]

I don't think anyone including myself would suggest test gear isn't a good thing. But I also think It's not the more important one. Obviously a meter is essential, but beyond that your ears are the best piece of gear.

Back on the original subject, i went back to the pre pi master. I liked the fact that it retained the tone as i turned up. But there was something about the feel that wasn't happening and i felt it was a lack of those fluid mids that make it sustain and have a great feeling attack. but i wasn't quite sure. So i left the BS master in and added the pre pi one back and i was also able to compare everything i've been playing with....the masters, the cut control, and NFB which i could only get to work with the pre pi master. So i would turn the BS all the way up and play with the pre master and tried it with NFB and with cut only, tried the BS master with cut. Tried the pre master with cut and NFB with and without presence etc etc etc. you get the idea. What i found was that i never really realized that NFB was the biggest culprit in holding my tone back from what it could be because i was able to compare them instantly and use the cut in place of it when i had no NFB. NFB had a constricting choking effect, while using the cut tamed the highs but left the basic tone character of a non NFB sound intact and usable. I also found that with a cut i can play up louder even with the pre pi master and get rid of that harshness that started this while thing. It does include some tone stack twiddling too, but when i tried that with the BS master the tone changed radically. And no matter how it was set, the BS master missed that one thing. I think what i originally liked about it was the very complex harmonic contend. But i'm getting that now with the pre pi master by using cut and zero NFB. But the frequency curve is much better with the mids i was missing.

So you know how it goes.....maybe this will change too after a few days of playing it. But i feel the tone is more "right" now. Both scenarios had thier benefits and disadvantages, but I think at this point it sounds and feels best and at the same time rectifies the loud vs low volume issue to some degree. That i have to test more but it's certainly better than it was before i stated this quest. I can't be sure just yet, but at this point I really think NFB was what was causing it to sound harsh as i turned up and not the master itself, just the way it worked with NFB.

By the way, whats up with this site? It can't be me because my home and work PC's are slow here. It took me about 7-8 minutes to get the pages to load and be able to reply.

[daz]

Just thought i'd update this because I'm getting a much clearer picture now of what does what. I have both variable NFB and cut control in the amp now and i've been listening to each seperately and comparing and trying various amounts of each simultaneously. I should say i was wrong about NFB being bad, it was actually the presence that was causing a lot of the harshness when i turn up. I took that out and i now see the nfb itself is a good thing. It does have good and bad points, but it's now much more usable without the presence. I was forced to do w/o presence with the BS master because the NFB loop just doesn't work with it, and i believe thats why it sounded much better when turned up than the pre pi master.

The most notable thing i have found comparing the 2 is that while both nfb and cut do the same sort of thing, NFB seems to take the tone and sort of throw a blanket over it to smooth the entire range, while cut tend to leave a ton of bass and the lower ranges stay open sounding while the top gets the blanket. NFB seems to be more usable all around and cut seems to give the tone a more scooped feel depending on the cap. I need to keep experimenting with caps, and at this point i really have no idea which i prefer or if i will ever come to that conclusion. I may just have to keep both, tho i'm not one who likes amps with a million knobs. Spend too much time turning knobs. But anyways, i think the presence may have been the reason i had this issue with harshness and volume in the first place. I'm finding that for more top when i'm using nfb i'm better off just turning the treble up. I used to think it was nice having a presence so i could add different a range of treble than the treble control to fine tune the top, but I think it's just not a good thing.

[Chuck H]

But anyways, i think the presence may have been the reason i had this issue with harshness and volume in the first place. I'm finding that for more top when i'm using nfb i'm better off just turning the treble up. I used to think it was nice having a presence so i could add different a range of treble than the treble control to fine tune the top, but I think it's just not a good thing.

You might say it's just not a good thing for YOU or that particular amp as you have it or possibly just for playing at lower volumes. The presence control isn't just a treble boost. In fact a if you turn it up with an amp at full clip you even hear any additional high end, but you WILL hear more high end harmonics and perhaps a little more note definition. It's as much an effect as a tone control and so not applicable to every situation. I too find trying to use the presence on Marshalls at low volumes clumsy and usually unsatisfying, but I personally wouldn't want to be without one while overdriving. On a trivial note I've read that Angus leaves his presence at zero all the time.

[daz]

You might say it's just not a good thing for YOU or that particular amp as you have it or possibly just for playing at lower volumes. The presence control isn't just a treble boost. In fact a if you turn it up with an amp at full clip you even hear any additional high end, but you WILL hear more high end harmonics and perhaps a little more note definition. It's as much an effect as a tone control and so not applicable to every situation. I too find trying to use the presence on Marshalls at low volumes clumsy and usually unsatisfying, but I personally wouldn't want to be without one while overdriving. On a trivial note I've read that Angus leaves his presence at zero all the time.

No, i don't mean that the presence sound harsh at low volume, i mean i think it's a big part of if not the whole reason the amp sounded harsh as i get louder. I think that was at fault, not the MV. I liked using presence at low volumes. Without it i seem to be able to turn up loud and retain a much better balance of tone and that feel i like at low volumes as well if not better than with the bootstrap master.

I also added a switch to cut NFB altogether because i am finding even with a 33k R and 250k pot in series theres still enough NFB to hear it's effect. With NFB removed completely i find I can tweak the tone controls and cut and get a great sound thats more alive sounding than with NFB. One of my main tone tests that usually tells me i'm on the right track is to slide up to the A note on the B string 10th fret and let it sustain with my gain around 2:00. If it sustains till it breaks into harmonic feedback at the low levels i usually play at, I know I'm really on the right track. It does that right now with NFB off and a .0047uf cut cap with the pot cutting just enough to smooth it out for a perfect balance of highs without harshness. It will sustain indefinitely set like that at a volume just a bit louder than typical TV volume. That is the one test that always works to let me know things are in good balance. Using NFB tends to sound more constricted in the mids and not as alive sounding. When using cut only i also have a ton of lows, too much actually and i have to cut them a bit more than usual. I'm really diggin it right now in any case, and it's really tweakable as is. I may just leave both cut and NFB knobs in for max options.

[daz]

Just to update this, I must say that i am really glad i started this thread because not only has the original issue been solved, but in the process it caused me to make some other changes that in the past never sounded right and now have not only worked but solved other long time imperfections i have always been bothered by. And at this point the amp is truly the best it's ever been in the many years i've been obsessed with tweaking it.

First, the master issue as i said seemed to be a product of the presence circuit. With that removed and NFB still in place i could turn it up w/o the radical change as it got louder. Then due to that i began to see that i could now do without NFB altogether and just turn the tone controls down as i turn up. In the past that never worked because there was no point where i could control the harsh high mids without the tones being so low the sound lost all of it's aliveness and usefulness. Then at that point the changes led me to try a larger voltage divider between stages 2 and 3 and remove the 500pf treble peaker, something that never worked before because it would become too muddy and inarticulate.

Even with a 25k pot the presence affected all these things, and without it I was able to make changes that in the past i knew it needed but wouldn't work quite right. Now the amp retains it's sound better as i turn up, but the even bigger benefit is that The high end is now as i have always envisioned it. Everything just works so much more "right" now and the range of tones are more usable. So much so that i'm finding i don't seem to need either the cut control or variable NFB, both of which are hooked up to no load pots now so at thier extreme clockwise positions they are 100% out of circuit. Had i tried that in the past it would be painfully bright. I attribute the fact i can now run w/o any high end control in the PI/PA to the loss of that treble peaking cap, one of 2 i have in the pre. (3 if you could the treble bypass on the gain pot, but that depends on setting of course) I was surprised how much different that one cap made, and even more surprise i can now run w/o it and retain clarity in the lows. It's also no longer giving me a nasty staticy distortion in the very top of the high end when i turn the guitar down to clean up like it AND my MV marshalls have always had. I think because i lost that the peaking cap i can run the PB bright, whereas marshalls have a lot of NFB and therefore need a lot of treble in the pre, and the excessive peakers cause that nastiness in the very top. The whole key here seems to be losing the presence and maybe NFB altogether and tweaking the preamp circuit to compliment the PI/PA like that.

In short, the map is at a new level, one that i was always shooting for but due to a couple issues i could never solve are now gone. This is what i was after when i first began building these....to create a marshall like i've used for decades but without the several tonal details that always kept them from being perfect. We will see how i feel in time, but at this moment i feel like this amp is the perfect MV marshall. Surely better than i have ever had in this amp or any other to date. I am working on a old schematic i have that hasn't been updated for a long time and once updated to reflect the current state i will post it if anyone is interested in seeing it.

[Jazz P Bass]

Thanks for the update Daz.

I for one, am in awe at how you stuck at it.

What a cool Christmas present.

An amp that you can like & enjoy.