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

[Chuck H]

Can someone post a schem?!? I did find someone elses claimed circuit and it looked for shiza.

Pete. Basically it's a 1M pot replacing the 1M grid load on the PI input side. Wiper feeds the PI input grid. For reasons I don't understand the TS ground is moved to that node (in the PI circuit where the bias resistor, grid loads and tail resistor join).

I'm not sure what you have wired up daz, but it sounds like you're using a 250k load on one side of the PI and a 1M load on the other right now. Also, did you move the TS ground lead to the above mentioned node? What is the other pot used for in the TUT circuit???

Perhaps the way you have it wired is doing some of what you want, but it's not correct and I wonder how much detriment there is to the amps actual function as a "whole amp" with the partial circuit your using now!?! Or if you'll still get the tone you're after with the circuit wired correctly.

The version I saw didn't look to have any noteworthy advantage over the typical MV. It still adds significant series resistance after the TS and it still tanks the PI input impedance. Unless something about the actual TUT circuit surprises me I have to say that I remain skeptical. But hopeful for daz's sake.

[daz]

Don't be Chuck. (skeptical) I just got done doing the proper mod as opposed to quickly and haphazardly tacking in a 250k in the input side only. This time i used a 500k dual pot and took the cap inputs on each side of the PI to the outer lugs of the pots, the centers going to the grids, and the other side of the pots tied together and going to the 470k/10k tail junction. I forgot to put grid stoppers on but i will. Treble center straight to the PI input cap and a 1M also from the treble center to ground.

Holy $hit Chuck, I know you can't see why it would work but trust me, the amp came alive. Big time. The low volume tone i loved before is even better. It's orgasmic. Raise it up and it does get brighter but the feel and tone are the same. The amp sounds phenomenal. I always felt that master was killing the top end, and i don't mean the AMOUNT, but the quality. But the amount is higher too, and that makes sense because i have always had to tweak the pre very bright to the point it made no sense that the amp would not be piercing. Now it IS. But i can EQ it out and it's just sweet as hell. So now i think i must redo some of the treble peaking stuff in the pre, and i believe when i do it will be incredible. Only thing is, the NFB is almost nil. I tried strapping a 470k across the pot that is on the input side hoping the other side will allow a larger ratio of nfb but no luck. Doing the one sided design like i first tried a few hous ago allowed more nfb but still a small amount. At this point my thought is that i either need to find a way to make the nfb work right or darken the pre as i suggested. I think the latter is probably the way to go.

I have some work cut out, but i feel like i made a huge leap forward and that this mod has opened up the gate to the degree of tone that i was beginning to feel this and maybe few if any amps are capable of. try it chuck. Maybe your ampsa are good enough that it won't make much diff. But the one you have with the pre pi master has got to improve hugely i think.

Can someone post a schem?!? I did find someone elses claimed circuit and it looked for shiza.

Pete. Basically it's a 1M pot replacing the 1M grid load on the PI input side. Wiper feeds the PI input grid. For reasons I don't understand the TS ground is moved to that node (in the PI circuit where the bias resistor, grid loads and tail resistor join).

I'm not sure what you have wired up daz, but it sounds like you're using a 250k load on one side of the PI and a 1M load on the other right now. Also, did you move the TS ground lead to the above mentioned node? What is the other pot used for in the TUT circuit???

Perhaps the way you have it wired is doing some of what you want, but it's not correct and I wonder how much detriment there is to the amps actual function as a "whole amp" with the partial circuit your using now!?! Or if you'll still get the tone you're after with the circuit wired correctly.

The version I saw didn't look to have any noteworthy advantage over the typical MV. It still adds significant series resistance after the TS and it still tanks the PI input impedance. Unless something about the actual TUT circuit surprises me I have to say that I remain skeptical. But hopeful for daz's sake.

[Chuck H]

Sweet! A little searching shows that a lot of people deal with the same MV troubles. The significant difference is that most players want more of the high MV setting tone at lower MV settings. Which is opposite of your issue. Either way, if it's a more consistent MV with less impedance flip flopping then it's better. I'd still like to see the TUT schem to get an imprint on it. But I suppose that's why the books are for sale

[g1]

Schematic or even a link to something similar would sure be appreciated.

[daz]

Sweet! A little searching shows that a lot of people deal with the same MV troubles. The significant difference is that most players want more of the high MV setting tone at lower MV settings. Which is opposite of your issue. Either way, if it's a more consistent MV with less impedance flip flopping then it's better. I'd still like to see the TUT schem to get an imprint on it. But I suppose that's why the books are for sale

Naaa, schematics for it are all over. heres one. I tried the 10k grid stoppers a hour ago and took them out. The ringing stopped after i put the dual pot in so no need for them and i found with them the tone suffered a bit so why bother. So i did it just like below tho w/o the stoppers and w/o that thing on the tail, which i guess is some sort of high cut? Anyways, like i said the NFB now doesn't work so i unhooked the 250k pot and added a .022uf and wired it up as a cut control. I hate those, and again it was a waste. Seems the mush out the tone if you go too far, and if you don't go far enough they don't cut highs and there seems to be no in between. (tried many cap values too from 500pf to .1uf) I tried snubbers and they help but kinda blanket things too much. So it will be another round of tweaking to find what compliments this whole new and very different master circuit.

Anyways, theres your schematic...



By the way Chuck, the reason most have the opposite issue is because it's really the same issue in that the master sounds very different in high vs low volume, but mine was designed and tweaked to sound good at low levels. If the amp is tweaked to sound good loud like marshalls, then the opposite happens.

[Chuck H]

I don't see anything about that circuit that should detriment the function of the NFB loop. I trust you didn't include the 100k 0V reference resistors after the PI? That circuit hanging off the tail WOULD be a presence circuit IF there were a NFB reference. As it is it wouldn't do much.

If the circuit is disabling your NFB I think something may be incorrect.

[teemuk]

I don't see anything about that circuit that should detriment the function of the NFB loop.

I do. The BS master volume has control on PI's overall gain and therefore open loop gain of the entire amp. This is what also detriments the function of the feedback loop.

Since the control directly effects the amount of NFB it ensures the frequency response and damping characteristics of the amp vary across the entire dial of the master volume. If daz had a problem where master volume past certain settings affected amp's tone too much he now has a master volume that ensures amp's done will be different on EVERY setting of the dial.

Basically, at low settings of the dial the open loop gain is so low that there's is very little difference to closed loop gain. However, this difference is the range on which tone controls inserted to feedback loop can work on. At low volumes the low open loop gain hinders feedback-based tone controls - as well as the negative feedback - ineffective. When feedback is decreased the amp's response changes drastically enhancing gain at frequencies where speaker's impedance rises: upper mids and highs, as well as the resonant frequency. Response at low volume settings is totally unflat. Hindering feedback also increases distortion (or basically the loop corrects less of it)

At higher settings of the dial the open loop gain increases closer to its usual magnitude. Difference between open and closed loop gain becomes larger and feedback-based tone controls have more range to work on and turn functional again. Higher amount of NFB corrects distortion and improves damping: The amplifier has much flatter response and doesn't change its gain as drastically at frequencies where speaker's impedance increases.

The BS master volume, similarly to post PI setups troubled by the very same issue of interactive negative feedback ratio, blends between the extremes.

Another effect comes from the potentiometer itself, similarly to usual master volume setup. At low volume settings a considerable series resistance is introduced by the potentiometer. This series resistance is practically adding to the grid resistance. 1 Meg logarithmic pot dialled halfway means there will be about 900K or 700K grid resistance (depending on taper), which introduces a RC circuit with grid capacitance amplified by miller effect. For many, this is a considerable treble rolloff. Again this characteristic varies across the volume dial, being decreased towards higher volume settings as series resistance at grid decreaees. Decreasing resistive value of the potentiometer can reduce the effect but simultaneously it goes on more or less defeating the purpose of the bootstrap in PI in the first place.

I'll be damned....it retains the tone when turned loud !

No it doesn't. The tone actually changes gradually at every setting and differences between loud and quiet are huge. It's your ears failing as a proper instrument to measure what's happening in the amplifier. This is one of the good reasons why proper analysation actually requires either theory to understand fully what happens in the amplifier or, the swear word of this thread, measurement gear.

[teemuk]

For reasons I don't understand the TS ground is moved to that node (in the PI circuit where the bias resistor, grid loads and tail resistor join).

Ah, the extremely silly "bootstrapped tone stack" circuit. It's a nice example how someone can draw all kinds of strange circuits that look cunning on paper but in practice turn out to be total flops. Wiring a tonestack that way ensures so limited range of control that the circuit is practically useless in comparison to standard tonestack with usual ground reference.

[daz]

No it doesn't.

Good grief ! If i said the world was round you'd say it's square ! Look, NO amp in the world sounds EXACTLY the same when turned up. Read between the lines. If you take everything 110% literally then you'll just keep butting heads with people. Does it sound the same when turned up? No, not exactly. More than with the pre pi master? Well, lets just say it's not even remotely close. If 10 is exact and 0 is as far away as possible from the low vol tone, it went from 1 to 9. Maybe your precious instruments will say different, but if they do they're making a case against you because in the end it's what you hear that is ALL that matters. It is the end all this is a means to.

[daz]

[QUOTE=Chuck H;326970] I trust you didn't include the 100k 0V reference resistors after the PI? That circuit hanging off the tail WOULD be a presence circuit IF there were a NFB reference.

No, in fact till you mentioned it i didn't even notice that ! I don't understand it but if you think they are necessary i'll try it. I never even heard it mentioned in any of the threads i read about it in a google search. Not sure any of the other schematics show that either.

[frus]

Ah, the extremely silly "bootstrapped tone stack" circuit. It's a nice example how someone can draw all kinds of strange circuits that look cunning on paper but in practice turn out to be total flops. Wiring a tonestack that way ensures so limited range of control that the circuit is practically useless in comparison to standard tonestack with usual ground reference.

+1 (I tried it )

[teemuk]

Look, NO amp in the world sounds EXACTLY the same when turned up.

Yet you say it retains the tone. Either it does or doesn't.

If you keep on claiming there are attributes in tone or operation of the amp that do not really exist there in the first place then how can we even trust you on anything you say?

Maybe your precious instruments will say different, but if they do they're making a case against you because in the end it's what you hear that is ALL that matters.

Look daz, these circuits follow strict laws of physics. I practically explained every thing you heard an experienced after making that mod (loss of feedback, feedback-based tone controls seizing to work, variable damping, increase in distortion, spurs of oscillation, frequency response differences, etc.) by theory of how such modification effects the amp's operation. The effects you heard are all closely tied to tampering with amp's open loop gain and making it variable across the potentiomter dial. This is well-established theory.

Yet you keep on making claims that the amp sounds the same. ...Or did it even? That doesn't put my ears to question, it puts entire theory of electronics to question and at that point I will rather question how accurately you can hear and actually identify certain effects happening in the amp.

[Chuck H]

I figured it to be something like that. Or perhaps some local NFB rather than 0V reference.

[Chuck H]

I do. The BS master volume has control on PI's overall gain and therefore open loop gain of the entire amp. This is what also detriments the function of the feedback loop.

Gotcha... I was viewing it as "Wouldn't putting less drive signal into the PI do about the same thing!?! What's the real difference between changing the amount of signal the PI is amplifying vs. the amount the PI can amplify it WT the NFB loop." But Now I see where the loop gain gets lost.

P.S. You do know that was just good natured ribbing above, right? Pretty funny too I thought!

[daz]

I'm not going to argue with you because you have no ability to understand the simplest things unless they are represented on a screen on a scope. Example: I tell you it's the same and you have to argue that even after i tell you ok, it's 99% the same. What do you want? Sheer perfection beyond question? That doesn't happen in real life. But you just can't read between the lines, you take everything 100% literally. How you get on socially would have to be a hell of an interesting case study. You can only argue because thats what you do. I've seen your posts elsewhere many times and all you do is troll for an argument so i'm not going to feed you anymore. I'll just leave it at this....it sounds close enough that i've never owned another amp that retained it's tone better than this and many not as good, vs how it was with the pre pi which was unusable at anything other than bedroom volume. I'm done responding to you...