treble bleed with resistor

[daz]

I tired this long ago and forgot about it, but i just tried it on my current amp. It's odd....people use the resistor in guitars with treble bleeds to bring back some of the lows and keep the cap from thinning out the tone. But it does something very different when you use this trick on the gain knob on a cascaded amp if it has a bleed cap. I know it should just change the overall series resistance and just add or subtract the amount of effect of the cap, but it seems to do more than that. It seems to accentuate the mids and give it that very fluid feeling midrange response that never happens without it no matter where the gain knob is set. I tried a 220k, a 470k and a 1M. Lower values accentuate it more. I'm not sure whether to leave it, as i like it a lot but it's very different than that more natural crunchy more full range sound. This tone is more midrange as i said and very focused and has that very light fluid feel with gain. I wanna make it switchable but i already have switchable 250/1000pf bleed caps and i'll have to get another pull pot to do that. Between the 2 caps and the option of the resistor or no resistor, you can really dial in the tone and feel.

I wonder why i never see this in amps? At least i don't recall seeing any amps with it. I think people who don't care about rhythm tones and just want a fluid focused lead tone would like this a lot. It's still good for rhythm but i think w/o it might be better in that regard. Anyone else tried this?

[pdf64]

Putting a variable resistor in series with a bright cap is similar to a simple tone control, eg http://bmamps.com/Schematics/fender/..._Schematic.pdf
Having some sort of tone control in the early stages of an overdrive pre-amp seems a good idea, eg to help achieve a suitable distortion character with different guitars.
As it's pretty much what the regular tone controls do in an overdriven non-master volume amp
I seem to remember various amps with an 'edge' switch, or some other control to do this sort of thing in a cascading gain overdrive channel.
I suppose that the Boogie Mk series 'lead' mode is an example of taking this idea to a logical extreme, ie putting a whole tone stack after the input stage http://bmamps.com/Schematics/Mesa_Bo...Schematics.pdf

As you seem to be a Marshall guy, if you've not got your head around it before, it may seem a bit weird, eg to facilitate bass being boosted at the beginning which would act to make blocking distortion harder to keep under control. But the flipside of that is it can put control of the degree of 'fart' in the hands of the user, rather than the designer.

[Dave H]

It's odd....people use the resistor in guitars with treble bleeds to bring back some of the lows and keep the cap from thinning out the tone. But it does something very different when you use this trick on the gain knob on a cascaded amp if it has a bleed cap.

Is the resistor in series with the cap or in parallel? You sometimes see a 150k in parallel with the cap on guitar volume controls. It works differently in a guitar. When the vol is fully cw the inductance of the pickup makes a resonant peak with the capacitance of the cable to the amp. When the vol is turned down the added series resistance dampens the resonant peak and makes the guitar sound dull. The capacitor and 150k parallel resistor is an attempt to compensate for this and make the guitar as bright sounding with the vol on '5' as it is on '10'. I think that's how it's supposed to work anyway. In an amp the bright cap makes the amp sound brighter when the vol is turned down (as you'd expect). The resistor will reduce the brightness.

[J M Fahey]

Amplifiers are active and you have many ways to equalize them, so in general you just add a bright cap and leave it as is, if you have way too much highs, not enough mids or whatever you can compensate somewhere else, specially by parts choice in the tone stack; you do no have that luxury in a (passive) guitar so that lonely resistor is the crude way to achive *something*.

* Resistor in series with cap: it atenuates excess brightness.
* resistor in parallel wih cap: increases perceived mids and makes volume "jump" faster.

Not the same.

Which is "best"?
Let your ears decide.

[daz]

Just to clarify, what i'm saying is the resistor is in parallel with the bleed cap. As to lows at the beginning of the gain stages, i agree 110%. Not the place to allow lows in a cascaded preamp, and it's why my amps always have a lot of things to eliminate it like .68uf/1.5k cathodes on the first 2 stages and lots . But thats why i'm saying it's odd....it DOESN'T have the effect of more bass, at least thats not the way its perceived. It sounds like mids and the bass sounds like it's no different than w/o the resistor. In a guitar it sounds like low end is added back, but it;s working very different for my amp's gain stages.

I have at times tried a R/C filter like marshalls use with the typical 470k resistor but instead of the usual 250 to 1000pf cap i'd use a .0047uf and it gives you a perceived mid boost that has that same sort of fluid mids i'm talking about here. But somehow putting that resistor in parallel with the cap on the gain pot which in effect should be the same thing as those filters but just variable, but instead it makes the 250 to 1000pf caps sound like a R/C filter with a.0047uf. Thats what i don't get. I have to assume it has to do with the fact a filter NOT on a pot isn't varying both the series resistance and the load at the same time? You know me, i know pretty much nothing about theory but i'm trying to understand why this acts so different. I would have expected it to do nothing more then change the ratio of whatever highs the cap passes too the rest of the frequency range depending on where the pot is said. Instead it seems to change the frequencies the cap passes. I understand thats not the case, but thats what the end result sounds like, and thats why i said this sounds odd.

By the way, i don't see it in that boogie schematics, tho maybe i missed it.

[Dave H]

The plots below show the effect of adding a 220k resistor in parallel with a 1n bright cap on a 1M pot. The pot is set at 100k from the ccw end (mid rotation on a 10% log pot)

no resistor


with resistor

[Old Tele man]

GOOGLE "shelving network".

[daz]

Like i said, shouldn't act like this and those seem to agree. But this is one of those cases where theory just doesn't follow the ear.

The plots below show the effect of adding a 220k resistor in parallel with a 1n bright cap on a 1M pot. The pot is set at 100k from the ccw end (mid rotation on a 10% log pot)

no resistor
[ATTACH=CONFIG]41137[/ATTACH]

with resistor
[ATTACH=CONFIG]41138[/ATTACH]

[daz]

GOOGLE "shelving network".

I see you didn't try it, did you? Give it a shot, you may be surprised.

[J M Fahey]

But thats why i'm saying it's odd....it DOESN'T have the effect of more bass, at least thats not the way its perceived. It sounds like mids and the bass sounds like it's no different than w/o the resistor

Donīt know what are you hearing, but curves agree 100% with whatīs predicted: almost no difference on highs, say 1kHz and above (fractions of andB), HUGE difference in lows (+10dB @ 100Hd) and evenly proportional at intermediate frequencies.
So theory and measurements match.

IF bass is severly cut, as usual in guitar amps, plus speakers are bass shy, deep bass increase wonīt look like much, now mids are always present in a guitar signal, thatīs why I talked of

increases perceived mids

[daz]

Ok, i suppose i do hear increased lows, but it doesn't sound drastic and really it's not near as noticeable as the apparent mids for whatever reason. This is why i said it's one of those times where the technical theory doesn't seem to match what i notice. Also you gotta consider the fact this is overdriven tone which acts very different than clean given the same EQ treatment.It compresses the content so unike a clean tone where this may have accentuated the lows big time, this more or less evens it all out. Maybe it's a case of the way mids are affected by overdrive vs highs and lows. If tech theory and charts like that gave a accurate portrayal of what you're gonna hear, you could build a killer amp without ever listening t it till it's done. But it only gives you a general idea of whats going to happen, and with overdrive i think there is a lot of black magic that charts can't predict accurately as compared to the result you hear.