Tube parametric eq-add cut and boost?

[diagrammatiks]

that original circuit looks great. you should build it and check it out. maybe I will.

The feedback/phase issue is a nonissue in a guitar amp since the tone stack will never be within a feedback loop anyway.

actually i just looked at it a bit more and it's easy as heck.

All it is 2 stages enclosed in a local feedback loop.

The unmarked control is a gain control between v1a and v2b.

If you cover up the 2 frequency dependent feedback loops with your hands it just becomes 2 gain stages cascaded.

not that complicated or difficult at all.

I might breadboard it tomorrow in the morning.

[Chuck H]

I want to come clean on this. I was only speaking dryly about my own preferences and didn't really mean any disrespect. There are just so many easy to implement and good sounding EQ circuits that I personally try to keep things simple. If I want a variable and narrow filter I'll just plug in a wha pedal. But I concede that my own design preferences aren't the subject at hand and digress.

A filter like the one discissed could be a fun addition. Certainly you want to target a different mid point for cleans and overdriven tones. And with so much cut/boost you could do really cool resonant effects. I'm reminded of the guitar tone at the end of "Once in a Lifetime" (Talking heads). I love that tone.

[diagrammatiks]

ya, the thing is is that that circuit above has limited application in a guitar amp...

it might make a good pedal but it looks like the eq just falls apart if the tubes are overdriven.

[Steve Conner]

My circuit? I cheated. I fixed the gain structure so that the input wouldn't see more than 20Vp-p or so with the previous stage overdriven right to a square wave.

The second tube will overdrive a little when the boost is turned up. I made sure it would clip before any of the op-amps do. But the circuit isn't intended to be overdriven hard, any more than the Mesa graphic EQ was. As Chuck has pointed out before, too much distortion after EQ tends to negate the EQ.

[J M Fahey]

You hit the nail in the head.
Tubes do wonderful things when overdriven, with no or *very* little NFB involved.
Active equalizers (which, by definition, are high and complex NFB stuff) , are not meant to be overdriven but to do "other things" and, beyond that, overdriving them is self defeating.
So they must be used "clean" for best effect.
Once you have that clear, SS (Op Amps or discrete) becomes the best option, because of flexibility, elegance, practicity and last but not least, cost.
In my view: let tubes overdrive when necessary, and pre and post process "clean" with SS stuff.
Use each technology where it shines.
(Ahem !!! ..... did I say Steve Conner ... ?)
jm2c.

[Chuck H]

Yup. Like an effects loop or graphic EQ. Make sure not to overdrive the circuit. Pad and re amp.

As to EQ being distorted. True. If you want the EQ to actually adjust the audible frequencies then more than just a little clipping after the EQ circuit can spoil it's performance. If you want the EQ to modify distortion character then post EQ clipping is great.

One reason you can't play modern metal on a vintage amp is that to get the needed gain the amp must be cranked. And this eliminates the EQ function and any ability to get the mid scoop wanker sound. Something I've done for kicks is to crank a vintage style amp into an attenuator, then take a line signal from there to an EQ with the mids cut and the bass and treble boosted (very high treble also cut) and then to another amp that is run clean. Instant wanker tone.

[Steve Conner]

I noticed the same thing. On the amp that has this EQ circuit, it'll do the mid scoop wanker tone very well. (that was why I built it after all ) But it only works at moderate volumes. Cranking the master volume up enough to distort the power stage, the tone falls apart into a farty mess. When playing really loud, setting the EQ for a bass cut and mid boost works better. It goes louder and cuts through the mix, but it doesn't have the modern metal sound any more.

I don't think this indicates a problem with my circuit, it is just the nature of things.