Ideas on how to impliment a single knob cut that is linear?

[12AX7]

I wasn't sure how to describe what i want in the title but heres what i mean. I have a NFB on/off switch on my 2204 type amp with a 6V6 output. Of course when i turn it off the amp sounds alive and great but is crazy bright and turning the tones way down isn't enough to counter it. Due to a preamp designed to cut lows in the cascaded stages, and with NFB engaged it's just fine as far as EQ balance. But i want to be able to use it w/o NFB too so i also added a on/off switchable cut control across the power tube grids ala vox etc to cut high end when using the amp w/o NFB. But the problem is that after trying many cap values from as low as .002 to .02uf i find that because the roll off happens at a given point in the freq range thats dependant on the cap value, the tone of the amp changes pretty radically. I also tried hooking it up between ground and various signal points after the preamp but the results were no better. I suppose what needs to happen is a roll off that is a flat line from the very top to very bottom as if you took a graphic and had the bottom slider at 0 dB and the top at some very low dB point and the line between then straight because otherwise it seems like there is a peak at the rollof point creating mid boosts or other depending on cap value.

So the question is, does anyone know of a way to accomplish this without  adding another tone stack or some other equally non simple solution? If i can't do it simply i'll just live with it, but i'm just hoping there might be a location i can hook it to are a simple method to add another component to make it more of a bright control that doesn't create peaks that lead to tonal peaks at some point.

[sluckey]

Experiment with different pot values and tapers.

[12AX7]

Thing is, i'm using a 1M pot and at the far end of resistance it's not doing much and at the other end too much. So the value is fine, and taper would only decide where in the travel a given sound happens. So it's not going to do anything to change the sound it gives, just where it happens and sensitivity of the knob, neither of which is an issue as is.

[sluckey]

You have no path to happiness.

[jjasilli]

12AX7, this is your 2nd recent thread on finely tweaking a narrow frequency band, which is an interesting topic.  To my mind this is called a Notch Filter, or sometimes a bandwidth filter. But I'm no expert.  A notch filter consists of an RC network or networks (though inductors could also be used).  The circuit could be a simple L, or a more complex T or H, etc.  It might be in series with or a shunt to the signal path.  Then you can use a simple SW or Pot to toggle or dial it in & out of operation. 


In a thread some time ago, HotBlue explored this topic in detail.  I think it was part of a thread by Tubenit on designing an acoustic guitar preamp. 

[12AX7]

12AX7, this is your 2nd recent thread on finely tweaking a narrow frequency band, which is an interesting topic. 

Not at all. Just the opposite in fact. I want to roll off the entire freq range with lower frequencies rolled off least and treble most so that if you did this with a graphic EQ it would look like a straight line from the lowest to the highest frequency slanted downward from lowest band to highest. Or to try and make it clearer, say you set the highest band at zero dB and set the lowest all the way down, then adjust all the other sliders to create a straight line between them. The pot would be laiek that at it;s furthest left point and flat at the furthest right. Thats just an example and it dosn't have to be that big a cut nor as perfectly tonaly linear from low to high as that, but i used it as an example of what i'm want to get close to rather than a notch as you thought i meant. 

[shooter]

I want to roll off the entire freq range with lower frequencies rolled off least and treble most

Take a look at attached schematic.  I did this because Highs were "bigger" in amplitude than lows.  I tweaked it some and got both low and high to equal.

[12AX7]

I want to roll off the entire freq range with lower frequencies rolled off least and treble most

Take a look at attached schematic.  I did this because Highs were "bigger" in amplitude than lows.  I tweaked it some and got both low and high to equal.

Not sure where i could put that In a 2204 type circuit or whether it could be adjustable. Looks like it just affects that tone control but i don't want to make it that complicated to add yet another tone circuit. I'm just looking for a way to make that cap and pot work a little less like a notch so i can use no NFB w/o the amp sounding ultra bright.

[Ed_Chambley]

I have had a lot of luck using a pot in-place of the slope resistor.  Also, snubbers from plate to cathode will tame highs.  For instance, most amps I have where the tonestack is very responsive, I have way more highs than I will ever use so having the ability to adjust them is not even necessary.  The lack of dampening created by the removal of the NFB does release some crazy overtones, but sometimes adding a pot in series to replace the resistor in the NFB will give you the ability to tweak out some of the objective high end hash and still keep what you are referring to as lively.  On off does not give any tweaking ability.  I use locking nut pots and keep them just tight enough so I can turn them with a flathead so if I am using different cabs I can dampen when needed.


Also increasing the cap value returning to the bass has given me a rounder sounding high when combined with an adjustable slope.


I cannot remember the thread, but member Tubeswell posted a schematic for a notch filter and adjustable slope.  Give the search a try.

[jjasilli]

12AX7, this is your 2nd recent thread on finely tweaking a narrow frequency band, which is an interesting topic. 

Not at all. Just the opposite in fact. I want to roll off the entire freq range with lower frequencies rolled off least and treble most so that if you did this with a graphic EQ it would look like a straight line from the lowest to the highest frequency slanted downward from lowest band to highest. Or to try and make it clearer, say you set the highest band at zero dB and set the lowest all the way down, then adjust all the other sliders to create a straight line between them. The pot would be laiek that at it;s furthest left point and flat at the furthest right. Thats just an example and it dosn't have to be that big a cut nor as perfectly tonaly linear from low to high as that, but i used it as an example of what i'm want to get close to rather than a notch as you thought i meant.

Ok, but maybe the notch filter idea will help in your other thread.

[12AX7]

Ya know, i think this idea isn't going to work and i'll tell you why. After trying many ways to do this I found that it seems like when you removeb the excess high end, the tone becomes no more lively that using NFB with the same degree of high attenuation via the tone stack. In other words, i think that excessive high end in the tone when NFB is removed IS the lively tone that you get w/o NFB. And when you tame that it;s like you came full circle and now it sounds about the same as with NFB enabled. And i think the tone is better with NFB. So this little experiment is over!

[PRR]

I realize you are moving on.

> a roll off that is a flat line from the very top to very bottom

That's actually a very tough thing to do.

Circuits are naturally flat or naturally 6dB/octave. 10dB in 10 octaves, 1dB/oct, is UN-natural.

It can be done. Look for "pink noise filter". This is a 3dB/oct slope. It is built on an allowable error and a cascade of staggered 6dB/oct slopes. It can give ~~1dB error over 9 octaves with about 4 or 5 R-C filters.

You would almost always be better off doing a few slope/shelfs in several stages.

And because it is un-natural, you gotta wonder if something else is wrong. Annoying speaker? Bad listening room? Shrill strings?

[SnickSound]

Have you tried simply reducing NFB instead of defeating it entirely?

On a 2204 I made for a friend, I put a 3 way On-Off-On switch, on one side is just a single 56k resistor from the 8 ohm tap, on the other side a 100k is added (total 156k) and in the middle the NFB loop is disabled.

The 156k is by far the most interesting sound, it's a lot more dynamic, but not as aggressive as total NFB defeat.

If that doesn't do it, how about this: a "reverse presence control"?

The presence control works by removing high frequencies from the NFB loop, so you could get the reverse effect by only leaving high frequencies in the NFB loop. For example, a small inline cap between the output transformer and the resistor would form a high-pass filter. A bigger cap is often used in this position to get a low-end boost from the NFB line, a big enough cap would have no impact at all, but a small enough cap would be almost the same as no NFB except for what little frequencies do go through.

For example, if I did the math correctly, assuming a 100k NFB resistor, a 220pF cap would have a corner frequency of 7.2KHz, meaning the negative feedback loop would only work on frequencies above that point. And you could always counteract it with the Presence control if need be.

One could even go crazy with a multi-pole HP filter setup, but that's getting out of my area of expertise :)

[12AX7]

yeah, I've tried all that. In fact i used to have a variable NFB pot all the time but i found myself either dialing it to the sweet spot or dialing it out altogether (no load pot) like i'm doing now with a switch. Haven't experimented a lot with inline caps but i've done a bit of it with nothing good coming from it. I just need something switchable that darkens the output section w/o any peakiness. I thought about a triode/pentode switch but looking into doing that with 6v6's revealed a post where someone said every time they have tried that with a 6v6 amp it hummed badly.

[HotBluePlates]

... I thought about a triode/pentode switch but looking into doing that with 6v6's revealed a post where someone said every time they have tried that with a 6v6 amp it hummed badly.

Then they had some other issue in the amp causing the hum.  Triode mode in itself doesn't cause hum, but may allow more hum voltage to be dropped across the load presented by the OT.

... I have a NFB on/off switch ... when i turn it off the amp sounds ... crazy bright ...

Unless you have something odd going on with your presence circuit, the "extra brightness" you describe sounds a lot like "distortion."  Welcome to pentodes/beam power tubes without negative feedback...

[12AX7]

Well, then i think i'll try it. It's only take a minute to wire it so what the heck. Then if it seems worthwhile i'll but it on a switch. I never liked T/P in most amps but in one (silver jube)  i actually preferred it and gigged it in triode mode all the time. Maybe this one will be the second, hopefully.

[12AX7]

Done, i like it ! Most amps seem to sound dull in triode but this one certainly doesn't. It really tames the top end but i can still dial in plenty. It has a mode balanced frequency range with better mids. Just not sure how it will fare in a band scenario, whether it will be loud enough. But i think i like it enough that i wanna put it on a switch. That is, if i can find room. May have to start rearranging jacks and pots on the back to make room.