Dual gain pot that changes cap values for better clean?

[12AX7]

Something i've thought about for a long time but never seen it implemented. Has anyone either done this or seen it in a schematic? I'm speaking of something along the lines of a gain pot that would be a dual ganged pot so that when you turn the amp's gain down one of the pots would gradually change a cap value like for example a cathode cap on the first or second gain stage in a cascaded pre. It might for example gradually add a paralleled cap to the permanent one to bring the value up as you turn down to get cleaner. Tho i imagine if you tried this a custom ganged pot would be in order, not one with the same value/taper on both. We use very bright design details in preamp gain master volume amps to tame what would otherwise turn to mud with much gain. Then a clean amp always has big cathode caps like a fender does because for clean they give a full sound but bass goes to mud as gain goes up. It would be nice to be able to give one channel the clean treatment as the gain is lowered and visa versa.

To date i have yet to see a totally killer preamp distortion circuit that sounds equally as good clean or even close. So i would think someone would have tried this before to at least give the lowered gain positions a better clean tone if not as good as the  higher gain sound. But i've never seen it done, at least never noticed it in any schematic i've seen. And my example was just that, and i'm not necessarily talking about this as tho it must be done to a cathode cap. Obviously couplers are out tho. But maybe there are other ways people have found to do this? I'm very happy with my homebrew's distortion, but i wish at about 1/2 way down on the gain know it could be fuller sounding. Seems like someone must have figured out some way even if not a ganged pot, some other idea i haven't thought of. Anyone seen or themselves figured out a design to address this?

[HotBluePlates]

Look at the schematic of Jojokeo's Tiny Giant.

The Gain pot doesn't change the cap value, it changes the degree by which the cathode resistor is bypassed by the cap. No bypass has less gain than fully bypassed, and the pot allows any setting between the 2 extremes.

Now, that pot might typically be more like 5-10kΩ when across a 1kΩ cathode resistor, but whatever gives the range of adjustment desired is a good value.

Switches that add/remove a bypass cap are in a lot of amps over the last 2 decades, usually lableled "Fat" or "Gain" or some such.

[plexi50]

I built a few amps in my early days that used a 6 way rotary switch for a total of 5 additional cathode resistor tone (gain) settings. In the end there are too many choices and IMO best just to find a value that makes you happy. Too many bells and whistles can make you unhappy after the intial thrill wears off. But is is fun to experiment just the same

[loogie]

Sure you could.  I think there are all sorts of possibilities.  I've played around with a number of things and I've often wondered why we don't see it more.  Simple things.  Right now I'm using a linear pot and a resistor from wiper to ground to give it an audio taper.  Its really just a loaded voltage divider.  Normally we just figure the load isn't great enough to take into our calculations, which it generally isn't, but a 1M post loaded with, say, 220k is noticable.  Its total resistance goes down as its turned up.  So the load on the driving stage increases and the 3db point goes up with volume -- at least on paper, and as it turns out its audible. (or maybe I'm nutz.  Very possible)  This could be combined with some other bass attenuation scheme like bypassing the cathode cap or the coupling cap.

I think you have a good idea.

[12AX7]

The cathode pot is similar to what  was saying except i was suggesting it be done with a ganged pot thats ganged to the gain pot. And i was also suggesting ADDING capacitance bu having the pot add in a paralleled resistor. That pot gradually removing the current cap would help, but not much. It would need to add a lot of lows rather than remove highs like that because i've lifted the .68uf cathode caps many times and true it reduces highs which in effect give more mows in the balance. But not nearly like i mean. Adding a 22uf  on v1 for example does just barely enough. Doing both the 1st and second stage would maybe be closer, but i'd need a triple ganged pot, something that afaik doesn't exist.

But the biggest issue i think would be getting the 1m gain pot ganged to a 2nd pot who's value and taper would be a huge task to get right. it would need to be a custom affair. Thing is, there are many ways i could do it with switches or whatever, but my goal was to make the gain pot do it.

[silverfox]

Digital. Use a gain pot to latch a particular transistor on at the desired level. That transistor would switch in the desired capacitor. The circuit could use those bar graph drivers as your switching point.

Is it possible to switch a single capacitor on and off to simulate smaller capacitance values? I essence creating an electronic capacitor. As it is switched off there would have to be a taper to the charging.

Since I'm not an electrical engineer I can only hypothesize the use of a FET and an OP Amp to create an electronic capacitor.

Perhaps this is how modelling amp designs got started.

Fox.

[VMS]

KOC uses 1Mlog bass pot on his Soma84 amp in TUT5. It has two coupling in series, one big and one small, and the pot is wired to bypass the other cap.
Maybe something like that could work.

[Bluemeany]

Although not an amp design the good old Klon Centaur works like this - as gain goes up a dual gang pot deals with increasing gain and simultaneously lowering bass/lower MIDs. In fact there are a few fx pedals that work like this. If you place the gain control of an opamp booster not in the feedback loop but on the feedback path to ground it'll have this effect (lower bass as gain increase). Admittedly you can't implement that technique exactly but it shows some proof that this technique works and sounds good.

Why not have an amp where you have 2 channels - one set for clean one for drive and as you go up in gain you actually blend between the two channels?

[HotBluePlates]

The cathode pot is similar to what  was saying except i was suggesting it be done with a ganged pot thats ganged to the gain pot.

Try it. The only way to see how it behaves is to do it.

[12AX7]

The cathode pot is similar to what  was saying except i was suggesting it be done with a ganged pot thats ganged to the gain pot.

Try it. The only way to see how it behaves is to do it.

I might if i can find a 1m ganged locally. 

[loogie]

Doug has them.

Doesn't the coupling cap give you more control over bass?  The bypass cap gives a shelving response, right? -- although reducing it introduces more negative feedback which might clean up the remaining bass?  In either case I'm sure you could make something work from the cathode cap if you like.  I would try both coupling cap and bypass cap -- or maybe you can get a triple pot?  Nice.