Tone stack not very responsive

[Rob's Radio-Active]

Hi, all,
I have two Fenderish style tone stacks in this new amp build. The clean part of the preamp - V1a and b - has a tone stack that is not very responsive. This is the left stack on the schematic. The overdrive section has much better range of control but with bass and treble set midway, also sounds more trebley than the clean section of the pre-amp does.
Is this an impedance mismatch issue to the stacks, perhaps? Thanks!
Rob

[Leo_Gnardo]

Hi, all,
I have two Fenderish style tone stacks in this new amp build. The clean part of the preamp - V1a and b - has a tone stack that is not very responsive.

It's that 100K "tail" resistor, with a value that large you have mostly bypassed the tone stack. In a Fender circuit (the well known Fender tone, "blackface" or "silver face"), that's a 6K8 resistor. Those with mid controls, 10K pot. Sometimes a customer wants expanded midrange, I'll put in a 25K pot but have to warn that the treble & bass will seem to have little effect with the mids cranked all the way up.

Swap in a lower value. Experiment with resistors from 1K to 22K, pick one that makes your ears happy. Same with the overdrive channel stack.

Are you using linear or audio taper pots for your lead channel treble control. Generally best to use audio taper. Even with audio taper, the treble really starts to take off past 3 and definitely dominates by the time you're dialled up to 5. Hi frequencies may be even more pronounced with all the overtones you create by distorting the signal.

[Rob's Radio-Active]

Thanks, Leo, that is very helpful. I am traveling at the moment so cannot access the amp, but I will post back when I get to experiment.
Rob

[eschertron]

Hi, all,
I have two Fenderish style tone stacks in this new amp build. The clean part of the preamp - V1a and b - has a tone stack that is not very responsive. This is the left stack on the schematic. The overdrive section has much better range of control but with bass and treble set midway, also sounds more trebley than the clean section of the pre-amp does.
Is this an impedance mismatch issue to the stacks, perhaps? Thanks!
Rob

[ATTACH=CONFIG]40056[/ATTACH]

Hi! I love what you've done with the place

Agree with Leo's assessment of the 'mid' resistor on the first tone stack. I did a build using a 50k pot in that location, it's more of a 'drive' control than a tone control once you get much above 10k or so.

I'm really interested in the PI. Are you looking to maximize drive current to the 6L6s? Why not use a 12AT7? It's different enough from anything I've seen before to wonder what the inspiration is. Can you share? Thanks!

[Enzo]

Agree^^^

That 100k resistor is where the mid control is usually. You don't have a mid control. But one way to experiment with other values is to mount a small "trimmer" pot wired as a variable resistor, in place of the existing 100k. A sort of internal mids control, if you will. Then you can dial in any value you like to find where you like it best. Then you can remove the trimmer, measure the setting, and replace the trimmer with a resistor of similar value. Or, just leave the trimmer in there, it can sit untouched or you might get a hankering to twiddle it in the future.

[Rob's Radio-Active]

Thanks for the ideas, guys!

eschertron: The PI is just a typical cathodyne splitter with a leftover triode used in parallel, so the resistance values halved.

I figured that since I wanted to keep all the tube types the same, I would use the "leftover" triode section to delay grid clipping a little bit on the 6L6 pair.
That way (by sticking with a 12AX7A) I could rewire the PI for an extra pre-amp gain stage if it turned out that I needed it. Since I didn't need the extra gain, I could put in 12AY7 or 12AT7 and lower the resistance values further for more drive current, but it is nice to have all the same tube types sometimes, so I am undecided as yet on that.

Rob

[Rob's Radio-Active]

Well, I changed the tail resistor to a 15K and it seems to help, but I am still not satisfied with the tone stacks yet. I'm going to do a little experimenting with different component values and see if I can get there.
Rob

[J M Fahey]

Search and download Duncan TSC ; it allows to simulate different part values with precision.

Try "Ampeg"/James tone control, way stronger than Fender/Marshall/Vox ones
No need to mess with it, itīs so powerful and versatile that it covers many areas with ease.

[Rob's Radio-Active]

Thanks, Juan,
I used the Duncan TSC sim to come up with these current stacks. Using the Baxandall James/Marshall crossed my mind at the beginning of the build but was forgotten.
I think I will take your advice and try it.
Rob

[Rob's Radio-Active]

Follow up: I made some changes and got the tone and tone controls I want. I did not end up going the Baxandall route, but may do that on my next build. It was just that I didn't want to rewire a lot (and order new pots) as long as a little modification would do it. Changed around some capacitors and added three...two in the stacks and one bypass before the OD stack to cut the highs a bit. It was too tinny on OD.

The only other thing was having a friend play through it (cause I suck), and he said (and I agreed) that it needed a bit more clean headroom on the clean channel. I re-biased the pre-amp before the master volume control and limited the clean output to the PI a little with a 330K in series with the master volume pot.

It sounds great now, and I think I have a buyer!
Thanks again for all your help!
Rob