what is this doing?

[daz]

I put a 100k R across the outer lugs of my 250k treble pot. It's a marshall style CF/tone stack but with a 33k slope and 270pf treble cap. rest is typical marshall. I put the R across it because i felt it might simulate a pot about 1/2 the value. But looking at it i'm confused because i knows it's not that simple and there are other things going on. Can anyone explain exactly whats happening? The tone improved immensely, and in fact that hi-mid harshness i've been complaining about forever seems gone or at least darn close. And the bass has become a punch in the chest as tho the resonance cap i have in the NFB has had it's impact doubled. I can even turn the treble and presence up quite a ways now. So i want to understand what exactly is happening with that R there in hopes of realizing exactly what was causing the issue in the first place. Rarely have i changed anything that has caused this big a change and i'm very intrigued to say the least.

[tubeswell]

The 100R from the input lug to the ground lug reduces the overall resistance of the pot (resistors in parallel), but the pair of resistors will still have no resistance when the pot is 'cut'. So I'd say you are decreasing the range of the tone control.

For other variations of pot and resistor combinations see the attachment

[daz]

It's doing a lot more than that. The tone is totally different now. But when you say from the input to ground lugs, realize that this is the treble control and none of it's lugs are grounded. So it's doing more than just decreasing the range. and like i said, the lows have changed dramatically, not in how much or little there is, but the way it feels/sounds.
Oh, and i click on that attachment but nothing happens.

[tubeswell]

Sorry mate - the attachment's a .pdf - should light up with acrobat reader. If not, try changing it's file properties to acrobat reader, or typing '.pdf' at the tail end of the file name. (I've just realised that my Mac might not be posting the file right. I'll try again).

So the ground lug of that tone control is not connected aye? Still it would seem to be putting the two resisors in parallel, except the wiper is only coming off one of them, so when the wiper is cut, both resistors are shorted, but when the wiper is fully wound out, you still theoretically have two resistors in parallel, which reduces the overall pot resistance

[daz]

No, it's connected to the treble cap at one end, middle goes out to the PI, and the other side to the bass pot and cap. thats why i said theres too much going on for me at least to figure. i know it's not just changing the pot value because i tried a 100k pot before and it didn't do anything like this. It's amazing tho....the more i play it the more i realize this one thing transformed the amp into a very close representation of what i've been trying to achieve after uncountable other tweaks.

[stokes]

Since the resistor is across the entire pot,it is changing the value of the pot and altering the frequency response of the pot.I am sure there is more to it than that,but that is surely part of it.

[daz]

and altering the frequency response of the pot

That makes sense because as i said it sounds very different. In fact, this is a big day for mme because after 2 amps i finally nailed it with this one today. That tweak plus a couple more that i could tell would make that one even better nailed it. It went from a good sounding amp with issues to a killer marshall. easily as good as the best i've owned, better than most. It's just killin ! I will likely mess with it more, but mainly out of bordom and for fun. But i will always keep the schematic of the way it is at the moment and return it to that state should anything ever not sound quite right again.

Now i'm going to start building the 1-12" combo cab and make the faceplate. I'm finally totally thrilled with an amp i built.

[Steve Conner]

To a good first approximation, yes, it just makes the treble pot look smaller, like about 75k.

[Bruce / Mission Amps]

That makes sense because as i said it sounds very different. In fact, this is a big day for mme because after 2 amps i finally nailed it with this one today. That tweak plus a couple more that i could tell would make that one even better nailed it. It went from a good sounding amp with issues to a killer marshall. easily as good as the best i've owned, better than most. It's just killin ! I will likely mess with it more, but mainly out of bordom and for fun. But i will always keep the schematic of the way it is at the moment and return it to that state should anything ever not sound quite right again.

Now i'm going to start building the 1-12" combo cab and make the faceplate. I'm finally totally thrilled with an amp i built.

I just tried it on an old 50 watt Marshall I just happen to have on the bench here using my closed back 1x12 with a Weber ceramic Silver Bell and with the treble pot set to about 50% to 65%, it does seem to very slightly increase the low mids and some bass but it also drops some of the dynamic, snappy lower treble and upper midrange crang response.... at 25 feet away it sounds the amp is turned around facing away from me and the sound is bouncing of the back of a lightly blanketed wall. Kind of odd but in a good way and interestingly, it sounds pretty good with the treble pot dimed.
My gut feeling is that I don't think this would be a great live, stage mixed sound... but who knows.
However, maybe my high freq hearing is going away faster then I thought or .... you just like a darker sound then me.

[daz]

My gut feeling is that I don't think this would be a great live, stage mixed sound.

Well, don't forget this amp is a lot different than a marshall in a number of ways. But one thing i DO know very well is what sorta tone translates thru a live mix. I used marshalls for years while gigging quite regularly, and one thing i came to be able to recognize is what sorta tone it is that cuts the right way in a mix. This one has that in spades, so i have no worries about that. I no longer play out, but i do occasionally play a bakyard BBQ with my friends and i can already hear that tone that cut so well for me in this amp. This amp is also tonally much more versitile than my marshalls were. they only did one sound. i mean, i could set the sound differently, but it would be too harsh or soft or whatever. There was always only one setting that worked. With this thing, between the presence and tone controls i can get several different sounds from that range from more american to more brit, and it sounds smooth and punchy with any of them. they are all usable. I'm not foolish enough to swear it's all going to work great live because i have plenty og years behind me trying new gear at gigs only to find certain things sound awesome at home and like crape at the gig. But i do know this will cut well and the chances are real good i can use more than one setting as witth my marshalls. They only had one good setting even at home let alone at the gig.