changing slope resistor to shape mids

[phsyconoodler]

I have a D-Style build out there that the owner is unhappy with the mid hump that it has.Can I just shift it a bit by changing the slope resistor?He isn't comfortable with the mid push these amps have.

[HotBluePlates]

I'm looking at a schematic of the Mini-D HRM amp that DaGeezer and Tubenit had discussed at some length a while back, and it has a tonestack I'm assuming is representative of the D-style amps.

When I've modeled this an equivalent circuit of this tonestack in the ToneStack Calculator, it doesn't have a "mid hump" but does have a flat midrange when the mid pot is full-up; that however, does sound like very pronounced mids given that we're all used to blackface-style mid-dip (or Bassman-style, Marshall-style, etc).

This circuit has a 150k slope resistor (in the form I'm looking at), which then feeds a 0.01uF mid cap and a 250k pot to ground. When I modeled the 250k set to 0, the familiar mid-dip was present. I would first look at reducing the value of that pot, maybe by using 150k in series with a 100k pot. Maybe a smaller pot, more series resistor. That will give the ultimate effect of the stock circuit with a mid pot that can't be turned up until the flat midrange (mid-hump) occurs.

In my opinion, if you make major changes to the tonestack, you might as well ditch it and install the typical blackface stack. The only real difference I see is some tweaking of cap values to put control action in the right ranges, and some trickery to allow a wide-range mid control.

[bluesbear]

If you're using a 250K pot for a mid, it's actually a mid/raw control. 25K is a good sized mid pot. Anything over that and you're beyond mid and into a Dave Allen style raw control. I like that but you can't turn the pot up more than about 2 or 3 before you're actually lifting the tone circuit from ground. That adds gain, a lot of mids, and makes the treble and bass controls less effective.
Dave

[OldHouseScott]

I modeled this tonestack in LTSpice and changed the mid cap to a 6n8 and the cap across the bass pot to a 10n in a recent ODS type build. This yielded a more Fendery type response, but pretty much negated the usefulness of the mid pot. It's now a very subtle effect, which is fine by me. With the PAB, you get all the mids you can stand.

[jjasilli]

Does the amp look like this?:  http://schematicheaven.com/dumbleamps/preamp.pdf

If so that looks reasonably similar to the Vox tonestack in the Duncan Tonestack Calc which might help.

Anyway, I think you are right that raising the value of the slope resistor will brighten the amp.  Will it be enough?  Ditto to all other comments on that.

Another possible culprit, on the attached schematic, is the large 500pF bypass cap on the vol pot.  +/-100pF might also help.

[phsyconoodler]

This tonestack doesn't react the same as a fender stack.I have a 500k mid pot in there now and have tried 100k-250k in other builds.The mid-dip doesn't happen in this amp with whatever pot is in there.
   Bluesbear,it does not turn it into a 'raw' control with a larger mid pot,because it's not like a fender stack.
I personally love the way the amp sounds but the owner is stuck on that mid-dip like a fender.It's way too much trouble to change the tone stack.I think I'll try a slightly smaller slope resistor and see what it does.
   jjasilli,
I'm using a 140pf bright cap,but it's on a switch,it's not in all the time.The 250pf is way too bright for me.
  The Dumble tone stack is pretty responsive.You turn up any knob and you really hear the changes in tone.Treble MEANS treble and bass means BASS.The mids are quite pronounced as well.There is a  volume increase when the mid pot  is turned up.
  He has swapped speakers quite a bit already,going from a Mojotone Greyhound to a Gov'nor to a 1218 Legend.I'm thinking he should try a vintage 30 or a G12 65 Celestion myself.I absolutely love the G12 65 for this amp.It's paired with a Celestion Vintage 10.
  It's got a 10 and a 12 in the amp.
 When I hear him play it I absolutely am floored with the tone I hear,but this guy is an incredible player and he wants what he wants.
    He uses an Anderson Cobra tele style guitar with splitable humbuckers.What a nice guitar!I like my 72 tele Deluxe better,but each to his own.
  I like the mids because it cuts through any mix like a knife.

[jjasilli]

Sounds like we might be in the subjective area of customer satisfaction.  You know enough about amps, and building them, as anyone needs to know IMHO.  Maybe this customer is not destined to be happy with tone -- there's always something else tonewise, no matter how good a player he is.  A player could be attracted to a sexy name, like Dumble, but his ear wants something else.   If he doesn't like the tone of this amp  -- though you think it's a great tone in itself -- then maybe he needs a different amp.  Can he specify the name(s) of particular amp models that he does like?

[phsyconoodler]

I hear ya.This guy is on never ending search for tone,but he is rather flighty in a way.He can play the strings off pretty much anything but he changes gear like I change my socks.Every time you turn around he has something different.
  He goes to extremes sometimes,going from the very fine stock pickups in his custom shop strat to a set of SM antiquities and then dumping that guitar and going to the Anderson.
  It's like he has some band playing in his head and he isn't going to stop until he hears THAT tone.

I'm going to try a different approach with him and get him to try different guitars rather than mod the amp to suit his ear with that guitar.
  Maybe a mid-scoup switch on the guitar? 


  

[topbrent]

Phsyconoodler, I would send an email to Henry at RedPlate amps or PM him over at ampgarage. 
-  heisthl is his screen name.

Of any of the Dumble oriented builders out there, he would be the guy to help you in this situation.
His BlackVerb and BlackLoop amps have a more fender oriented scooped sound on the non OD channel.

Garth Webber playing a BlackVerb: http://ampgarage.com/forum/files/rp_gw_demo_01_155.mp3

&feature=related
http://vids.myspace.com/index.cfm?fuseaction=vids.individual&videoid=58406191

[phsyconoodler]

I wish I could post a clip of mine.It's every bit as good as that clip.
  I'm thinking that the guy needs to let his ear adjust to it for a while before we mod the thing.
 It really sounds marvelous.

[topbrent]

I wish I could post a clip of mine.It's every bit as good as that clip.
  I'm thinking that the guy needs to let his ear adjust to it for a while before we mod the thing.
 It really sounds marvelous.

I love the tone of the OD Rocket on your website.  Really Great stuff!

http://www.claramps.com/rocketsound/rocket01.mp3

http://www.claramps.com/rocketsound/rocket02.mp3

[phsyconoodler]

Yeah the tone is ok but the songs are not really ones that show off the real potential.
   My latest one is very Marshally with a JCM800 OT and a pair of Sylvania 6CA7's.Oh and reverb too.
All in a nice snakeskin covered head cab.
  I'll post some pics soon.

[HotBluePlates]

The mid-dip doesn't happen in this amp with whatever pot is in there.

No mid-dip with the mid pot on zero?

My suggestion was about not changing the circuit, but using a smaller-valued pot in series with a resistor, with a total series resistance equal to the original value of the pot. It mimics the original circuit, but with a pot that can only be turned from 0 to (say) 4, which then sounds like more mid dip than the stock circuit.

You could combine a slope resistor change with this alteration.

[phsyconoodler]

Yeah the mids dip with the pot turned down,but it takes away quite a bit of girth at the same time.It's like the stack's total range is where it likes to be with the existing pots but I want to retain the fullness of the sound without the middy flavour,I think a slope resistor change would do it.I guess I'll just change it and tweak it where he likes it while he waits.

  If it gets closer to where he's happier I'm going to stop there.Any more tweaking will make it a different amp.I don't want it to lose any of the incredible tone it has already.
 

[12bz7]

I think with the D style amp/skyline tone stack the mid cap almost works in reverse. A .047 mid cap will get you closer to the mid scooped blacksface sound, .02 more mids, .01 typical D style fat mids. Easy to swap caps and see if it helps.