James tone stack w/ seperate mid control

[cobaltu]

Hello all.

I'm modifying an amp with 2 standard 12ax7 preamp gain stages (5751 at the moment actually), a long tail pair phase inverter, and 2 fixed bias 6l6's.

The plan is to situate the James tone stack in front of a 1meg volume control. The circuit looks like this:


I want to omit the shift control in place of a fixed 100K resistor on load it on to the volume control (1Meg). Then after the second gain stage, I want to place a bridged-T mid cut in between the second gain stage and the phase inverter.
For anyone who has read Merlin Blencowe's book on guitar amp preamps, I'm planning on following the James tone stack he outlines (uses primarily 10n caps) and the Framus bridged-T mid cut.

Is this a good Idea? Are there some impedance concerns I should know about (I hear the bridged-T middle cuts are sensitive to surrounding impedance)?

Any help is appreciated. I'll try and find a picture of a bridged-T mid cut.

 

[tubeswell]

That's out of Merlin Blencowe's Designing tube preamps book.

I've built that circuit a few times, but prefer to replace the 'shift' pot with a resistor that can be shorted by a 'slope' switch. (I found that the promise of the pot didn't really add much from a practical guitar amp point of view (in my view).

I've also added a 1M resistor between the tone stack ground returns and the actual pre-amp ground return and put a switch across this so that I could lift the TS load (i.e. a raw/boost switch).

See schematic attached (Note that I scaled the TS to use 1M pots and resistors (whereas Merlin's schematic shows 500k pots). This just adds a touch more signal (albeit at the expense of a bit of extra resistor noise - but what the heck, its a geetar amp)

 

[cobaltu]

Thanks tubeswell! it's good to know that someone has actually tried this exact same tone stack and - more importantly - likes it.

I found a picture of the Framus bridged-T mid cut (no apparent boost here, so '10' neutral):

I was going to use the one in Merlin's book which is scaled differently. His has a 1Meg pot, 100K ground resistor, and, .001uf caps. Again I was going to place this after second gain stage, before the phase inverter.

Does anyone know how the scaling influences the circuit? I'm assuming that the response is the same as the one in Merlin' book b/c everything is scaled by a factor of 10 but is there more insertion loss?

 

[tubeswell]

If you increase the resistances all round, there is less insertion loss, but more resistor noise. (You have to inversely scale the capacitors at the same time to get the response the same)

 

[FenderLover]

Merlin's graphs show the insertion loss is only a couple of dB, except where the notch is engaged which is where you want the cut, obviously.

Examples of the circuit in Framus, Gibson, and Matchless circuits use the control as the plate load as well, so I wouldn't be too concerned about scaling the circuit for higher impedance.

 

[waparker4]

The Carvin Belair has a passive baxandall with mid

 

[cobaltu]

Oh, the picture didn't seem to load:

 

[tubeswell]

The amount of insertion loss with that tonestack becomes apparent if you have a switch you can flick to lift the tone stack from the ground return. Its quite noticeable.

 

[cobaltu]

I bet it is. But I intend to use it with a 1Meg pot, 100K ground resistor, and 1nF caps.

 

[JD0x0]

The amount of insertion loss with that tonestack becomes apparent if you have a switch you can flick to lift the tone stack from the ground return. Its quite noticeable.

You ever try this with a standard Blackface type stack? Huge losses, there, too. Maybe a tad more with the Bax, depending how it's set.

 

[FenderLover]

The amount of insertion loss with that tonestack becomes apparent if you have a switch you can flick to lift the tone stack from the ground return. Its quite noticeable.

If it really cuts 15dB when dialed in, certainly, it'll change about 12dB if the graph is accurate.

That's what passive EQ's do, but I meant in terms of rescaling the circuit for less loading with higher impedance. Using 500K or 1M pots would be fine. I was unclear with my comment.

 

[cobaltu]

JD0x0: I actually did try a many iterations of the fender tone stack - including marshallesque territory - but I never bonded with it. The mids always seemed too scooped, controls too interactive, and the bass seemed too soggy. That's what lead me too this tone stack, it doesn't have may of the pitfalls the fender design has.

I'm actually surprised many are concerned about there being too much insertion loss. I've been worried about there being too much gain.

Any other thoughts, particularly about the bridged T control?

 

[JD0x0]

The mids always seemed too scooped... ..and the bass seemed too soggy.

I have had the same perceptions. I find the Fender TS is a good platform for some component value changes, though. I tend to prefer using a higher value mid pot, so I can dial out the 'scoop' use a 47n for the 'bass cap' instead of 100n to kill some of the sogginess, and maybe a lower value 'treble cap' to tame the upper mids a tad. Sometimes a lower value slope resistor, if I need to fatten the amp a bit more.

The Baxandall/James has always interested me, and I'm surprised at how uncommon it is compared to the Fender/Vox/Marshall type stacks.

 

[FenderLover]

The insertion loss of the EQ can be integral to the gain staging of the amplifier depending on what you are trying to accomplish, a clean amp with lots of headroom or an amp with lots of drive and little clean headroom.

That said, the advantage of a Baxandall (James) is that it doesn't really need a Mid control, IMO. The James is flat at -15dB or so. If you cut bass, mid, and treb, it's essentially flat at -30dB. There is not too much benefit to add all the circuitry to cut 15 dB and still be flat.

If the bass mid and treb are boosted, the mid is still cut 15dB relative to bass and treb, because it is all passive and the James will be -15dB in the mid band when the mid has no cut at all. Again, no real advantage except that you can cut another 15dB in mids and be -30dB relative to bass and treb if they are boosted. Super-scoop.

With the James taken by itself, the mid can be +20/-15db relative to the bass and treb without a separate mid control at all.

Compared to Fender/Marshall, the mids can only be +0/-15dB relative to bass and treb.

**All numbers approximated for the sake of conversation...

 

[jhundt]

can't comment directly on your question, but I wanted to say that I added that Blencowe-modified Framus control to a home-built Matchless Spitfire-inspired amp, and it is a very useful addition.

 

[cobaltu]

jhundt: That's good to hear. Where did you place the control in the circuit?

FenderLover: I can definitely appreciate why you would see the mid control as redundant, but I still think it would add a nice layer of flexibility. The idea here, is to crate a tone stack that can mimic a fender one without being locked-in to its traditional pitfalls. In order to get an appropriate mid scoop I would have to essentially crank the bass, treble, or both controls and given the relatively low loss of this circuit I feel like that could get overbearing. The added mid control would allow me to be able to carve out some mids without having to drastically modify my bass or treble content.
Besides, if your right and this idea turnout to be crap or redundant, I can always remove it.

 

[cobaltu]

Oh!

Does anyone have any idea's about what my bass and treble cutoff frequencies should be?

I'm planning on using the Merlin modified Framus mid cut that is in his book and happens to be notched right at 500Hz - alla, Fender tone stack.

 

[jhundt]

I put my mid control in series after the volume control and before the following tube stage.