James tone stack for a Tweed Princeton

[g-man]

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[tubenit]

That should work OK.   Use a Duncan Tone Stack calculator to figure out what values of resistors and caps you want.

With respect, Tubenit

[g-man]

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[tubeswell]

FWIW I did a supro t-boltish build last year with a goosed-up front end that included a James TB tonestack. This configuration has really wide-band treble response.

[g-man]

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[g-man]

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[kagliostro]

if you are still in the testing phase



or



K

[g-man]

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[Stankfut]

Okay, I'm going to let my inexperience show, but the diagram in the first post has the tone stack before the volume, am I seeing that correctly? Can this tone stack be placed in the usual location? How about using it with a cathode follower?

Sorry for the questions!

[HotBluePlates]

... Can this tone stack be placed in the usual location? ...

"Usual location" for what amp?

How about using it with a cathode follower?

You can if you want, but what do you expect to gain from that? James tone circuit = Baxandall tone circuit, without the feedback loop.

The appeal of the James circuit is that when you set the Treble and Bass controls to half, everything is equally attenuated, giving flat response (we're so used to mid-scoop that this will sound midrangey). Turning either control down cuts that frequency range, while turning either control up appears to boost that frequency range.

Rather than real boost, the James does what every other tone control does, it gives less loss which then sounds like a boost. If you did something to reduce the amount of loss the circuit imposes at mid-settings, you also reduce the amount of "boost" the tone controls can provide, which will make them seem less effective.

The Baxandall circuit takes the James tone control and situates it inside a feedback loop around a single tube (could be a triode or a pentode). This gives cut or a real boost relative to the signal level with no tone control in the circuit.

But guitar amps are designed to have excess gain, so the modest gain return of using the feedback stage often doesn't seem worth the added complexity involved. I'd ask myself if the amp really lacks gain if I have the amp and guitar volume dimed.

[Fresh_Start]

So I tried a few different tone controls (original 5F2A single knob, tweed E series bass/treble, Danelectro Centurion bass/treble, and James bass/treble). I guess any of them will do the job on a small single-ended amp like this, but the James gives the most useful sweep to my ears.

I also jumpered in a tone bypass switch to see if I want to add one. It's a good way to hear exactly how much signal loss there is in a tone stack...quite a lot. I may add it just to have a raw boost mode.

Is the cathode of the second triode still un-bypassed like the Tweed Princeton? 

If so, you can bypass part or all of the cathode resistor to make up for the tone stack loss. Making the bypass switchable opens up all kinds of alternatives. 

[jjasilli]

Okay, I'm going to let my inexperience show, but the diagram in the first post has the tone stack before the volume, am I seeing that correctly? Can this tone stack be placed in the usual location? How about using it with a cathode follower?

Sorry for the questions!

Some points of clarification at the risk of redundancy.  Re "usual location", you show the 5F2-A, with the tone control after the vol pot.  The 5F2 has the tone control before the vol pot.  In either case, the vol pot serves double duty:  1. vol. control & 2. "grid leak resistor" for the next preamp stage. 

Signal gets attenuated by tone controls; this is known as "insertion loss".  (Except for a true Baxandall circuit which uses a gain stage(s) and hence is active, not passive.)  As a result, the "tonestack" needs to be followed by a gain stage, called the "tone recovery stage".  This amp has a simple topology with only 2 gain stages.  Hence the tone control needs to be placed in between those 2 stages. 

A cathode follower is used to drive a tonestack, not to recover from one.  There are not enough preamp stages in this simple amp to use a cathode follower.  If the dual-triode tube were used as a cathode follower to drive the tonestack, there would be nothing left for the tone recovery stage. 

[Fresh_Start]

jj - you could use a MOSFET for the cathode follower before the James tone stack.  However, it would affect how the first gain stage behaves as well as the "insertion loss". 

[g-man]

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[jjasilli]

"In stock form the NFB switch makes a fairly large difference in gain, but it has almost no effect when the extra bypass cap is in place. Is that because the gain increase from the bypass cap is larger than the NFB gain so it doesn't really add anything more, or is there something more to it?"  

When the bypass cap is added to this V1b cathode circuit, it automatically defeats NFB.  All or most of the NFB signal bleeds to ground through the cap.

To add a cathode resistor by-pass cap, AND keep NFB, you need to split the cathode resistance into two resistors in series, and redirect the NFB insertion point.  This is easy to do.  Just copy the NFB loop of the Fender Champ or Vibro-Champ AA764.  

N.B.:  The AA764 circuit assumes an NFB voltage derived from a 3.2Ω OT secondary.  If the output impedance of your OT is different, so will be the NFB source voltage:  If the output impedance is higher, then the NFB source voltage will be higher.  This might be OK, or you can vary the value of the NFB resistors to reduce it.

EDIT:  The Princeton Reverb AA1164 shows NFB resistor values for an 8Ω OT secondary.

[Fresh_Start]

JJ got it spot on.

Somewhere I've got a switching diagram for going from un-bypassed cathode resistor to bypassed with the appropriate change in the NFB insertion point and change in the NFB resistor. IF I can find it easily, I'll come back and post it.

Cheers,
Chip

[Stankfut]

Just to clarify, "usual place" meant after the volume control. 

... Can this tone stack be placed in the usual location? ...

You can if you want, but what do you expect to gain from that? James tone circuit = Baxandall tone circuit, without the feedback loop.

The appeal of the James circuit is that when you set the Treble and Bass controls to half, everything is equally attenuated, giving flat response (we're so used to mid-scoop that this will sound midrangey). Turning either control down cuts that frequency range, while turning either control up appears to boost that frequency range.

Rather than real boost, the James does what every other tone control does, it gives less loss which then sounds like a boost. If you did something to reduce the amount of loss the circuit imposes at mid-settings, you also reduce the amount of "boost" the tone controls can provide, which will make them seem less effective.

The Baxandall circuit takes the James tone control and situates it inside a feedback loop around a single tube (could be a triode or a pentode). This gives cut or a real boost relative to the signal level with no tone control in the circuit.

But guitar amps are designed to have excess gain, so the modest gain return of using the feedback stage often doesn't seem worth the added complexity involved. I'd ask myself if the amp really lacks gain if I have the amp and guitar volume dimed.

That's the kind of information I'm looking for! I've only dealt with TMB and 5E3 style tone stacks. To be honest, I didn't really know what the differences are.

There is a lot of good information here, Thanks all of you guys! Now that I've read it, it will have to stew in my brain for a while, and eventually I will have a better understanding!

BTW, sorry for the mild hijack g-man!

[mike45]

I build a Princeton like amp with the tonestack from A Komet Songwriter. It has nearly no gain loss in TS. See schematics.

BR

Mike

[jjasilli]

I build a Princeton like amp with the tonestack from A Komet Songwriter. It has nearly no gain loss in TS. See schematics.

BR
Mike

To me this tonestack resembles the Fender "E-style" tonestack.  With the bass and tone controls in the middle its frequency curve resembles that of a James tonestack with its controls in the middle: a pretty flat frequency response with little insertion loss.  To get some mid cut with the  "E-style" tonestack, the bass and tone controls need to be dimed.  See the Duncan tonestack calculator.  The same is true for the James style -- lacking a mid control, its mids are "cut" by "boosting" (throwing away less) bass & treble, leaving the mids lower by comparison.

The modern Fender TMB tonestack has a deep mid cut, more overall sweep, and more insertion loss than the E-style.  Insertion loss is a requirement for any passive tonestack in order to get a mid cut, and to get more sweep (as Hotblue stated above).  Some signal has to be thrown away to shape tone -- in the sense that a flat frequency response has not been "shaped".  More ability to shape requires more signal loss (or active tone controls).  Insertion loss is not a bad thing; it is required for user-adjustable tone shaping.  The lost gain can be made up elsewhere.   

[Stankfut]

Okay, sooo.....what does the "shift" or "tilt" control do in the drawings that kagliostro posted do? From the graph, it looks like it cuts top end?


Bear with me....I'll get it one day!

[kagliostro]

what does the "shift" or "tilt" control do

Look to the picture

K

[HotBluePlates]

I build a Princeton like amp with the tonestack from A Komet Songwriter. It has nearly no gain loss in TS. See schematics.

To me this tonestack resembles the Fender "E-style" tonestack.  ...

The Treble control is exactly the same as the tweed Princeton or Deluxe; the Bass control is very different from the Fender "E-style" tonestack. Is is simply a small coupling cap (which shaves a lot of bass) with a variable resistor placing a large coupling cap in parallel. When you turn up the Bass control, you're lessening the resistance isolating the large coupling cap, and shaving less bass.

It has nearly no gain loss in TS.

All tone controls have some form of gain loss, because that's how they change the balance of frequencies. What you're hearing is a lack of midrange attenuation from this tone circuit.

Look at your Treble control, or the single tone control of the tweed Princeton or Deluxe. Either a small cap bypasses the volume control (which passes highs here with no attenuation from the volume control), o a larger cap passes signal to ground (now highs & upper midrange, giving the impression of a darker, bassy sound). Midrange itself is not directly impacted, and is one of the reasons these small tweed amps sound midrangy compared to, say, a blackface Fender.

Those amps have a steep midrange cut (to create an overall tone shaping), and you can dial more treble or bass back in. But I've never known anyone to turn all tone controls on a blackface Fender to 10, because that balance usually isn't too pleasing. The resulting midrange scoop that's left is what gives you the impression of gain loss (because it's there, both in absolute and relative terms).

[jjasilli]

Okay, sooo.....what does the "shift" or "tilt" control do in the drawings that kagliostro posted do? From the graph, it looks like it cuts top end?
Bear with me....I'll get it one day!

The "shift" or "tilt" control changes both the slope, and the "center" or resonant frequency of each curve.  Perhaps this is most noticeable with the pair of brown vs. blue curves showing the deepest mid cuts.  Though these curves drop to about the same depth, there are 2 important differences:  

1.  Slope.  The walls of the blue vs. brown curves are at different different angles from one another.  Even subtle differences in the visual shape of the curve indicate large tonal differences in what we hear.  

2.  Resonant frequency.  Note that where the curves bottom-out, they are centered around different frequencies.  The blue curve is shifted to the right, centered around 650Hz.  The brown curve at around 450 Hz.  (Note that the X-axis of a frequency graph is logarithmic, so the spacing may seem uneven at first.)  This changes the tonal flavor.  

I build a Princeton like amp with the tonestack from A Komet Songwriter. It has nearly no gain loss in TS. See schematics.

To me this tonestack resembles the Fender "E-style" tonestack.  ...

The Treble control is exactly the same as the tweed Princeton or Deluxe; the Bass control is very different from the Fender "E-style". . tonestack. Is is simply a small coupling cap (which shaves a lot of bass) with a variable resistor placing a large coupling cap in parallel.

Do you think this materially changes the analysis?

[tubeswell]

The James tone stack treble control is also like a tweed princeton tone control, whereas the James tone stack bass control is a pot that bypasses either one or the other of the (4n7) caps creating either more of a 'treble' bypass, or alternatively more of a 'treble' bleed, depending on which way you rotate the bass pot.

[HotBluePlates]

Do you think this materially changes the analysis?

Yes, because the circuit from the Komet amp won't provide any mid-cut.  EDIT: Maybe you were talking about the E-style circuit & James circuit, despite the quote regarding the Komet circuit. In which case, no, it wouldn't change your analysis.

[g-man]

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