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Vox Pathfinder 10 - Looking for EQ help

Started by Leftysquire, July 11, 2013, 02:12:34 PM

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Looking to install a Mid control.

Which one should i replace, and with what Value/taper: R12 or R11?

What values for caps should i put in parallel with the C9 & C11 to do a mid shift down to 400hz like Fender? I've included the tonemender schem to give the idea for the mid shift I want to implement.




Not quite that simple.  R12 only sets the limit of the bass control range.

Download and play with Duncan's Tone Stack Calculator.

Just a point; the Vox circuit looks like it runs from split +/-ve rails, while the Tonemender runs from a single supply and has an artificial mid point "Vref".  In the Vox "Vref" will actually be ground.

{If it were mine I would also be changing the 4558's for LM833's while I was in there, but that's a whole other thing.}
If you say theory and practice don't agree you haven't applied enough theory.


Thanks for the info Roly.

So replacing R11 with a 1.5K pot would work then? Log or Linear, what would you recommend? I'll try that first and see if I want to shift the the frequency of the mid dip.

I have the TSC software, but I don't know the input impedance, nor the load.

Why would R12 be needed, the speaker size? I am tempted to try and squeeze and 8' speaker in there...

What would the difference be between the 4558's and the LM833's be? is it just straight swap, or do other components need to change too?

I have some electronics background, but mostly guitar wiring and 1 year college electronics technician course, 20 years ago...


Quote from: LeftysquireI have the TSC software, but I don't know the input impedance, nor the load.

The output impedance of an op-amp can be assumed to be zero, an AC short circuit to ground, so the source impedance into the tonestack with either be zero or the value of any resistor in series with the output.

Similarly the non-inverting (+) input of an op-amp is assumed to have an infinite input impedance, so the effective input impedance will be any resistor connected to this to supply bias.  In the case of the Tonemender this will be the 1 Meg resistor following the treble control wiper.

Quote from: LeftysquireWhy would R12 be needed?

It is to provide a residual signal when both bass and treble controls are turned to zero.  Without it there would be no signal at all under those conditions.

The primary difference between the 4558 and LM833 is noise, but the 833 also as a much greater bandwidth so some rolloff (such as a 47 or 100pF cap in parallel with the feedback resistor) may be required if there was any resulting instability.  TL072' or 082's represent a half-way-house in both noise and bandwidth.
If you say theory and practice don't agree you haven't applied enough theory.


I've now loaded the tone stack in TSC, and it's all starting to make sense. C1 shapes the mid response. Doubling it's values would seemingly shift the mid dip into Fender range, while keeping the rest intact.

I'm tempted to put sockets in for the tonestack caps and resistors to play with it. Or hijack the Clean/OD switch to parallel a 100nF cap with C1. The sound is a bit muddy, probably a combination of my pickup and cabinet design. I use the amp as my clean basic sound as I'm not fond of the OD on the amp. The cleans on the amp are really good for low volume playing (I keep below 5 on the master).

I may just socket the IC's and play with them, just to see. But I have no issues with the noise wit this point, i"m using a Dimarzio Fast Track T in my Tele, so it's good moderate level signal going in. Which resistor is the feedback resistor, R7?

I have an extra 1 to 1.5" on the top of the speaker before I hit the chassis top, could a 8" fit in there?  Probably, right? I'll try plugging into the Pro Jr I'll be selling off to see if a speaker change would help in getting a bit more in the low end.

Thanks for all the info, I'm getting a better understanding of how this amp works. I am proceeding slowly and with caution on these mods. Lest it gets out of control... :)


That's one of the great strengths of TSC; it makes it easy to quickly diddle with values and plot the result so you get almost instant feedback on the effect your change has made - it's a great teaching tool as well as for design.

I got into electronics before protoboards so I learned to "breadboard" circuits as "birdsnests" or "aeroboard", all the components soldered together on full length leads in free space.  As a result I find most plug/socket arrangements an unreliable complication and just tack solder components as required.  But if you want to fit some sort of socket and mount some different caps on some sorts of plugs (e.g. header pins) then this is a reasonable way to make changes quickly with fairly reliable contacts.

Quote from: LeftysquireWhich resistor is the feedback resistor, R7?

In the Vox PF10 circuit the feedback resistors are R7 and R15, and C7 is the cap that limits the top end bandwidth of op-amp U1B.

The corner frequency, fc, where it starts to have an effect is the frequency when the capacitive reactance, Xc, (or the effective AC resistance of the cap) is equal to the value of the resistor in parallel, 1Meg in this case.

Xc = 1/(2 * Pi * f * C)

transpose to make f the subject;

f = 1/(2 * Pi * Xc * C)  (hertz, ohms, farads)

   100pF  =  100*10^-12  =  10^2*10^-12  =  10^-10

we want to know f when Xc = R7 = 1Meg, therefore;

f (hertz) = 1/(2 * Pi * 10^6 * 10^-10) = 1591.5Hz

This is quite a low frequency, far too low for Hi-Fi, but not unreasonable for tenor guitar.

The effect of a speaker change depends on not only the diameter of the speaker, but the volume of the box behind it, and if it's sealed or open back.  More diameter should mean more bass, but it would be compromised by a sealed box that was too small, or an open back where the air path from the front of the cone to the rear is too short.

Since this is an open back cab, take a length of string and run it around the shortest path you can find from the back of the driver cone to the front.  This represents half the wavelength of the lowest frequency this cab will support, and any attempts to make it go lower are a waste of time.

(Speed of sound in air, c, ~1000ft/sec.  f = 1/wavelength.)
If you say theory and practice don't agree you haven't applied enough theory.


Again, thanks for the info Roly.

I think I will end up using the Clean/OD switch to parallel a 100nF cap to the mid cap. I'll try the lower freq for the mid dip, hopefully it'll solve my problem.

Great explanation of the op amp circuit, I'll keep it my back pocket if I do decide to go with LM833's, should the noise start bothering me.

I'll try the string trick, but my assumption is that with this small a cabinet the the amount of bass I ultimately may gain will be negligible. The point of diminishing returns is probably not far from where the amp is now.

The next step will be building my pedal setup, that should be interesting to say the least... I'm thinking an Axis Fuzz http://shop.pedalparts.co.uk/Axis_Fuzz_clone/p847124_6532845.aspx fed into a Peppermill http://www.runoffgroove.com/peppermill.htmlfor my fuzz sound, some sort of 80's metal tone (maybe http://www.runoffgroove.com/grace.html with a tone control) and a slap delay to thicken up my sound. I'll probably buy the delay as they seem to be expensive to build correctly. Trying to build a small, but good sounding setup for quiet home use.


As an alternative to string and math you can try hooking the amp up to a borrowed sealed cab with a 12" or two and see what that sounds like.  If it sounds okay (sufficient bass) he  it suggests that the open back cab is the basic problem; but I would check out your cab with the string &c because if you mod the amp to drive the cab with low frequencies it can't handle you are inviting driver failure through over extension of the cone.

From on-line pix it looks like it has a 1/3rd opening across the back, and you may actually get better results by fitting a piece of wood across the opening and sealing it, but measuring the front-to-back air path will give you a point of reference.

Quote from: Leftysquirefor quiet home use.

With respect to bass, the tone contour of your ears changes with the absolute level of the sound and your ears tend to be less sensitive to bass at low listening levels.  Hi-Fi amps are sometimes fitted with "loudness" controls which basically boost the bass a bit when the voluem is turned down, so for low level work specifically you may need a bit more bass response again over louder levels.

"interesting" might be the right word.  Have a go by all means, but I have a few problems with some of the circuits on Runoffgrove, specifically these using FET's where there seems to be a habit of using a trim pot with each FET (or at least those in gain stages).  I have covered this at some length elsewhere here, but essentially;

- if you know what you are doing you don't need trim pots,
- where they are unavoidable you don't put them in the Drain circuit.

He has a couple of projects in particular called "Eighteen" and "The English Channel" which have three FET's and three trim pots, and five FET's and four trim pots respectively which I consider to be poor design.
If you say theory and practice don't agree you haven't applied enough theory.


Thanks for the info Roly.

I'll test it out with my Pro JR cab to see how the low end changes.

As for the pedals, thanks for the heads up. I figure to use the Axis fuzz straight into a low gain OD, housed in the same box. The fuzz faces and derivatives seems to work better with slightly distorted amps.

What would you recommend as a DIY pedal for some 80's style dirt (old maiden, Rhoads era Ozzy, etc...)?


Look, pedals in particular are such a matter of personal taste I wouldn't dare; all I'm trying to say is that a lot of the circuits on the net are attractively presented, with all sorts of glowing assertions and endorsements, but aren't really up to scratch - so, have fun building and exploring, but if something doesn't work out it's even money it isn't your fault, even if you were a tech you couldn't get it to work, and if you did it would still sound shite.

You learn to build pedals, etc, in much the same way you learn to play guitar (or do anything really) and that is by first following the well trodden path of copying, e.g. cloning the Tube Screamer, and once you have done a few of the favorites, branch out off-Broadway.

But if you want to go bush-bashing right away, don't get disheartened if it doesn't work out right away.
If you say theory and practice don't agree you haven't applied enough theory.


What would be the normal expectation if I were to make the cab Closed back vs the current open back? How would the frequency response typically shift, generally speaking?


Well there's an awful lot of "it depends...", mainly on how much internal volume the cab has, but very generally sealed enclosures tend to have more bottom end than open back, "fuller" or "duller" depending on your taste and prejudice.

When gigging the rear spray from an open back tends to spread the sound around more while a sealed cab tends to be all out front.  The practical upshot may be that the other members of the band can hear the lead better when an open back is used, but care with positioning and aiming can overcome these problems.

Making a full back for an open back cab is reversible - try it, and if you don't like it restore the old back.

If you say theory and practice don't agree you haven't applied enough theory.


   Is it possible to see the whole Pathfinder-Schematic? :)