Vox Pathfinder 10 mod galore

dimkasta__

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That's my second ongoing small guitar amp project after the Fender Champion 40.

I just love the small pathfinder. With a "few" mods and a cab it sounds great. Plus it's a DIYer's dream, being a full old-school circuit with a transformer PSU and all through-hole parts. Keep in mind that the pcb is crappy though. It's easy to lift pads if you are not careful.

The schematic has been around for almost a decade, but there still might be things that need to be fixed.


My goal is to have a nice platform to use with my pedals, but mostly diy itself since I have other amps. This could be a bit over the top :)

So here is the list of my mods, gathering info from around the internet, plus results so far.


Super easy things to do:

[✔] Remove the hard clipping LEDs. This is very common mod, and the first one that anyone should do. Just snip the LEDs and you immediately get rid of that "bees in a can" sound when the boost is on. Highly recommended mod. The only issue with it, is that when the boost is on and the gain is set high, the amp is VERY loud. The volume goes from 0 to OMG very fast.

[✔] Removing C12. This cap acts as a low-pass filter on the final stage. It rounds the sound a lot, and it kills lots of clarity. Without it, when the boost is on, the sound gets a bit harsh. But the clean sound is just amazing. Something especially evident in harmonics and pick attack. Perhaps it makes sense to experiment with different values if you have kept the final gain stage (read below), but without it, you get a nice percussive and super-rich sound that is almost JazzChorus-like.

[✔] Add a 1/4 jack to allow connection to a cab. This is super easy and very straightforward. I have soldered a cable directly to the speaker. And repurposed the old hole to hold the 1/4 socket, which is soldered to the old wire. This way you can use any cab with the amp, but you can still plug in its own speaker when you need to.



Mods that require some more effort with soldering tools

[✔] Changed the tonestack to match the voicing of classic VOX circuits. I have checked the curves of the classic Vox tone stack using the Tone Stack Calculator application. The curve of P10 was waaaaay off. P15r is a bit better, but still off. So here are my changes, using values of parts that I had. The pots do not track the same, but it's close enough. And the sound improvement was amazing.

Classic Vox tone stack

vox.jpg

Modified P10 tonestack.

p10_candidate.jpg

In detail

R10 - 15K
R11 - Removed and added a wire from its C11 pad, towards the junction between R12 and P3-pin3
R12 - 1K
C9 - 1nF
C10 - 560nF
C11 - 150nF

[✔] Changed the last opamp stage into a buffer. The power stage is a TDA2030a running at +-20V which gives 18W power and 12V peak to peak output on an 8ohm speaker and an input sensitiviy of 900mV (which amounts to ~2.5V needed if you count the attenuation from the tone stack). This means that the amp has too much gain since the two front stages are more than capable of this. This is evident from the amp going to OMG LOUD very fast. So I removed R15, R16, R17, C12 and C13, and then bridged R15 and R17 so that pins 1 and 2 of the opamp are connected. This results in a much more manageable volume especially at home levels. It can still go loud if you want, but now it remains super clean for much more. You might feel like you are not getting enough speed for how much you punch the gas pedal, but the top volume is still there and with a powerful pickup you can still get full volume if you need it. You can even get some overdrive action at home volumes (sounds a bit meh). Removing C12 also removes a lowpass filter that kept the amp muffled (as described above).

[✔] Replace the plastic boost switch with a DPDT toggle switch. My switch was getting scratchy. The panel now looks much better
:)
The switch fits only if it's mounted on its side so that it flips horizontally. You do not have to solder all 6 pads. The top right is not connected to anything, and the bottom left is no longer used since we removed the LEDs.

[✔] Eliminated the boost. I will repurpose the button at some point, but so far my goal is to make everything work through the gain knob

[ ✔ ] Add a simple series FX loop right before the power amp. This is very easy with two switching jacks. And we probably do not need a buffer either. It might cause gound loops though.

Abandoned:

[✘] Removing R8. I have tried this but I am not sold on it. It gives another boost in gain on the cleans that is immediately audible as extra noise. And it seems to also affect the voicing a bit since it includes C8 and R9 in the feedback loop (probably to compensate for adding R5 on the first stage).
I restored this since I do not need any more gain on the clean channel and I am not sure I like the change in tone.
This is going to be part of a bigger change to rearrange the gain structure of the amp

[?] This one is rather obsolete now that we know that the last stage can be changed into a buffer without affecting the tone negatively. The original mod was to change the linear volume pot P4 to one with an audio taper. The difference was more evident without the boost on. When you engage the boost, the amp still went loud fast, but you got some more volume resolution than before.

[ ? ] Change C28 to a bigger cap. I was wondering how starved the chipamp could be without some solid power capacitance close to it. So I changed C28 to a 1000uF one. I have not used a cap this way before so I was not sure about the effect. The truth is that I did not hear any noticeable difference. I will revert it in a few days to check again.


TODO:

[ - ] Reworking the entire gain structure

[ - ] Experiment with R27 and R28 to see how lower or higher rails on the preamp could make the amp's volume range more usable.

[ - ] Add some soft clipping diodes to get some mild clipping when digging in. I will try some stuff, but I am not 100% sold on this. A good pedal in the front or in the FX loop makes more sense.
This again will be part of a bigger rearrangement of the gain structure

[ - ] 100nF ceramic bypass capacitors between the power rails on all opamps. This might not be critical, but I have seen a big difference in other applications, so I plan to try it. It's cheap and very easy to do on the underside of the pcb. It could make a bigger difference if you plan on using the amp on higher gains, especially on the first stages.

[ - ] In general, experiment with PSU optimizations, like CRC, better rectifier diodes, a quasimodo snubber, perhaps a bigger transformer with lower V to reduce the Wattage and make it more home-friendly

[ - ] General experimentation with different caps and resistors on various positions. I am not going fancy, just going to try different parts that I have in my stash. Changing the gain might not be easy without changing the voicing though.
 
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bluescaster72

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Have you finished this project or is it still a work in progress? I have a ten and did the LED mod which really opened the amp up. Overdrive pedals really work well though it I think. The clean channel really favors strats for some reason. Keep us posted I'm interested in hearing how difficult this is .
 

dimkasta__

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It's still ongoing, although at this point I am probably in "going too far" territory

It's not hard to do most of these if you are handy with a soldering iron. The biggest issue for most people will probably be desoldering the old components. The pcb is very crappy and it's super easy to lift a pad if you don't have proper tools.

I have rearrangend the initial post a bit to make things clearer

The easiest wins that are super easy but still have a very profound effect are
removing the leds, removing C12 and adding a socket to use it with a cab
 

NTC

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The tone stack is more like a Fender or Marshall topology.

Nice work! I like different, including no tubes...
 

dimkasta__

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The tone stack is more like a Fender or Marshall topology.

Yeah the p10's pcb is a weird fusion. It has the left leg resistor R11 going to ground like old-school Vox, but follows the 90s vox layout on the bass pot, which is used as a variable resistor instead of a divider. All modern ACs and the 15r follow this fenderish layout.

And since I could not do old-school Vox directly without messing with the pcb, for simplicity and because I was a bit lazy, I went full modern-vox-style

For those that are fearless and want to go full old-school, you will have to find a way to separate the wiper on the bass pot and make the relevant connection. In which case you can also try to include how the pots track, by taking the tone stack of your favorite AC amp, and then accommodating for the 50k pots by just dividing each resistor by 20 and multiplying each capacitor by 20. You will have the same curve but a few dBs louder unless you also handle the output of the previous stage and the loading of the next stage to match those changes. (suggestion is courtesy of the awesome ppl at diyaudio)
 

gusfinley

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Nice write-up!

I keep hoping to find a good priced used Pathfinder 10 on local online classified ads. So far no luck.

I have toyed with the idea of turning one into an AC4. With the help of 3D modeling, I have found that the cabinet is Just big enough to house the power tube.

The Pathfinder 10 is small enough to act as a head. It would be interesting to test it out with different speakers - the 6.5" speaker has got be its major tonal weakness.
 

dimkasta__

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Cool idea. Yeah there should be plenty enough space, especially if you remove the speaker. I am thinking something similar for my Champion 40
 

dimkasta__

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Oh that fits so nicely :) What about the trafos? Do they fit inside? Or at the bottom?

What's the serial of the chassis? I might need one soon :)
 

dimkasta__

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For anyone looking for a clean slate chassis, the code is 1444-18 and it seems to fit exactly

wrong measurement
 
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gusfinley

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For anyone looking for a clean slate chassis, the code is 1444-18 and it seems to fit exactly
I'm not sure about the chassis height - I know that one of these in a Pathfinder 15 cabinet will have a slight gap between the top of the chassis and the cabinet.
 

dimkasta__

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I'm not sure about the chassis height

My bad, I measured in a haste from the front. The back has some extra mdf, so 1444-18 won't fit.
P10's metal chassis is 300x100x51mm, but the back gap in the cab is 309mm. The top opening for the panel is 240mm, and the opening at the back is around 200mm tall.

So there are some options but nothing that fits perfectly, unless I don't see the full catalog. For most we would have to cut a new mdf cover for the back, since they are wider than 100mm.
 

Long Tom Cash

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Interesting mods. I am glad you are sharing them. I am curious about your tone stack mods if they made much of a difference.

I have done a few mods to my Pathfinder 10 and even bought a second one, which I later sold, to use as the control amp. Since I don't have any formal training in electronics I just tried to follow what I could find on the web.

I've made the following changes:
Added an output jack
C3 470n
C6 470n
R4 2.2K
R5 82K (wanted to try 91K but did not have one)
R6 3.3K
Removed the two red LEDs in the hard clipping circuit.
Created a soft clipping circuit: Removed R7 and jumped it to a mini board with 470K and 220K resistors, the two red LEDs I had removed, and two 1N4448 diodes (see attached diagram)

It still sounds close to the control amp in clean mode but now no more fizzy buzz when the overdrive button is pressed. I also put in a one-off 6.5 heavy alnico speaker from PMR/Hempopotamus just because I've never owned an alnico speaker before. I have some specs but not the magnet weight.

I have been wanting to modify my Pathfinder 15R to add foot switch for gain and reverb but that may be above my skill level. I also saw an effects loop mod that I may want to try but I might have someone in town do it for me. I've already done a simple reverb mod to it -- I added a larger tank and replaced a 47K cap with a 100K. The schematic shows 150K but I tried it and it seemed like too much so I went with the 100 that some other people said they used. I would also like to add some kind of switching to change from the built-in short decay tank to the Revisit long decay tank.
 

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dimkasta__

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I am curious about your tone stack mods if they made much of a difference.

The difference was quite obvious. Below you can see the response of the original P10 tonestack. If you compare it to the modified above you can see that the difference between mids-highs-lows is much more pronounced now. From ~7db to ~12db.

p10.jpg

C3 470n
C6 470n
R4 2.2K
R5 82K (wanted to try 91K but did not have one)
R6 3.3K
Removed the two red LEDs in the hard clipping circuit.
Created a soft clipping circuit: Removed R7 and jumped it to a mini board with 470K and 220K resistors, the two red LEDs I had removed, and two 1N4448 diodes (see attached diagram)

The first changes mainly affect the gain of the first stage at different frequencies. From a quick look, you should be getting more bass. But at the same time, because this is before/in the distortion stage, you might be getting some fuzz-like effect. I would imagine it something like how Boss BD-2 sounds if you get the gain too high.
Your soft clipping is interesting at R7. I will check the arrangement at some point.

It still sounds close to the control amp in clean mode but now no more fizzy buzz when the overdrive button is pressed. I also put in a one-off 6.5 heavy alnico speaker from PMR/Hempopotamus just because I've never owned an alnico speaker before. I have some specs but not the magnet weight.

Cool. But as said above, keep an eye on the bass from the first stages. If you go too high in gain, you might get some fuzz effect. Not that fuzz is bad if it's what you want.

About the speaker, TBH I do not know if a 6.5" one is worth the effort and money. An external cab or 12" speaker makes a huge difference. I have kept mine as a test speaker mainly.
 

dimkasta__

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And a short update for my own progress

I am currently going tooooo far. I will open a new thread soon since this is not a mod any more.

I have designed new PCBs and a new face plate. Unfortunately PCB fabrication takes too long. I already have newer versions of this. I just have to balance time, flexibility and cost.

And a teaser

Screenshot_.png

IMG_20220623_130156.jpg

Screenshot_(7).png
 

dreamingtele

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These posts make me wish I know how to work on amps and circuitry. Alas, my trade is working on designing roads and highways, but would love to be able to do this too!

Excited to hear this in its final form!
 

dimkasta__

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Thanks :) That's sooo encouraging :)

TBH I am a web dev myself. This is what keeps me sane during those weird times. And TBH I spend more time tinkering than playing guitar. If you can call all those half-learned riffs "playing the guitar" :D
 

acrvr

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I'm really considering buying a pahtfinder 10 just so I can try these mods. Love tinkering.

I have a pathfinder 15r that I'm lending to a nephew so he can learn electric guitar.
 

Long Tom Cash

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The difference was quite obvious. Below you can see the response of the original P10 tonestack. If you compare it to the modified above you can see that the difference between mids-highs-lows is much more pronounced now. From ~7db to ~12db.

View attachment 996846



The first changes mainly affect the gain of the first stage at different frequencies. From a quick look, you should be getting more bass. But at the same time, because this is before/in the distortion stage, you might be getting some fuzz-like effect. I would imagine it something like how Boss BD-2 sounds if you get the gain too high.
Your soft clipping is interesting at R7. I will check the arrangement at some point.



Cool. But as said above, keep an eye on the bass from the first stages. If you go too high in gain, you might get some fuzz effect. Not that fuzz is bad if it's what you want.

About the speaker, TBH I do not know if a 6.5" one is worth the effort and money. An external cab or 12" speaker makes a huge difference. I have kept mine as a test speaker mainly.
I have used my PF10 with both 10" and 12" cabinets and you are correct that it sounds much better than the factory Vox speaker.

I usually keep my PF10 at work so the 6.5 alnico speaker works great for me (see pics) and sometimes I forget it's such a small speaker. The other day no one was on campus and my office is kind of isolated, so i cranked it up and yes, you are right about the bass fuzz, good catch, though I only noticed it on the clean channel with the gain turned up somewhere past noon. I'll probably replace C3 with the original 220n.
 

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dimkasta__

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Slow day at work today, so I had some time to experiment.

There are two interesting parts in the first gain stage.

The first is the capacitor C5 and how it interacts with the gain pot. At first glance it looks like a coupling cap. But it's in the feedback loop, and interacts with the pot. As the gain increases, the upper part of the pot divider decreases, and the cut off frequency of the high pass filter is increased (less bass passes the filter). But because the filter is in the negative feedback loop, this means that low frequencies are increased at the output. This works exactly like a resonance control. But the problem is that bass increases with gain. Which is quite counter intuitive, especially at higher gains, unless you like fuzzy distortion. Removing C5 removes that weird interaction and high gain sounds tighter.

The second area is C4, R4, R5 and the switch. The switch acts exactly like turning up a presence control at high gain. R4 sets a minimum resistance so that things do not go too far. Ideally, we would replace R5 with a 100K or 50K pot wired as a variable resistor, and then remove this pole of the switch so that we have full control both at high and low gain. This would give us a very versatile presence-like control.
 




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