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Author Topic: Vox Pathfinder 10 as a modding platform  (Read 657 times)

dimkasta

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Vox Pathfinder 10 as a modding platform
« on: May 09, 2021, 05:52:05 PM »
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 very nice. Plus it's a DIYer's dream, being a full old-school circuit with a transformer PSU and all through-hole parts.

The schematic has been around for almost a decade, but there still might be things that need to be fixed.
https://imgur.com/gallery/IneD17t

My goal is to have a nice super-clean platform to use with my pedals. I do not expect it to clip or sound like a tube amp on its own, but it can sure sound awesome with pedals.

So here is the list of my TODO stuff, gathering info from around the internet, plus results so far. Ssguitar and tdpri forums have much useful info.

[] Remove the hard clipping LEDs. This is the first mod 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.
This brings me to my second recommended mod.

[] Change the linear volume pot P4 to one with an audio taper. The difference is more evident without the boost on. When you engage the boost, the amp still goes loud fast, but you get some more volume resolution than before.

[] Add a 1/4 jack to allow connection to a CAB. This is super easy and very straightforward.

[]  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 and types here to suit your taste, but without it, you get a nice percussive and super-rich sound that is almost JazzChorus-like.

[]  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.

[] 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.

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

[ - ] TODO: 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.

[ - ] TODO: Eliminate the boost circuit and repurpose the switch to control the clipping diodes.

[ - ] TODO: 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.

[ - ] TODO: Add a simple series FX loop right before the power amp. This should be easy with two switching jacks. And we probably do not need a buffer either.

[ - ] TODO: 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

[ - ] TODO: 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.
« Last Edit: May 14, 2021, 04:32:10 AM by dimkasta »

phatt

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Re: Vox Pathfinder 10 as a modding platform
« Reply #1 on: May 10, 2021, 07:18:18 AM »
If you can find space on the panel you can always replace R7 with a pot and re-solder the Leds.
That will give you much more control on how hard the leds (Or diodes if you prefer) activate.
I've never had this amp so just a thought.
Cheers, Phil.

mguzzo

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Re: Vox Pathfinder 10 as a modding platform
« Reply #2 on: May 11, 2021, 12:24:36 PM »
lots of good tips there. I haven't modded any of the circuit on mine yet. I did install a Celestion 8" in place of the 6.5" bulldog speaker. It fit quite well in the cabinet. My son has been using it with his string bass for small gigs (church, musical pit), the clean channel and speaker handle the string bass tone quite well in those small settings. My plan is to replace the 2030 with a 2040 and put in a larger transformer to drive a little more wattage.

what's the difference between adding a 1/4" for driving an external cabinet v. the headphone/line out jack?
« Last Edit: May 11, 2021, 12:30:49 PM by mguzzo »
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phatt

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Re: Vox Pathfinder 10 as a modding platform
« Reply #3 on: May 11, 2021, 06:12:22 PM »
The Difference between the 2 chips is not worth the effort.
Your ears would hardly be able to notice the Difference between 10 Watts and 15 Watts.
Those chips will now likely be hard to find anyway so *Just buy a bigger amp*.
Re the headphone out. Well it's exactly what it says,,
for driving headphones,,, different Z and Voltage.
If you plug in a speaker you won't hear much. :-[
Phil.

joecool85

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Re: Vox Pathfinder 10 as a modding platform
« Reply #4 on: May 12, 2021, 02:41:17 PM »
Great job!  I love modding these small amps, plus the Vox Pathfinder 10 looks mint!
Life is what you make it.
Still rockin' the Dean Markley K-20X
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dimkasta

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Re: Vox Pathfinder 10 as a modding platform
« Reply #5 on: May 13, 2021, 09:03:47 PM »
My plan is to replace the 2030 with a 2040 and put in a larger transformer to drive a little more wattage.

As Phil said, it's not really worth it. The 2030A is already at its max with +-20V, being able to give around 18W. That's plenty loud for a guitar unless you are playing big venues, in which case the limit is the speaker.

The easiest and probably the most effective way to get more output is to add the output jack and hook it into a cab with a more sensitive speaker. Even a 2x12.



LM1875 could be a good candidate to get more power. But it will need a new PSU and probably a beefy heatsink. For now, this is probably "going too far" territory for me.

For bass, the preamp is too mid-focused. Perhaps the FX loop would be a solution for you so that you can bypass the preamp with its highpass filtering and drive the output directly with a bass preamp or pedal. But to be honest for bass I would just get a fender rumble and call it a day.

what's the difference between adding a 1/4" for driving an external cabinet v. the headphone/line out jack?

The headphone output has series 1W resistors that limit the power to levels adequate for headphones.

dimkasta

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Re: Vox Pathfinder 10 as a modding platform
« Reply #6 on: May 14, 2021, 04:33:42 AM »
By the way, about R8, I just restored it and I will probably keep it as is

I also added another TODO point, to experiment with R27 and R28 to see if I can get a more usable volume and gain range