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Author Topic: Vox Pathfinder 10 mods  (Read 21758 times)

bluesky6

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Re: Vox Pathfinder 10 mods
« Reply #15 on: May 11, 2014, 10:59:32 PM »
Here's a schematic of the mod.

I thought it was easier to hand-draw and scan than to learn LTSpice or some other "modern" tool.   xP

GE means Germanium diode.

ehsan_zt

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Re: Vox Pathfinder 10 mods
« Reply #16 on: March 24, 2016, 04:32:50 PM »
Hi, sry for bumping this topic, but I have some questions regarding these mods, and if I wanted to post a new topic I had to reference to this topic more than once :loco

It's little confusing, does paralleling a 1M resistor to R7 has any effect or not ? (reducing gain when we are in overdrive channel)
Pot labeles on my amp are: B104 for gain,ALPHA B50Kohm for treble, A50Kohm for bass and ALPHA B50Kohm for volume. I think I need a log taper for volume,and since I can not find any log pots where I live, can I swap bass pot with volume pot ? (won't there be any problems with bass ? )
Should I use anti log pot for gain ? if so,how can I change a linear pot to anti log ?

blackcorvo

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Re: Vox Pathfinder 10 mods
« Reply #17 on: January 27, 2018, 03:15:10 PM »
Answering these, as I have some things to add to this topic. Thought it was better than making a whole new topic.

It's little confusing, does paralleling a 1M resistor to R7 has any effect or not ? (reducing gain when we are in overdrive channel)

Paralleling resistors makes the value lower. When they're the same value, you simply divide the value of the resistors by the number of resistors. So, 2 1M resistors would be, considering 1M as being 1000k ohms, 1000k / 2 = 500k.

Pot labeles on my amp are: B104 for gain,ALPHA B50Kohm for treble, A50Kohm for bass and ALPHA B50Kohm for volume. I think I need a log taper for volume,and since I can not find any log pots where I live, can I swap bass pot with volume pot ? (won't there be any problems with bass ? )

Yes, if you swap the Bass and Volume pots around you'll have better control over the Volume, but then you might not have great control over the Bass (or at least not as gradual). Try it out, worst case you'll have to swap the pots back.

Should I use anti log pot for gain ? if so,how can I change a linear pot to anti log ?

That's not necessary, the gain pot is pretty responsive as it is.

---------------------------------------------------------------------------------------------------------------------

So, besides the responses, I just wanted to add to this topic by mentioning some mods I've done to my own Vox Pathfinder 10.

Here's the schematic with the mods: https://drive.google.com/open?id=1cmLku3r3WP6RrpEsWeMjAIwxlQHuqlLA

So, what these mods have done for me:
- Changed C6 from 220n to 470n to up the bass response a bit, since I like using a treble booster with this amp;
- Removed the Red LEDs from the board and placed 2x White LEDs soldered directly onto R7, for a less harsh distortion even on "clean";
- Changed R8 to 100k, to get a bit more gain on "clean", which coupled with the White LEDs, sounds sweet;
- P3 was actually not a change due to seeking better tone, I bought this amp used and the Bass knob wasn't working, so I changed it with what I had at the time and never changed it back;
- Reducing R5 is just an alternative/suggestion to increasing R8, which I haven't tried (yet).

And here's a few sound samples of slightly different configurations. The guitar I'm using is a cheap SG copy with it's original pickups, and a self-made "Brian May Treble Booster" pushing the amp, all with the amp's overdrive switch On:

https://drive.google.com/open?id=1U020qi_7YmRRCM21xxvB2zgSsClibI6O
https://drive.google.com/open?id=1ztU0C0BBcWrmXaOP1azp4cRmpcetUanL
https://drive.google.com/open?id=1hApyLMedKybXe9PxBCcmxcbIyK1f-xOU

I'm really loving how it sounds now! Even without the treble booster, it sounds pretty sweet!
« Last Edit: January 27, 2018, 03:51:23 PM by blackcorvo »

j_flanders

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Re: Vox Pathfinder 10 mods
« Reply #18 on: January 30, 2018, 05:41:43 AM »
Here's the correct schematic by the way. (Edit: Sorry about the large pictures. There's a scrollbar at the bottom of the post to see the right most part of them.)
Green means 'correct component value', red arrow means 'the component was in the wrong place' or 'incorrectly connected'.

One correction not included is about the note: "Gain switch shown in Clean position"
The schematic shows one part of the switch in clean position, namely the diodes taken out of the circuit, but the other part shows it in distorted position, namely R4, which should not be connected to ground for Clean.



Here's the circuit board tracing.
Red is signal, black is ground, grey is power supply to opamps. White components are resistors, greens are capacitors.
I was only interested in the preamp section, so I stopped at C19, just before the power amp section.


I love this little amp. I've been playing it daily (nightly actually) for the past two years or so.
The clean channel is perfect, the overdrive channel not so much. I tried a ton of mods all over the preamp section. I'll post about them later on.
« Last Edit: January 30, 2018, 05:53:37 AM by j_flanders »

blackcorvo

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Re: Vox Pathfinder 10 mods
« Reply #19 on: January 30, 2018, 05:57:27 AM »
Here's the correct schematic by the way.

Thank you for the revised schematic! I didn't even notice the drive switch position thing until you pointed out. It's also wrong in my modified schematic... whoops!

I'm now thinking about possibly adding an FX loop to this amp, but I'm still debating how I should approach it.

j_flanders

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Re: Vox Pathfinder 10 mods
« Reply #20 on: January 30, 2018, 06:59:44 AM »
So, what these mods have done for me:
- Changed C6 from 220n to 470n to up the bass response a bit
Must be only a tiny bit as C6 (220nF) is in series with C5 (4,7nF).
Caps in series are like resistors in parallel, so the total capacitance of stock C6 and C5 is 4,601nF.
Changing C6 from 220nF to 470nF would result in 4,653nF, a change of 53pF or a 1% increase...

Now, C5 is in parallel with the upper part of the gain pot, so in reality things are a bit more complex and variable:
When the gain pot is maxed at 10, C5 is effectively shorted and only C6 is active. Turning down the gain slowly brings C5 into play, putting it in series with C6, lowering the capacitance, increasing the low cut / high boost.

Your mod will have most effect when gain is high and least effect when gain is low.

Changing C5 instead of C6 could give you more reach/control at lower gain settings because stock, it is a much lower value than C6.

Removed the Red LEDs from the board and placed 2x White LEDs soldered directly onto R7, for a less harsh distortion even on "clean";
It's still 'hard clipping' to ground, as these go to the inverting pin, but since they are in parallel with the feedback resistor R7 and cap C7, I do agree that it sounds a little less harsh. They will clip in clean mode, but you will have to turn up the gain considerably and by then you're also bosting lows a lot more, resulting in a flatter pre-clipping frequency response, which is a large part of the reason it sounds less harsh, more balanced.

Those green wires soldered to the back of the board you see on the PCB tracing picture was a similar mod, me trying different diodes, resistors and caps in the feedback loop.

blackcorvo

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Re: Vox Pathfinder 10 mods
« Reply #21 on: January 30, 2018, 07:32:05 AM »
j_flanders, I have only a crude understanding on filters, but in my simple terms, what I see with C5 is a high-pass filter, not a coupling capacitor.
The difference lies in the resistor (potentiometer in this case) in parallel with the capacitor, as it "leaks" the frequencies that can't get throught C5. What I did by changing C6 was allow more of these lower frequencies throught to the next stage.

About the clipping, the amp came modded with a Si+Ge clipper, so poorly added in there that the copper foil was coming off the board. I originally put it back to the stock red LEDs, but after having connection issues, I added screw-in terminals to that point of the board for testing. They're still there, in case I wanna mess with that in the future.
I decided for the soft clipper because I don't like how the op-amps sound overdriven. There's a harsh change between clean and driven tone I don't like, and using white LEDs (which take like 3v to conduct) that harshness is reduced a lot. I haven't tried mixing a hard clipper in there yet, but I don't feel it's necessary at this moment.

ehsan_zt

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Re: Vox Pathfinder 10 mods
« Reply #22 on: March 05, 2018, 02:39:23 PM »
Thanks blackcorvo for answering my questions. I'm tempted to try some of your suggestions.

I too have some things to add:
1- I've been mainly experimenting with drive channel and trying different diodes,I've made a topic over diystompboxes :
http://www.diystompboxes.com/smfforum/index.php?topic=114013.msg1057967#msg1057967
I've found out the most effective way to make drive channel better, without altering too many things is diode limiter (check above link). You can get a nice transition from crunch to distortion.

2- Another interesting thing I found out was this video:
https://www.youtube.com/watch?v=rRAv76ylAcI
what this guy did is he simply removed C12 2A274K!
I tried this,and it's AMAZING ! when you remove C12, amp will sound clearer and there is an increase in volume. I really suggest trying this and removing it or replacing it with something lower (like 2A272K).
I'd love to know the science behind it,if some one can explain?!

3- I changed my volume pot(B50K) with my bass pot(A50K),now volume is more versatile specially when in drive mode. It's a simple effective tweak. :dbtu:

blackcorvo

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Re: Vox Pathfinder 10 mods
« Reply #23 on: April 11, 2018, 08:16:09 AM »
2- Another interesting thing I found out was this video:
https://www.youtube.com/watch?v=rRAv76ylAcI
what this guy did is he simply removed C12 2A274K!
I tried this,and it's AMAZING ! when you remove C12, amp will sound clearer and there is an increase in volume. I really suggest trying this and removing it or replacing it with something lower (like 2A272K).
I'd love to know the science behind it,if some one can explain?!

C12 is part of a filter on the second op-amp. It goes in parallel with the feedback resistor R15, but isn't completely across the feedback loop because of R17.
I believe it's a low-pass filter (again, I'm not super well versed in filters), which would explain the change in volume. I'd guess it's there to remove some of the harshness coming from the preamp, and possibly to limit the volume some before it hits the power amp.

Personally, I'd rather play around with the filter instead of just removing that capacitor. You might be able to emulate a cab if you do some clever use of that op-amp. Perhaps copy the values from something like RunOffGroove's Thor output circuit? Or more appropriately, their Britannia circuit? Food for thought.

blackcorvo

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Re: Vox Pathfinder 10 mods
« Reply #24 on: April 22, 2019, 02:20:12 AM »
j_flanders, thanks again for the photo of the board. I decided to do a simple diagram on it of how to do the mod. I'll leave the coupling cap and bass pot I changed out of it, as I don't think it changes much in regards to the overall response of the amp.
I decided to recommend removing R8 along with the Red LEDs, as I think it muffles the amp too much.

https://imgur.com/qZBD5JF

phatt

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Re: Vox Pathfinder 10 mods
« Reply #25 on: April 26, 2019, 04:05:31 AM »
j_flanders, I have only a crude understanding on filters, but in my simple terms, what I see with C5 is a high-pass filter, not a coupling capacitor.
The difference lies in the resistor (potentiometer in this case) in parallel with the capacitor, as it "leaks" the frequencies that can't get throught C5. What I did by changing C6 was allow more of these lower frequencies throught to the next stage.


Correct,, C5 is almost irrelevant for bass response going to U2, C5 is just a treble bleed for lower gain settings. C6 sets the low freq roll off point that gets to the next stage.
Phil. :tu:

 

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