Vox Pathfinder 10 mod

[ehsan_zt]

Hi,

I know that this forum is for stompboxes but since I didn't get any replies in ssguitar forum I thought you guys can maybe help me.
I have a pathfinder 10, here is the schematic :

This amp have 2 problems :
A- volume is not very usable, and seems to be linear
B- when I switch to overdrive channel, gain suddenly goes up and altering the gain knob should have transition between overdrive to distortion, but goes to full on distortion even at 9 o'clock.

This topic http://www.ssguitar.com/index.php?topic=3440.0 answered a lot of my questions. since its old and OP's last visit was long ago I've decided to post a new topic.
as mentions in the other topic the main changes I should do is :
A) changing linear volume pot to a log pot.
B) changing gain pot to a anti log pot
Pot labeles on my amp are: B104 for gain, ALPHA B50Kohm for treble, A50Kohm for bass, ALPHA B50Kohm for volume.

1) my first questions is : can I swap bass pot with volume pot ? won't there be any problems with bass (other than it being linear)?
2)Should I use anti log pot for gain ? if so, how can I change a linear pot to anti log ?
3)also for the diode clipping stage I thought of implanting a limiter (diode ladder) like this :

which was posted here: http://www.diystompboxes.com/smfforum/index.php?topic=108510.0
so that I have better transition from crunch to overdrive and finnaly distortion. My question is how can I figure out how many stages I need and what resistor values should I use ?
4)Is there any other way of improving this amp that you can think of ?

I would be grateful to any help you are kind enough to provide.

[ehsan_zt]

BUMP!
[hoping someone could help me out]

[robthequiet]

Quick reaction: The thread you referenced in ssguitar seems to address your questions pretty nicely as far as the problems with gains and levels. As long as your pots are both 50K for bass and volume, the taper does not matter except that it might be more useful to meet your particular needs. Best way to convert a linear pot to audio or reverse audio taper is to buy another pot.

As far as the clipping diodes, I don't have one of these amps to try out but the suggestions put forth elsewhere seem to point towards simply adding other diodes in series to increase the clipping threshold thus hopefully reducing premature fizz. The diode ladder is interesting but you may want to consider the amount of current available to push through a bunch of diodes on that scale. Adding an extra identical red LED in series on either side of your clipper will allow more gain before clipping. Or even putting two red LEDs in series on one side and three 1N4001s on the other might be something to try.

The only thing I would caution about here is trying all the mods all at once. Before swapping pots out, try the diode substitution, then see what you have to work with. The other thing to consider is the use of a buffer or booster in front of the amp that might give you a better gain level to play with.

[ehsan_zt]

Thank you for replying.
Unfortunately I only have access to linear pots... that's why I wanted to replace bass with volume.
In the other topic it was suggested that I should parallel R7 = 1M with another 1M resistor to reduce gain when in overdrive is on, which I did and it did not have any effect on gain !
I have tried using several LED's in series, they sound harsh and not very effective. Gain pot have 10 steps from 0 to full gain. If I set it at 3, having multiple diodes makes it less harsh but if I go over 4 or 5, even with 8 LED's, they all are always on. On lowest gain my multimeter reads 2VAC and on highest gain it reads 11VAC. That's why I thought diode ladder would be better so that I can clip the signal in relative to gain, but I don't know how to change the diode ladder to suit my needs.

The diode ladder is interesting but you may want to consider the amount of current available to push through a b.unch of diodes on that scale.

How can I measure the amount of current available? (I mean I know how to use a multimeter, but I closed my amp and I only have 2 wires for testing clipper diodes, one connected after R9 and one from ground)

[robthequiet]

What I meant was that creating a diode ladder section might create a situation where the diodes happily turn on but may soak more current than just the two you have, not that anything terrible would happen, but as they soak up the current they are also possibly creating some noise. Since it is not a 9V pedal you probably have plenty of available power. What you could try to do is build the ladder with just four stages from the top, then see what you have. If it does not reduce the gain enough, add another stage. As shown in the image put up by composition4, the limiter will hold the level to about 8V. This could be a good project even if it does not solve the Pathfinder 10 problem, of course.

When you have Drive switched off, do you get enough volume from the Gain control to get some distortion or is it just clean?

In order to really balance the stages you may have to just try some of the mods suggested around the web, or do what a lot of people do and use the amp in clean mode with a distortion pedal in front of it which will actually give you more control over your tones, imo. Otherwise, I might suggest a clean booster because it might give you a way to provide a bit more signal for the Gain control to be useful at 1 or 2, and not have such a volume drop going back to clean.

[ehsan_zt]

When you have Drive switched off, do you get enough volume from the Gain control to get some distortion or is it just clean?

It depends, with my single coil pickup it's clean, but with my humbucker there's some distortion near the end of full gain.

I will try composition4 limiter , a quick question though, in vox circuit the resistor in series with signal path before clipper diodes is 220R, whereas in almost all diode ladders it is 1k, so should I change R9 in my amp  or is there a simpler way ?

This amp is pretty loud by default but I have a clean booster, and it can overdrive opamps in clean channel. Opamp doesn't sound so good, I like mild overdrives.

[robthequiet]

Yes, it sounds like this amp was designed to be a screamer. As far as changing R9, we don't know what effect it might have on the tone circuit as well as the impedance between U1B and the tone network, so I would leave it as is. I would start with just working with the diode ladder at first and see if it gets you closer to what you want.

If that does not have the result you need, I think you may want to try an overdrive pedal with the amp in clean mode, with an output level control, so that you can get the type of sound you want without pushing the opamps.

[ehsan_zt]

I ended up building all the stages of the limiter and it sounds good and has a very smooth clipping, but I might need to tweak it a little.
With default vox's red LEDs my DMM measured maximum 2 volts, and now with this limiter it's about 6. I might try a little more compression if I can figure out how this ladder works.

Thank you Rob for helping me out 

[robthequiet]

This is good news. How does the level compare with the clean channel?

[ehsan_zt]

It's much louder. With original Vox LEDs, overdrive channels seems to be 2x louder than clean channel, sound was a lot more compressed, but now with this limiter it seems to be 3x or 4x louder than clean channel and it's not as compressed as before and I can actually hear all the notes in a chord. Now I can really hear the transition between crunch, overdrive and distortion when I turn the gain up. I've found out that an easy way to get more compressed sound is reducing clipper stages. I will try different resistors and see if I can improve it.

I might be wrong here, but as I understand it seems that if you use diodes that their forward voltage is low, and you have a large signal, and you have many diode pairs, you can carefully control and shape how this signal is clipped, and how soft is the clipping knee.

By the way, this limiter is a cool and easy circuit to try out with any project that requires clipping, I recommend breadboarding it.

[robthequiet]

Yes. I had some doubt about how the circuit would work but you have proven that in this example it is a great solution. I have thought about using it in a future project but I would like to simulate it in software as well. My thought is that fewer stages will make the transitions more harsh, because the diode ladder will take whatever Voltage is at the top and drop as much to ground as possible, so the more stages, the better resolution. Much like an analog-to-digital converter. Just as an experiment, I might use a potentiometer between the ladder and ground to see if the compression works in a certain range, and then place it at the top of the ladder.