Vox Pathfinder 10 mods

[bluesky6]

Just got myself a Pathfinder 10 (the Pathfinder 15/R are going for way too much money for a mod platform).

After playing with it a little bit, I found that at high gain/distortion, there is a lot of speaker buzz on the little 6.5". No buzz with an external 12" cab. So I'm thinking of putting in a Jensen MOD6 to see if things improve. The Eminence 620h is tempting but costs more than the price of the amp...!

Question for those who have tried both the 15/R and 10: is it worth tweaking the 10 to recreate the pre-amp of the 15/R tone-wise? From the schematics, the two seem to have very similar gain and tone stages.

Other mods I'm thinking of:
- Change the gain resistor of the second stage to reduce the gain when the drive switch is on
- Use an analog taper pot for volume
- Zeners to "control" the supply voltage for the op-amps
- Remove/short the headphone resistors and use the headphone jack as a speaker out jack

[Roly]

Hi bluesky6, welcome.





A six inch speaker is worth replacing just because it's six inch, and a buzzy six inch speaker ...

- Change the gain resistor of the second stage to reduce the gain when the drive switch is on

That's R7 1Meg.  The overdrive from this stage is clipped by the following back-to-back red LED's.  You may find some tonal advantage in playing around with one of these LED's trying one with a different forward voltage (generally a different colour) with will give you asymmetric clipping of +ve and and -ve parts of the signal, which can produce a nice 2nd harmonic.

- Use an analog taper pot for volume

All pots are by their nature analog - I think you mean "audio" taper or log pot, which the volume control should already be.  Set it to 5/10 and measure the resistance from the wiper to each end.  If it's a linear taper then it will be about 25k to each end, but if it's quite different then it's already a log pot.

- Zeners to "control" the supply voltage for the op-amps

Do you have a reason/problem?  If you don't actually have a problem then I wouldn't bother, most op-amps have about 120dB of power supply rejection.

- Remove/short the headphone resistors and use the headphone jack as a speaker out jack

That should work, although you could always mount another socket somewhere for an external speaker and retain the headphone/Line Out option.

HTH

[bluesky6]

Roly, thanks for the suggestions and the schematic.

I'll look into replacing the leds, either with a single one of different color or with a couple of germanium diodes. There's a blog article where someone used a pair of 1N34As.

The volume control pot is a B50K which I assume means that it's linear as opposed to audio taper. An alternative to replacing it (since it's probably hard to get an exact physical replacement) is to replace R18 with a large value.

My problem is that of too much volume (hearing sensitive spouse and all that) 

Other mods I'm thinking of:
1. Move the AC circuit from the main PCB and use a fuse holder instead. Will mean drilling a hole in the chassis. This will make removing the PCB easier for tweaks.
2. Remove all the original audio amp components and use one of those PSU+AMP TDA2030 boards from Eb*y. These boards include power rectifier and amp. The question is of available space...
3. Putting in sockets for the op-amps to try out the lower noise ones

I'm going to try to reduce the noise level as much as possible so that if I don't do the speaker out mod, I can use headphones without the hiss. I've a Vox AC30 Amplug and all I hear (with gain >2) is hiss.

[BigPolishJimmy]

I've been curious about the pathfinder 10.  How do you like it so far?  Obviously you're looking to mod it, but is that more from dissatisfaction where it is lacking, or more from wanting to customize it to your liking?   

[bluesky6]

I've been curious about the pathfinder 10.  How do you like it so far?  Obviously you're looking to mod it, but is that more from dissatisfaction where it is lacking, or more from wanting to customize it to your liking?

To be honest, I spent about 20 minutes playing through the amp before taking it apart. While the cleans were okay (I have a Fender FM65R as a comparison...!), I didn't like the drive channel, even using my external 1x12 cab. Hence the thought of reducing the gain on the 2nd op-amp. But that's my personal preference: I like a dry compressed crunch without too much fuzz.

I did find that, like some others, using the gain as some kind of volume control worked pretty well.

[bluesky6]

Picture of the innards.

I don't like the incoming AC having to go through the main PCB.

The power amp on a separate PCB with a long 5-wire pigtail also looks nasty. The schematic Roly posted indicates a ferrite core but not on the amp that I have.

The big positive is the modability of the board. I have a Behringer GM108 (now for sale) which I wanted to mod but gave up on the idea because of all the SMT components.

[BigPolishJimmy]

Thanks for the reply, and the pics.  Wow, I'm blown away at the how nice the board looks.  Honestly I'm surprised at that price point that there are serviceable components in there.  It really seems to lend itself to modification. 

I wonder if these will develop the hiss problems that the valvetronix series had? 

I'm going to have to go play one to see what's what about the sound.  I play mostly clean-ish and have a pedal for when I want distortion, and when I want distortion I want sustain for days and I get that from my pedal

Thanks again for posting this and humoring my threadjack

~Jim

[Roly]

bluesky6

The trick with clipping diodes is to make them different, that's what gives you 2nd harmonic.  The LED's that are in there are clipping around +/- 2-3 volts.  A germanium diode will clip around 0.1V so you may have a problem getting enough signal into the output stage.

You can't assume anything from the taper code letter.  A fairly orderly system in the 1960's has been well corrupted by everybody baking their own cake.  These days the only way to be sure is the test I mentioned, but I'll be surprised if you don't find that whatever the code letter, it's already a log taper.

Trade spouse in on one that is a) deaf or b) mad for RnR.  My third one is both. 

1. Getting the mains off the PCB and well insulated is a very good idea, particularly if you want to tinker inside.   

2. that's not a "mod", that's a transplant.   

3. there are lower noise op-amps than the 4550's, but you soon strike the law of diminishing returns and that you won't notice the difference between modern cheapies like LM833's, TL07x-series etc, and das zupervundendorfer's at $$$$ each.  If you can't hear it fully cranked with your ear against the speaker then making it 20dB quieter is pretty pointless.

- most hiss originates in the first stage

- reducing preamp hiss lower than the inherent hiss of the power amp is pointless.

I wonder if these will develop the hiss problems that the valvetronix series had?

That's pretty unlikely in a fully solid-state amp.  Hiss generally comes from using a cheap op-amp in the first stage.  Spend an extra buck and quite large and lasting improvements can be made.


ed typotypo

[bluesky6]

I just ordered several sets of op-amps (2132, 2134 and tl072). One option I have for the amp is to use it primarily with headphones (due to aforementioned non-deaf spouse). So step one will be to see if I can reduce the noise level.

I have also been researching power transformers. Again, with the goal of reducing the volume, I was thinking of using a 10-0-10 transformer. This will "naturally" provide a lower voltage power supply to the op-amps without the voltage drop resistors. It will also reduce the max power of the audio amp. Or allow me to use a lower wattage and voltage transplant like a TDA7052A or the Velleman 3W amp kit.

I discovered several sets of transformers from Hammond that would just slot in to the existing mount holes.

Guess what: the compatible transformers range from 10 to 12VA power rating.

If the OEM transformer is of the same or similar capacity, that means the amp in factory configuration is really too underpowered to claim a 10W output, considering you really don't have 100% efficiency.

More thoughts to follow.

Roly, thanks for the pointers on the diodes. I'll start my experimentation with an green LED and may play with other types of diodes in series...

[Roly]

I have also been researching power transformers.

Sticking a tastefully chosen power resistor in series with the speaker is a lot cheaper.

If the OEM transformer is of the same or similar capacity, that means the amp in factory configuration is really too underpowered to claim a 10W output, considering you really don't have 100% efficiency.

Power rating can generally be estimated fairly well by the size of the transformer core.

As a general rule an amp will consume a bit more than double it's power output, or put the other way, you aren't going to get more than about 40% of the input power as output.

Your amp doesn't deliver the output power claimed?  That would have to be a first.   

{Maybe I should take my battery powered "400 Watt" computer speakers back to the shop.}

I'll start my experimentation with an green LED and may play with other types of diodes in series...

That's the spirit!  YMMV but others have had pleasing results.

[bluesky6]

Finally put back together the Pathfinder.

Mods include:
1. Green LED (see pic).
2. Jensen MOD 6-15 8ohm speaker

It was fun to play my usual power chord sequence while watching the LEDs. The green seems to clip earlier than the red: my interpretation from seeing the green LED come on earlier and brighter than the red. I can't speak to the improvement or lack-thereof from the LED change as I didn't spend that much time playing on the un-modded amp.

On the other hand, the Jensen speaker is a nice upgrade. Better, richer sound, and very nice chimey cleans. I can tell the difference from my Fender FM 65R.

I still don't like the following:
1. Jump in volume when switching from clean to drive
2. Usable volume when in drive is at the first notch. Yes, the volume control IS a linear taper pot. I measured it as suggested by Roly. An audio taper is on order

In drive mode, gain is good for crunch until 9am. After which it's fuzz overdrive. It's pretty similar to the FM 65R (dry crunch until 4, then fuzz overdrive...).

I'll be using the Pathfinder over the next week to see if the noise level is acceptable. It's definitely better than the Fender 15R (in the other thread): you really don't hear any hiss until you switch to the drive mode. If I don't like the noise level, I'll have questions about op-amp swap 

[Roly]

Ah, had a closer scrute...

First stage op-amp U1A uses the "gain" pot just as it suggests, it alters the gain by altering the proportion of Negative FeedBack (not the amount of signal fed forward at the Master Volume control).

In this case it may well actually require an anti-log pot if you want a smoother transition, or you can use a standard log wired (and working) backwards.

But I'd certainly give the simple audio/log path a go and see how it works.   

[bluesky6]

I managed to "finish" up the mods that I wanted to do on the Vox Pathfinder. It helped that the parts I ordered arrived.

Although not exactly the right parts: the audio taper pot was solid shaft instead of split/knurled. So I have to get a chicken head knob with a set screw in order to use the pot. The other option was to get the "right" pot from Eb*y with an unpredictable ETA.

The other new parts were a bunch of different op-amps. Woohoo!

So changes I made included:
1. Swap out the linear taper volume control for the audio taper. I instantly had a better, more usable volume range. Otherwise, usable volume = first mark on the dial.
2. Soldered a 1M resistor across R7 to half the effective resistance hence reducing the gain with Drive. The theory was to have Gain control with more usable crunch range before fuzz. Result: not really effective.
3. Desoldered the 4558DD op-amps and put in sockets. For newbies considering modding their amps: desoldering wick is your friend
4. I replaced U1 with an OPA2164 and U2 with an NE5532. The latter because U2A was just a post-tone pre-amp so low-noise and clean was important. The former because I'd used the same chip to replace a TL072 on the Fender 15R and I got a "better" crunch from that swap.

I don't have an experience enough ear to really tell the benefits of the op-amp swap. At least, the differences weren't as obvious and I didn't spend any time doing A/B testing before I put everything back together again. But the swap didn't hurt.

I'll use the amp over the next week to see how much more I like it.

[bluesky6]

And... Eureka!

One of the things that folks don't like about the Vox Pathfinder is the usability of the drive "channel". Turn it on and you more distortion than overdrive. To the point where, if Google is fact, the "LED mod" is the most popular mod on Pathfinders: where the LEDs that are used to clip or distort the signal, are removed.

I don't like the default distortion effect and have been looking for a middle ground: to have more adjustment in the gain control between high gain clean, crunch, and distortion.

I think I have found my middle ground. YMMV.

The combination is the following:
1. Swap the volume control for an audio taper pot, replacing the factory linear taper. "Part is Parts" has the right one that will fit the factory chicken head knob.
2. Solder a 1M resistor in parallel with R7 to reduce the gain in the second stage pre-amp. This gives more adjustability to the gain pot.
3. Increase the clipping voltage of the LED pair.

For #3, LEDs apparently clip at 1.6V or more (I've not managed to measure myself, but if Google is fact...). So the trick there, if you want to have more adjustability in the gain control and therefore more headroom, is to add more diodes in series with the LEDs to increase the clipping voltage.

My choice was a pair of germanium diodes which I got from Frys (part number NTE110MP). They had about 230+mV (measured) in clipping voltage. So putting them in series would increase the clipping voltage by that much. For even more headroom, one could use another set of diodes in series or a silicon diode.

Anyway, in addition to the germanium diodes, I added in a 220pF cap in parallel. This is, in theory, supposed to removed some of the high frequency fuzz. Not really noticeable, to my old ears, but the 220pF value was chosen because I thought the original 100pF I used didn't work that well.

Finally, I added a 100ohm resistor in series with the LED+diodes+cap. This is supposed to mellow the effect.

The outcome of all these changes is a much more usable drive mode. More crunch and a very marked reduction in fuzz and fizz.

Pics of the mod are attached.

IMHO, this is better than removing the LEDs all together as you still have an option to go all out in metal distortion mode.

I'll be putting everything back together again and using this over the next few days.

EDIT: Put in lower res pics

[Roly]

"Eureka!"  indeed.  Well done.  Thanks for the report.   

I'll just note that it has been found generally with diode signal clippers that a more musical result can be had by using asymmetric clipping, either different colour LED's (that have different cut in voltages) or more diodes in one leg than the other, giving the same effect.  This produces more even-order harmonics.