Muy Caliente VOX Pathfinder 15/15r Modification

Discussion in 'Amp Central Station' started by SnorkelMonkey, Jun 11, 2010.

  1. SnorkelMonkey

    SnorkelMonkey Tele-Holic

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    Someone recently was inquiring about mods for the Vox Pathfinder 15/15r amps and I had a look see at what could be done to improve the amp a bit. There seems to be a consensus (myself included) that the gain control is, well pretty useless as the distortion is less than favorable. Also the output is just there and not quite up to task to bring to a gig. I looked at the early schematic that's out on the web and found they used a square wave shunt clipper in the form of two LEDs right in the main signal path. Good in some fuzz circuits, I've never really liked them in amps. They have a high insertion loss (loss in gain = output db's) and I've never liked what they yielded as far as distortion when used in amps, certain fuzzes yes.

    Super simple mod here. Unsolder the two red LEDs shown in the photo and remove them for a noticeable increase in gain/headroom and a better sounding breakup. I'm not saying that the distortion is going to be just like yer favorite old tube amp but I found it much more pleasing to the ear than the LED sw clipper in this amp (use a tube screamer for something better if you really can't do without).

    Combine this mod with a Jensen C8r and whoa Nelly! Where the amp was maybe barely able to keep up with a loud drummer completely max out you should be able to keep up with some headroom to spare. I noticed a decent increase in gain ala output db's. Might even make a good small club amp (just did this today yet to try it in those duties)? Report back tell me what you think. Easily reversible but I don't think you will.

    Disclaimer: Make sure you discharge those filter caps. Don't shoot your eye out kid.

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  2. Joe-Bob

    Joe-Bob Doctor of Teleocity

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    Do the LEDS just bleed signal to ground? Well, looks like one of them does, what does the other one do? I don't understand their function in the circuit if they're both connected to a ground.
     
  3. Skrik

    Skrik Tele-Holic

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    LEDs are either on or off. When these are on, the signal goes straight to earth, meaning that the amp output is off. When the LEDs are off, the signal goes through the speaker. Because there is no state between on and off, the audio signal is clipped with a square wave (which is a harsh, artificial sound).

    Or something like that.
     
  4. highwayman

    highwayman Former Member

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    I gig my 2 stock Pathfinders quite a bit.I run an a/b/y box and have them both on at the same time.If you still need more volume mic it.
     
  5. SnorkelMonkey

    SnorkelMonkey Tele-Holic

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    I guess you won't need to mike them now? :D
     
  6. zook

    zook Friend of Leo's

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    Very valuable info. Thanks.

    Winnie
     
  7. SnorkelMonkey

    SnorkelMonkey Tele-Holic

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    My pleasure Winnie, I was taking mine for a spin this weekend and I'm lov'n it. I messed with the circuit some more but nothing beats simply removing those two LEDs. Cheers!
     
  8. Will Chen

    Will Chen Tele-Holic

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    I'm trying the same mod but subbing in some alternative diodes. In my experiments, it seems the boost channel has to be engaged in order for the signal to be significant enough to drive the leds to clip (they have a pretty high threshold in general). Placing a softer clipping device with a lower threshold (bunch of different silicon diodes, mosfets, etc), alters the response of the amp when cranked up to max (moreso when the boost is engaged), but the changes are fairly subtle (slightly tighter low end, increased compression) but the overall character remains the same.

    I also swapped out the power amp chip. Preliminary details here.
     
  9. alexpigment

    alexpigment Tele-Afflicted

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    So if I understand it correctly, the boost is really more of a volume boost than anything, but these LEDs take that volume and cause it to clip prematurely? By removing the LEDs you're turning what used to be extra distortion into extra volume?
     
  10. SnorkelMonkey

    SnorkelMonkey Tele-Holic

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    The LEDs are in the main signal path all the time no matter if the boost is engaged or not. The boost makes the distortion effect more prominent. What's used here is LEDs as a square wave clipper. Square wave (shunt style) clippers have a high insertion loss, meaning they can rob the overall gain of the amp. That translates to a much lower output. Volume loss. They also clip hard as a square wave, more like something you would see with fuzz pedal distortion not tube amp distortion. So removing them boosts the output and IMO makes for a better sounding distortion. I was cranking it over the weekend and it's much smoother but still has some of that ratty edge to it (like I said it's not going to turn it into a tube amp). To my ears cranked up it has a bit of that vintage Vox SS series vibe to it sounding like something off mid-late 60's Stones cut.
     
  11. alexpigment

    alexpigment Tele-Afflicted

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    I was putting two and two together here when I posted. I was really just trying to get clarification that the boost (when no LEDs are present) is more of a volume boost rather than an increase in overdrive. If that is the case, is it safe to say that the boost can be used for soloing or lead when the gain (and therefore volume) threshold has been hit on the "clean" channel? I like to have a little bit of dirt from the Pathfinder involved at all times, but I find that this caps my volume level and I can't do anything pedal-wise to increase volume for lead purposes.
     
  12. Will Chen

    Will Chen Tele-Holic

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    Clipping is basically super hard limiting...so hard that it alters the peaks of the waveform moving towards a square wave. Running a pair of inversed diodes to ground (one for each half of the wave form) after an amplification stage is a fairly standard method of generating "tube sound" clipping in a solid state device. I would disagree that an LED will sound specifically like a fuzz (actually early fuzzes used germanium diodes and most more recent use silicon diodes), that has more to due with the EQ of the circuit which feeds the clippers, where the clippers are located, and EQ stages after the clippers. For example, the famous red led mod of the Boss DS-1 didn't turn that pedal into a fuzz but rather significantly increase the output and low end of the circuit. I believe LEDs actually clip less (ie more headroom, less compression) than pretty much all the other diode clippers.
     
  13. Will Chen

    Will Chen Tele-Holic

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    Without the leds, the circuit still will still overdrive. I believe it is also using opamp clipping (which is the majority of the overdrive you hear). Google "op amp hitting the rails" for a definition...
     
  14. alexpigment

    alexpigment Tele-Afflicted

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    So the OP is claiming that there is a volume boost from removing the LEDs (I presume only when the boost is enabled), but you're claiming that there isn't one?

    Lemme rephrase my question with a real-world example. I have three ODs that I use in front of the Pathfinder. I keep the gain on the PF at 10-11 or so, which is just below breaking up. No matter how much I turn up the output level on my ODs, there is no volume variation after a certain point. I *like* that I can use pedal output volume as a means for more gain, but it doesn't leave me any option for a volume boost for soloing and the like. From what I'm reading here, I would have the ability to footswitch the boost on, which would give me a volume boost, regardless of whether I get additional overdrive or not. Correct?
     
  15. SnorkelMonkey

    SnorkelMonkey Tele-Holic

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    Not to argue with you but where is this a standard in guitar amps? And are you convinced that it provides "tube sounding" distortion?

    You might notice that there is a lot more going on in the DS-1 than just a shunt clipper? Have you had a chance to experiment with shunt clippers in guitar amps before? I have. You do know that they can have a high insertion loss and are a great candidate to be followed by a makeup gain stage? If you're fond of the signal loss that's cool but I'm just showing a mod that people may or may not like, it's subjective.

    LEDs actually are one of the hardest clippers.

    The output is increased both in boost and non-boosted modes because the clipper is located after the boost gain section. Try it yourself, it takes five minutes and you can solder them right back in if you don't like it. ;)
     
  16. alexpigment

    alexpigment Tele-Afflicted

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    I'm sorry, but I feel like I'm talking to a wall here. Please agree or disagree with Will's statement below:

    "In my experiments, it seems the boost channel has to be engaged in order for the signal to be significant enough to drive the leds to clip (they have a pretty high threshold in general)."


    If that's true, it seems to me that the boost channel *will provide extra volume above that of the volume of the non-boosted section*, regardless of whether there is extra distortion also added by the boost.
     
  17. Will Chen

    Will Chen Tele-Holic

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    Pretty much all ss clipping pedalsdo it in one of 3 ways: inverted diodes to ground, diode pair in feedback loop, or opamp drive. Couldn't tell you which is the standard in amps, but I would assume they follow the same methodology.


    Yes of course I realize a makeup gain stage is needed. I'm no expert, but my understanding is a led has a higher voltage threshold prior to clipping, so while their transfer cruve may be harder than other diodes, it takes a high signal to force it to clip. I yield to your knowledge.


    @alexpigment - Removing the leds should increase the headroom of the amp. But theoretically due to their higher voltage threshold prior to clipping, it won't be a night and day difference and shouldn't have a significant affect on the volume of the amp.

    @SnorkelMonkey - Did you try this through the stock speaker or is your comment regarding the increased output a sum of removing the diodes and swapping the speaker? I've got mine rigged right now with wires coming from where the leds are on the board using alligator clips to sub in different components and have experimented with no diodes, various leds, mosfets, and various silicon diodes in symmetrical and asymmetrical pairings and all the variations have been subtle having the most effect when the gain is cranked to max. With the gain at noon or less, I'm not 100% sure I'd be able to tell the any difference in a blind test. I swapped the power amp chip and feel it's louder but have yet to compare it to stock (should be happening this weekend along with A/B comparisons of different slipping combinations versus stock).
     
  18. SnorkelMonkey

    SnorkelMonkey Tele-Holic

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    True but after the mod the "boost channel" will have even more output db's

    Will, I noticed an increase in output volume with a little more headroom without the LEDs and using the stock speaker. The gain is still in your face but IMO it's more usable, and smoother.
     
  19. alexpigment

    alexpigment Tele-Afflicted

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    Ah-ha! Finally, the answer I was waiting for guys. I'll try this mod out for sure.
     
  20. alexpigment

    alexpigment Tele-Afflicted

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    Honestly, not looking for night and day volume. Just want to be able to hit the boost switch for a volume gain for lead and solos. If there is additional gain/distortion added, so be it, as long as it's not crazy amounts.
     
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