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Vox pathfinder stripped

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Rutger:
Hi,

My first guitaramp was a Vox Pathfinder that I really liked, but sold it to buy something else (as usual). I'm still thinking of it once and a while, and wonder if I could build the preamp and stick it into my diy combo? I allready found the schematic in this thread (you need to be logged-in to download it).

The thing is, I would only like to build the 'normal' channel of it. So I don't need the boost or the tremolo, nor the poweramp (for I allready have one). I can do an educated guess but I'm not that good at reading schematics to say what parts exactly can be left out and what needs to stay in. Can you help me out please?

n9voc:
Just a quick observation:  The Schematic (attached) shows only one input CHANNEL, it only goes into distortion when the "gain" control is "cranked up".

That being said, I suggest the following changes on the "top row": from left to right - cut right after R5 (22k) and right before C11 (4.7/50).  Drop the entire section cut off your new schematic.  Join the two sections of the "top row". Eliminate LED1 and LED 3.  Combine the values of R15 and R16 into one resistor of 1.5 kohm. Cut right before R19 and after C18. Drop the "cut section" out. Join the two sections together.

These mods lower the overall gain, eliminate the crunch diodes and remove the tremelo from the preamp.  It may need further "tweaking" - but you at least have one opinion on what to do now!

Good luck!

Rutger:
Thanks!

I'm sorry for the confusion, my bad. The amp has just one channel, but I tried to describe that I just want the preamp without the extra's like the boost (hate it) and tremolo.

So the 'second stage' represents the boost? But how does it "switch in", for this stage is always in the signalpath as far as I can tell.
I thought that the LED's are there for the overdrive, or does this preamp use IC clipping? I really like the amp with a little od.

The 'middle section' of the schematic is the tremolo, am I right?

n9voc:
You are correct in that the "second stage" is always in the circuit.  However, when the boost switch is NOT engaged, Q1 (the FET in the feedback path to the second amplifier stage) is turned "on" making the feedback resistance equal to approximately 22 kohms (value of R8) - effectively superceding the value of R6+R7.  The gain of this stage (as all op amps) is determined by R(feedback)/R(input)  - the input value in this case is approximately the value of the 22k resistor right before the capacitor attached to pin 6 of this IC.
All that being said, with the boost switch "off" the gain of this amplifier stage  is approximately unity or "1".  Turning the boost switch "on" shuts OFF Q1, and the gain then becomes  (150k [R6] + 470k [R7])/22k [R5] or (to work the math) approximately a gain factor of 28.  (HUGE difference!)

Indeed the LEDs work in the Overdrive drive function.

The Tremelo is the center portion of the circuit, which is coupled to the preamp by the optomodule (note between R19 and C18).

So, all that being said -  and to keep all you like, losing ONLY what you don't the modifications become easier:
Remove Q1.  Place a wire in the spots formally occupied by Q1 drain and source.  Remove the optocoupler,  run a connection between R19 and C18 (where two pins from the optocoupler used to be.

Now, you have the preamp without the boost, and without the tremelo.  In circuit form, you have set the Trem setting to ZERO, and the boost switch to "OFF" permanently.

Good luck and happy building!

Rutger:
Thanks a lot, you're a great help! And thanks for explaining the circuit.
I'm glad that it's not that hard after all.  :)

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