The Vox Pathfinder 10 is an inexpensive amp (close to AUD $100 new if you know where to look) which makes itt a great platform for modding. This post is mainly for my own records, but also for anyone out there who knows what they are doing. This is the circuit diagram, not drawn by me!
Here are my mods in decreasing order of importance … Please keep in mind that I’m only really interested in the clean channel.
External Speaker Jack
The single most significant modification you can make is to add an external speaker jack! The tiny 6 inch Vox speaker in the cabinet just can’t take advantage of all the power the amp has to offer. I installed a simple jack and added a plug and cable to the internal speaker. Another good idea is to add a switched jack, leaving the internal speaker connected when the external speaker jack is not connected.
Famous R8 Mod
The next mod which will make the amp sound huge is something I’ll call the Famous R8 Mod. You read it here first. Notice when you gently press in the Overdrive button, there is a huge increase in loudness. As you continue pressing in the Overdrive button, the clipping LEDs engage, the sound becomes more distorted, and (depending on the Gain level) the overall volume might in fact decrease. But before the LEDs are engaged, the switch is simply taking R8 out of the circuit. One way to make this permanent is to simply clip one end of R8 from the circuit board. It doesn’t matter which. Voila! Instant loudness. Turn up the Gain control too much though and you’ll start hearing (I suspect) op-amp clipping, which is not as nice as I expected in this case. By changing R8 from 22k to 100k I was able to keep the extra volume comfortably without the extra ugly clipping. (With the Overdrive button engaged, the amp will still sound exactly the same as before.)
Low Thump Mod
I also added a bit more lower thump by shorting out C6. C6 forms part of an active high pass filter with a cutoff of 153Hz. It makes sense to keep this if you only plan to use the internal speaker, but for an external speaker I’ll have those frequencies back please.
Fender Tonestack Mod
I wasn’t a huge fan of the tonestack and I wanted the amp to sound a bit more Fender-y. “Why not buy a Fender amp then,” you might say. You’ve got some nerve, buddy. I changed the following components so the circuit approaches a standard Fender solid state tonestack:
R10: From 8.2k to 22k
C9: From 2.2n to 1000pF / 0.001u
C10: From 100n to 0.47u
C11: From 100n to 0.22u
Note .. the tonestack topology is not entirely classic Fender (or Vox for that matter). The circuit routing could be modified, especially around C11, R11 and R12, to make it even more Fender-y, however I’m not convinced it would that much of a difference.
LM1875 and Transformer Mod
For some final mods of questionable effectiveness – but great potential – I changed the stock TDA2030 power amp chip to an LM1875 drop-in replacement (the pin connections are the same) for slightly more power (15W vs 20W). And to make sure the amp could actually deliver more power, I replaced the stock transformer with a higher-rated 60VA transformer (Jaycar MM2005), using the 30V, 15V and 0V secondary connections for the original +15V, 0V and -15V connections respectively. I had to drill one new hole in the chassis to fit the new transformer. Hard to tell if these mods in particular were worth it, but what the hey.
It sounds quite good now through my 2×10 cabinet. An amp you would quite happily take along to a gig as a backup, but secretly not be upset if you had to use it.
Notes for other mods I dismissed:
C7 forms part of an active low pass filter with a rolloff frequency of 723Hz. I experimented with removing C7 altogether but the result was a bit too bright, so I put it back. I believe it also prevents high frequency oscillation at higher gain levels. Decreasing C7 will increase the rolloff frequency (eg. 100p would push out the filter to 1.59kHz) which might leave in more higher harmonics when using a fuzz pedal.
C5 bypasses the Gain control for higher frequencies (sort of), so the frequency response of the amp actually changes as the Gain is increased, that is the treble rolls off a little. Ultimately I kept this in, as a “feature” of the Pathfinder 10.