Vox Pathfinder 10 and Pathfinder 15 Simulation

I've got a Vox Pathfinder 10 amplifier that I bought a couple of years ago, and I love it. It's a great little amplifier, but I don't care for the distortion channel, it just sounds harsh to my ears. There are quite simple modifications that can be done to this amplifier to remove the clipping LED's and increase the boost of the clean channel (check out Ben Craven's mods for details). I'd been considering doing some of these, but whilst researching the subject, I discovered that the currently out of production (as at Nov '18) Pathfinder 15/15R generally receives a lot more love than the Pathfinder 10.

I found schematics for both the PF15 and the PF15R online, and saw that there were a lot of similarities between the PF10 and the PF15, particularly the earlier non-reverb PF15. The PF15R (with reverb) is quite a bit different after the first two opamp stages. I thought I'd model both the PF10 and PF15 pre-amp stages in LTspice to see how they compared.

There are some of differences that should be noted

• The PF15 powers the opamps with +/- 15V as opposed to +/- 13V on the PF10
• This means the PF15 has a little more headroom available.
• The PF15 always has the clipping diodes (LED's) in the signal path, whereas on the PF10 they are only in the signal path when the overdrive switch is engaged.
• The PF15 has the "light" resistance of the vactrol (tremolo circuit) in the signal path at all times.
• I've allowed for this in the simulation.

In the simulations below, the Treble and Bass controls are set at 50kΩ (~50%), and the Gain control is shown at 1Ω, 25kΩ, 50kΩ, 75kΩ and 99.99kΩ (~0% to ~100%)

Simulation settings

• Both
• Bass - 50kΩ (~50%)
• Treble - 50kΩ (~50%)
• Input - Sine Wave 800Hz, 100mv
• PF15 
• vactrol resistance set at 16kΩ (Tremolo OFF)
• boost switch OFF

First up, the Pathfinder 10 on the clean channel

Pathfinder 10, clean channel, gain at 0%, 25%, 50%, 75%, 100%

Next up, the Pathfinder 10 on the overdrive channel

Pathfinder 10, overdrive channel, gain at 0%, 25%, 50%, 75%, 100%

Finally, the Pathfinder 15

Pathfinder 15,  gain at 0%, 25%, 50%, 75%, 100%

This is just looking at the signal voltages, but it's easy to see there are significant differences. For one, the PF15 puts out a much stronger signal from the preamp compared to the PF10 on either channel, so it's going to be louder assuming the power amp stage is the same (it is the same chip TDA2030A, but there are some differences to how it's configured). The PF15 distorts more gradually as the gain is increased, whereas, the PF10 quickly gets very compressed on the overdrive channel. I'd expect the PF15 to have a more "natural" distortion sound from looking at these results, but I'm just guessing a bit at this stage.

I'm curious enough now, that I'm considering looking for a cheap PF10 that I can modify to PF15 specifications. It shouldn't be that difficult to do, and I have all the required parts available. Watch this space!

Update 2-Nov-18
I've added a simulation of the Pathfinder 15R below. As I mentioned previously, this one differs quite a bit as the final opamp stage is replaced with NPN transistors and a CA3080. In my simulation I used 2N2222's for the NPN transistors and a LM13700 in place of the CA3080, but it should be close enough for jazz.

Pathfinder 15R

Pathfinder 15R,  gain at 0%, 25%, 50%, 75%, 100%