Vox Pathfinder Reverb V9168R, P15SMR and P15408 Amplifiers
A Look "Under the Hood"


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The V9168R, P15SMR and SMR408  Pathfinder Reverb amplifiers were some of the most popular and enduring designs in the Vox amplifier line. Here's a peek under the hood.


Figure 1 - Single spring analog reverb delay line



Figure 2 - 8" Speaker with Vox "Bulldog" label

Chassis
The steel chassis for the Vox Pathfinder Reverb was folded into a "C" shape. The control panel was located on the outer top edge of the chassis, the output and foot switch jacks on the lower edge. The power transformer, main circuit board, power amp heat sink, output jack circuit board and the spring delay line were mounted inside the chassis.

Solid State Analog Circuit
The Vox Pathfinder Reverb featured an analog solid state design that included five ICs (integrated circuits). Here is a list of those ICs and the function of each.

IC1
NJM072D
preamp gain stage one
IC2
CA3080
preamp gain stage two
IC3
NJM062D
tremolo
IC4
TDA2030
15 watt output amplifier
IC5
NJM4558
reverb drive and receive

Power Supply
The Vox Pathfinder Reverb did not include a mains selector to adjust for the various AC line voltages offered in the world. Vox addressed this issue by offering the Pathfinder Reverb with one of three different mains transformers. For Japan and other countries with 100 VAC mains, Vox installed a power transformer with a 100VAC primary, p/n 057-VFJ. For the US market, a 120 volt transformer was installed with the p/n 057-VFU. For the UK, Europe and Asia, a 240-240 volt transformer was installed, p/n 057-VFE. All had a 12 VAC secondary output.

The 100 and 120 volt versions of the amplifier used a 250V-T500ma fuse. The 200-240 volts versions used a 250V/250ma fuse.

Reverb Delay Line
The Vox Pathfinder Reverb utilized a chassis mounted single spring delay line (Figure 1). A label on the pan stated that the input transducer resistance was 600 ohms, the output transducer was 2200 ohms.

Cabinet and Speaker
The Vox Pathfinder Reverb cabinet was constructed from MDF (medium density fiberboard). A single 8," 8 ohm speaker of Oriental origin sported a Vox "Bulldog" sticker (Figure 2).

15 Watt Power Amplifier Chip
Rather than designing a power amp stage using discrete electronic components, Vox chose to utilize a TDA2030 integrated circuit power amplifier chip for the Pathfinder 15 amp. It functioned as a complete solid state 15 watt Class AB power amp. The chip's internal circuitry included over 40 electronic components. All that was required for operation was a 40 VDC supply, an audio input signal, an inverse feedback loop connection and a ground. The TDA2030 is not designed to accept speaker loads less than 8 ohms. To protect against excessively low speaker impedances, inserting a cable into the extension speaker jack disconnects the internal 8 ohm speaker.

Modding the V1968R, P15SMR and SMR408 Pathfinder Reverb
The Vox Pathfinder Reverb has drawn considerable attention for aftermarket modification. The Vox Showroom has selected several Pathfinder mods from various web sources for presentation below. The Vox Showroom has not tested and does not endorse these modifications and we accept no responsibility or liability for damage to person or property resulting from these changes. Proceed at your own risk and please remember that amplifier circuits can store lethal voltages. All work should be completed by a qualified technician.

Disabling the Limiter
The Pathfinder Reverb circuit used a pair of PC mounted LED lamps as a level limiter. These LEDS are labeled "LED1" and "LED2" and are located side by side on the main PC board. LED1 places the anode in the preamp signal path while LED 2 places the cathode in the preamp signal path. The opposite ends of LED1 and LED2 are grounded. The limiter modification requires that both LED1 and LED2 be removed from the PC board. Nothing replaces these parts once removed.

The same effect as removing the parts may be achieved by installing a ground lift switch on the LEDs.

Removing these LEDS from the circuit gives the amp a little more headroom and affects the overdriven tone. If you don't like the sound of the amp after the modification has been completed, the LEDS can easily be replaced into the circuit.

The circuit board must be removed from the chassis to extract LED1 and LED2. This mod requires a high level of electrical and mechanical expertise and should only be performed by a trained technician.

Add a Three Spring Reverb Pan
The Pathfinder Reverb came equipped with a chassis mounted, single spring reverb pan (see Figure 1 above). The tone of the reverb can be significantly improved by replacing the factory unit with a three spring reverb pan. An Accutronics or Belton "short" reverb pan, part number 8EB2C1B, is ideal for this modification. The new reverb pan would need to be mounted to the inside bottom of the cabinet.

This modification requires that shielded cables be soldered to the reverb drive and receive connectors on the main circuit board. One can easily convert a standard "stereo pair" hi-fi cable for this application. One end of the stereo cables will provide the RCA style plugs that will connect to the drive and receive jacks on the reverb pan. Clip the RCA plugs from the opposite ends of the stereo pair cable and solder these to the PC board mounted reverb connectors. The PC board reverb connector with red and black wires is the "drive" (or 600 ohm) side of the reverb circuit. The PC board connector with white and black wires is the "receive" (or 2200 ohm) side of the reverb circuit. This mod requires a high level of electrical and mechanical expertise and should only be performed by a trained technician.

Change the Speaker
Many have replaced the factory Vox "Bulldog" speaker with an 8" speaker from Celestion, Jensen or Weber, among others. Any replacement speaker should be rated at 8 ohms and have a minimum power rating of 15 watts. As the front panel is easily removed from the Pathfinder cabinet, some have fabricated a new baffle to accommodate a 10" speaker. I have read that someone even squeezed a 12" speaker into a V9168R Pathfinder P15R.




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