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V9158 transistor guitar amplifier

I bought this amplifier new in 1999 to pair with a 12-string Seagull acoustic guitar. It was perfect for my needs: 15W transistor output, small size, and a nice vintage look. At the time I was experimenting with a Dean Markley acoustic pickup and DOD Chorus pedal. It sounded great but after a while I moved on to another project and this amp ended up in storage. Incidentally, VOX only produced this until about 2001 when it was upgraded to the V9158R, a model that included a spring action reverb circuit. This original V9158 has no reverb.

Fast-forward to 2017: Out of the blue I got the bug to buy an electric guitar. I've been playing strictly acoustic for years and the idea of a hollow body electric to play some jazz improvs suddenly appealed to me. I remembered my VOX amp in storage and pulled it out of the closet. I was pleased to read online that this little amp with 8" speaker has achieved somewhat of a cult status (!) There were quite a few reviews praising the sound quality of the amp, as though VOX engineered a bit of magic into it somehow. But after nearly 20 years of sitting idle I suspected that the electrolytic capacitors in mine were a bit dried out. So I took it apart for an assessment and to see what could be modded.


BELOW: The amp with the head removed. The black staining on the the rear panel is paint overspray from the factory, not burn marks.

BELOW: The amp contains one 8" blue driver.
Manufactured under license from VOX, it was specially voiced for this amp. I measured it at 6.8 ohms.


This is a transistor amplifier, not a tube version like it's big brothers. The design is well laid-out with excess wiring zip-tied out of the way. The output op amp is heat-sinked with thermal grease. Parts quality is very good for this price point. The board is marked with the orientation of componants, nice touch.



There are 8 small polarized electrolytic capacitors (.47 to 47uf) on the board. The power supply holds a pair of 2200uf 35V and a pair of 220uf 25V. To round things out are four green 224J (.22 5%) film caps in various places. The electrolytics are all Sanwha SGs listed as "Standard series for general purposes, high performance and high reliability". I will be upgrading all the caps.

The resistors all look to be good quality but are said to be 5% carbon films. A pair of carbon composites are used in the power supply.

- Three JRC 4558D " high performance monolithic dual op amps" are visible. These chips are well-known in the guitar industry and are frequently used in effect pedals. Given their provenance I'm leaving these alone.
- The main output device is a 5-pin TDA2030 14W hi-fi audio amplifier. Its rated for high output current with very low harmonic and cross-over distortion. Again, given its quality I see no need to mess with it.

To the right of the large power supply caps are four diodes creating a bridge rectifier. These generic types can be upgraded to Schottky's to eliminate rectifier switching noise.

The volume/gain circuit uses a pair of red LEDs in the Boost circuit to add distortion as the volume gets turned up. A popular mod is to remove the LEDs but I'll leave them to retain the factory circuit.

While this amp is well-regarded because of its tube-like sound, it doesn't appear to be due to unusual part choices. In the guitar world the "sound" of vintage parts is steeped in mystique. But there's a big difference between the sound of a weathered humbucker and a generic part with poor sonics. I want to hear all the harmonics and punch that my guitar can provide. So my goal is to make the amp as musical as possible by making the signal path more linear. MY POINT: filled with general purpose caps and resistors I can upgrade the parts quality with no guilt. Keep in mind that the range of an electric guitar is a narrow 80-1200HZ.

- To begin I changed all the small electrolytic caps to brown Elna Silmics of the same value. I like the Silmic sound, an open, warm, dynamic presentation I use them in signal paths when an electrolytic is needed.

- In the power supply I changed the 2200uf 35V caps to 3300uf 35V Nichicon PWs. The 220uf 25Vs I changed to a 330uf 25V. Physically identical they give a modest increase in capacitance, help filtering and beef up power reserves slightly. The amp only puts out 14W so no need to go haywire here. I like the PW series, they are listed as "Low lmpedance, High Reliability For Switching Power Supplies". I also added bypasses to each electrolytic to filter out noise.

- The green .22 film caps are likely polyester. I replaced them with Russian K73-11 .22 160V PETP caps.

- To lower switching noise I replaced the four diodes in the power supply with 1A 60V MBR160G Schottkys.


VOX created a wiring oddity (below). From the board a 22 gauge (#1) speaker wire carries the high-level signal to the jacks on the back panel. This is standard practice to let you use the head to power a separate speaker cabinet.

The oddity is that a skimpy wire (#2) returns from the rear jacks to carry the signal down to the 8" driver. The issue is that this return wire is very thin, half the diameter of the other. I realize the Pathfinder only outputs 14 watts, but choking down the wire going to the driver doesn't make it adds unnecessary resistance and inductance to the output signal. Considering the attention VOX paid to the rest of the amp this is a puzzler. Did VOX use a tiny wire to alter the frequency response intentionally?

So I went and bought a length of Marshall Sound Runner W8116 16 gauge speaker wire. Its features are listed as " High density, non-plated, oxygen-free copper (OFC) strands with low capacitance provide extremely low power loss, wide bandwith, and excellent transient response. Low wattage signal will be enhanced from this series of cables by assuring good dynamic range and damping factor of the amplifier". I like how the last the last line reads.

An easy mod, all I did was remove the skimpy return wire and solder the Marshall wire directly to the board's output....and run it down to the driver. I left the back panel and headphone jack connected.


I used K73-17 1.0uf PETP bypasses on the main (3300uf) power supply caps. K73-11 .1uf bypasses were used on the 330uf caps.


BELOW:The final layout with Nichicon PW, Elna Silmics, Schottky diodes, and K73-11s.
This upgrade went very smoothly.


I hadn't used this amp in a decade so I didn't have a reference sound to compare to the modded version. However...replacing capacitors with sonically better versions, beefing up the power supply, reducing rectifier noise, and correcting the wiring are all well-known mods that produce audible results. Using my 12-string and old acoustic pickup the amp has a clear, powerful sound. Transients are open and musical, I can't wait to try to with my upcoming electric guitar.



NOTE: The 54 watt rating is for power consumption not output. The amp only outputs 14 watts.

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All mods are illustrative only, perform at your own risk.
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