VOX V847 MOD : Get the Vintage Italian V846 Sound!

I am not sure what type of VOX wah Stevie Ray used. I don't think he used American Vox wah V846 with infamous TDK inductor. So I expect he used Italian Vox Wah with Halo inductor or Filmcan inductor. I got an Italian Vox Wah V846 with Filmcan inductor and the sound is amazing! It sounds like Stevie Ray!

Difference between 1969 Italian Vox V846 and Reissued Vox V847

There is a significant sound difference between 1969 Italian V846 and current Vox V847. Italian V846 has sweeter sweep wah sound like "Coowahoo", but current V847 has the sound like a frog "Geh Geh". V847 retrieve only 30% of Italian V846 sound. It's difficult for us to describe musical expression with V847.So I compared the parts of both products.

1969 Italian V846 Current V847
Inductor L Filmcan toroidal core inductor Coaxial core inductor
Transistor Try1,Tr2 2N5172 Unknown
Parallel resistor for inductor Rl 36K ohm 33K ohm
0.01uF cap Cin, Cf Aluminum sheet type cap Unknown
Emitter resistor Re1 for Tr1 470 ohm 510 ohm
0.22uF cap Cc1, Cc2 Unknown material ARCO Unknown
POT ICAR POT Jim Dunlop Hot Potz-1
Electrolytic Cl 4uF Bipolar 4.7uF Polar

Like above, there are some different parts on them. I try to make V847 sound become closer to Italian V846.

Replace Inductor

From left, V847's coaxial core inductor, Jim Dunlop's Reissued Red Fasel toroidal core Inductor, Jim Dunlop's Reissued Yellow Fasel cup core Inductor. Jim Dunlop has a license agreement with Fasel to produce Fasel brand inductor. We can purchase them at Antique Electronic Supply. I replaced V847 inductor with Jim Dunlop's Red Fasel. The sound became sweeter and had a saturation sound on latter half of sweep range. Next I used Yellow Fasel inductor. The sound was better than V847, but not so interesting, not so characteristic. So, I think we must use toroidal core inductor for getting vintage wah sound.

Right side is a Halo type replica from www.wah-wah.co.uk. It is a toroidal core inductor and has the sweet saturation sound like Fasel.

Replace 0.01uF caps with Copper sheet type polystyrene caps and replace 0.22uF caps with GermanERO 1862 poly carbonate caps.

V847 has muddy sound around high frequency and thin sound for mid frequency.So, I replaced 0.01uF caps with Copper sheet type polystyrene caps and replace 0.22uF caps with Roederstein (ERO) 1862 polycarbonate caps. The sound became pure and thicker. Caps with calm and wet sound are good for wah sound rather than caps with bright sound.

Replace transistor

Italian V846 has 2N5172 transistor. We can purchase Fairchild 2N5172 and Central Semiconductor 2N5172 at Mouser online shop. I bought both of them and replace V847's unknown transistor with them. The pin assignment of Fairchild 2N5172 and Central Semiconductor 2N5172 are different from V847's transistor. V847's transistor's pin assignment Emitter, Collector and Base(from left to right). And Fairchild 2N5172 and Central Semiconductor 2N5172's pin assignment are Emitter, Base and Collector(from left to right). So I bend the legs of Collector and Base of the transistor to cross each other without touching. First, I replaced the two transistors with Fairchild's 2N5172. The sound was almost the same as V847's transistor. Next, I used Central Semiconductor's 2N5172. The sound became fat for mid frequency and has natural saturation. They has Vintage vibes ! The sound started to sweep like "Coowahoo"

I tried BC109 transistor as well. They are good also. However, 2N5172 has more vintage vibes.

Replace POT

Italian V846 has a ICAR pot and it does not have linear curve. And almost of other pot don't have the same character. Fulltone produces ICAR replica pot and I bought this one at Antique Electronic Supply. Jim Dunlop Hot Potz-1 sweeps almost at the first step. Fulltone ICAR replica pot sweeps slower than Hot Potz-1

WAH POTS
I tested various replacement pots. The result is that Fulltone ICAR replica is the closest to vintage ICAR pot.

Comparing the frequency character of Italian V846 and V847 with Halo inductor.

It seemed like the frequency character of Italian V846 is different from V847 with Halo replica inductor. I thought frequency peak of V847 with Halo inductor is narrower than V846. So I checked the frequency response by using a PC software frequency analyzer.

Frequency peak point of V847 with Halo replica inductor is shifted higher than V846 with Filmcan inductor.At the sweep low end, V846's frequency peak is 680 Hz and the V847 with Halo is 800 Hz. And V847 with Halo has steep and narrow peak.

Shift the frequency down

For shifting the frequency lower, I added two 0.002uF copper type polystyrene cap parallel to the 0.01uF feedback cap. Total capacitance is 0.014uF. The frequency sweep became almost the same as V846. See the charts below.

The frequency become lower but the curve is still steep and narrow. See the width at -10dB, the width is narrower than V846.

Let the Q down

To make the frequency peak wider, it means to make the frequency Q down. First, I replace the 4.7uF electrolytic cap with 4.0uF. It is difficult for us to obtain 4.0uF cap now. So, I used 4 pieces of 1.0uF Bipolar caps parallel. Then the frequency Q become wider a little bit, but it is not enough. Next I replaced 33Kohm resistor connected parallel to the inductor with 27Kohm. The result can be seen on the next chart. Width of -10dB became almost the same as V846.

Gain Up

Frequency character became closer to V846. However lower Q made the sound calmer and less saturation on the latter half of sweep. So, to make the first transistor's gain up, I replaced the emitter resistor with 470 ohm. I could retrieved the wildness of the sound.

Now, the modification with Halo replica is completed!

Fasel inductor case

I checked the frequency character with Fasel inductor as well. I replace the Halo inductor with Fasel inductor . See the chart below, peak points of frequency are almost the same as Halo, but the Q became higher than Halo. Width of -10dB is narrower than Halo.

So, I change the resistor connected parallel to the inductor with 22K ohm. Q became similar to the Halo case. See the chart below.

Gain up for Fasel again

The sound with Fasel is less wild than Halo. I changed the emitter resistor lower again. 390 ohm is the best for Fasel case.

Now, the modification with Fasel is completed!

Summary

Italian V846 V847 V847 MOD Halo V847 MOD Fasel
Cin 0.01uF aluminum sheet type polystyrene cap 0.01uF 0.01uF copper sheet type polystyrene cap 0.01uF copper sheet type polystyrene cap
Rin 68Kohm 68Kohm 68Kohm 68Kohm
Tr1,2 2N5172 Unknown 2N5172 2N5172
Rc1 22Kohm 22Kohm 22Kohm 22Kohm
Re1 470ohm 510ohm 470ohm 390ohm
Rlc 470Kohm 470Kohm 470Kohm 470Kohm
L Filmcan Coaxial inductor HALO replica Red Fasel replica
Rl 36Kohm 33Kohm 27Kohm 22Kohm
Rg 100Kohm 100Kohm 100Kohm 100Kohm
Cl 4.0uF Bipolar 4.7uF polar 4.0uF Bipolar 4.0uF Bipolar
Cc1,2 0.22uF ARCO 0.22uF 0.22uF ERO 1862 0.22uF ERO 1862
Rs 470Kohm 470Kohm 470Kohm 470Kohm
Rc2 1Kohm 1Kohm 1Kohm 1Kohm
Re2 10Kohm 10Kohm 10Kohm 10Kohm
Cf 0.01uF aluminum sheet type polystyrene cap 0.01uF 0.014uF copper sheet type polystyrene cap (one 0.01uF and two 0.002 uF parallel) 0.014uF copper sheet type polystyrene cap (one 0.01uF and two 0.002 uF parallel)
Rf 1.5Kohm 1.5Kohm 1.5Kohm 1.5Kohm
POT ICAR Jim Dunlop Hot Potz-1 Fulltone POT Fulltone POT

Finally, I got 98% of original V846 wah sound!