I built an Axis Face using a 2N2369A, hfe=66, and a BD139, hfe=122. It sounds almost like my Ge clone that uses 2SA49 PNP Toshiba (hfe1=92, hfe2=168, leakage under 50uA). Phillip chose a pair of winner Si transistors. They are a more expensive but they worth the extra cents. I had no problem getting the BD139 but the 2N2369A is another story. So the BD131, BD237 and 2N1613 may be a replacement I guess. Note that there are two types of BD139: type 10 with gain between (63,160) and type 16, (100,250). I like the sound of the Axis Face, it's smooth and warm. I built clones in the past using BC549C and 2N3904 but the sound was horrible, thin and harsh. One mod i did was to place a OA91 Ge diode from Q2 base to collector. It gives an extra Ge touch. Also, a Ge diode like the 1N34 can be placed between Q1 collector and Q2 base. This mod adds some compression and reduces the bass response a little. I took this idea from Joe Davisson's Vulcan overdrive.
Glad to hear that you're enjoying your Axis Face, Mac! I love mine...I keep looking for ways to make it better, but the only thing that I think would make a big improvement would be to have a 1K reverse audio taper pot for the "attack" control instead of the typical linear taper, which really bunches up the fuzz rage at the end of the pot's rotation.
I think Steve at Small Bear is going to help me out, though ;)
I read that you don't like the bass response of the fuzz face. Me neither. Well, if you place a 0.47k - 1k resistor between Q1's emitter and ground, and a big elect. capacitor in parallel, then you can control de amount of bass hitting Q2. I modified my fuzz faces and sola tone bender MKII pro this way. I guess that the same circuit after Q2's emitter, the 1k pot and the 22uF, can be placed at Q1's emitter, acting as a pre gain and as a bass cut. Hey Jimmi, what dou you think???
I was just fooling around with some transistors in the Axis Face some more this afternoon. I was using a couple of higher gain BC108B transistors in the pedal, but now I think I like using a 2N4400 for Q1 (hfe = 74) and a 2N2222 (in the metal can like the BC107/108/109, hfe = 125) and I really like it.
Usually the 2N4400 has a higher gain range, most people end up using it for Q2 since they're usually around 120 - 130, but I guess the lot that I got from Mouser had lower-than-normal gain ranges.
Overall a very nice, smooth sound with the lower grain transistors! Just the way that the Axis Face is supposed to sound ;)
1) I forgot to say that if a resistor and a parallel capacitor are placed after Q1's emitter, Q2's emitter resistor (the 1K pot) should be increased. I have a little program to calculate the bias. Send me a msg if someone wants to try it.
2) central semiconductors (www.centralsemi.com) has a database with a large number of transistors with the proper gains for Q1 and Q2, 70 - 130. Or go to datasheet.com. If someone is interested send me a msg.
I corrected a mistake I made when I posted the mods I did to my AF:
I wrote: ..." Well, if you place a 0.47k - 1k resistor between Q1's base and ground, and a big elect. capacitor in parallel, then you can control de amount of bass hitting Q2."...
The resistor MUST be placed between Q1's emitter and ground.
I tried the 2N2369A in a Sola Sound ToneBender MKII, a cousin of the Axis Face. I used them in Q1 & Q2 position. For Q3 I used a Motorola MPSA42, hFE=95, but I guess that the BD139 could be used too. The first stage must be modified when using silicon transistors. A 1 MOhm pot should be placed between Q1's base and collector and adjusted to get around 8V at the collector. Start with the pot at 1MOmh!. Ge transistors do not need this resistor because the internal leakage acts as bias I guess. Following Phillips' MindBender guidelines and mods, I have a very good sounding Si ToneBender.
