Axis Fuzz (Roger Mayer) Questions

[Khas Evets]

I'm planning to build an Axis Fuzz. In addition to the Drive and Volume pots, I'm going to add a 100k pot (wired as a variable resistor) in place of the 47K feedback resistor (and maybe a small resistor in series to set the bottom limit) and a 100k pot (wired as a variable resistor) on the input for smoothness. I'll also scrap the 820k and 180k resistors and replace them with 1M.

I'm confused about the purpose of the 220R and .001uF between Q1 collector and Q2 base. It looks like it should roll off highs, but the corner frequency is 723KHz. I've read it only appears in the U.S. version and has to do with U.S. voltages being at 120V. I've also read that it should be 220K making the corner frequency 723Hz (more sensible). Can someone explaing the purpose of this?

Also any other mods you've done would be good to know before I build it.

http://www.diystompboxes.com/pedals/axisfuzz.gif

[b_rogers]

if i remember correctly, it is to help reject radio freq but i may be wrong..also the axis sounds like crap unless you run it into a cranked overdriven amp or another overdrive. BUT it has some unique sounds and textures with some volume and tone knob manipulations.

[Khas Evets]

I'm planning to run it through a Marshall. I thought the samples on Roger Mayer's site sounded pretty good.

[brianwenz]

Hello Hello--
      Yeah, the Axis was MADE to run through a Marshall........probably wouldn't sound too great through a Super  Reverb!
When I was messing around with that res. a few years ago  [the one Khas is talking about]  I found that a 50K pot produced very useable sounds.
Brian.

[Khas Evets]

I don't have a 50k handy, so I'll just throw in a 100k for now. I guess I could add a 100k resistor across the outer lugs later. Do I need to add a series resistor to set bottom limit, or is it O.K. with just the pot?. In other words, do things get weird when the pot has zero resistance?

[Doug_H]

Leaving out a resistor will give it more negative feedback and lower the overall gain when the pot is set to minimum.

Doug

[Khas Evets]

That's what I was hoping. In my spice sim, the wave form started to look weird below 20k, but that's probably a result of less clipping (less of a square wave). I like to build extremes into my pedals anyway.

[Eb7+9]

I'm confused about the purpose of the 220R and .001uF between Q1 collector and Q2 base. It looks like it should roll off highs, but the corner frequency is 723KHz. I've read it only appears in the U.S. version and has to do with U.S. voltages being at 120V. I've also read that it should be 220K making the corner frequency 723Hz (more sensible). Can someone explaing the purpose of this?

yeah, I think it should be 220k not 220R // sounds poor otherwise ... the 220k and 0.001uF combo in parallel with the 100k Base-ground resistor would form a shelving circuit to either stabilize the loop so it doesn't oscillate at higher freq's (it's not an RF shunt in this case) or simply to cut down on harmonic rasp ... with these values the treble shelving only drops the gain by about -3.3dB above 723Hz ...

the Axis is derived from one of the four basic two-transistor feedback amplifier topologies - in this case the trans-conductance version or Series-Series (voltage in / current out) amplifier ... while the Fuzz-Face is derived from the Shunt-Shunt (current in / current out) amplifier ... the original Fuzz-Face could howl if the bottom plate wasn't screwed tight - these circuits (esp. the Axis) can produce lots of loop gain - so a good quality box is recommended here ...

I would try to keep the 820k, 180k, 680k bias arrangement ... you may want to replace the 47k feedback resistor by a 100k pot instead of just adding in series - this allows for values below 47k for less loop-gain than stock, which can be useful with some device pairs ... I didn't find going above 100k of great use in mine ... you may also want to lower or make switcheable the NPN emitter bypass cap from 22uF to 4.7uF (or something) for less sag and a tighter response ... socketing your transistors is a given ...

I was restoring 1960 Bassman when I built mine and they were simply gorgeous together ... I'll second what Brian says about running it into a Super, I think you want an amplifier with little front headroom ...

Question of the Day :

... with a Zin less than 1k some people think the Fuzz-Face input should "load" a PU - but it doesn't ... why? 

Some reading material for the out house :

http://users.ece.gatech.edu/~mleach/ece3050/notes/feedback/fdbkamps.pdf

[Burstbucker]

So, the Axis Fuzz only sounds good through a dirty Marshall?  Dang!!!

My amps are a Fender Twin Reverb, a Peavey Classic 50/410 and a Marshall solid-state 5210 112 combo.  I just put in an order at Small Bear for the stuff I need to build myself an Axis Fuzz.  Oh well, maybe it'll work if I put a decent overdrive box after the Axis Fuzz.  I have many good dirt boxes such as a Tonebone Classic, Marshall Guv'nor(original), etc.......

 

[brianwenz]

Hello Hello--
      The Axis can sound great through lots of amps but NOT great through the older Fender  black-face or silver-face circuits  [Twin, Super, Bandmaster, etc...]    Any newer amp with pre-amp gain stages and a master volume will probably sound fine with the Axis.
Brian.

[b_rogers]

you can also run it into another overdrive and that will help quite a bit. i think the axis is either a love/hate kind of thing. i love mine, and i use a ts9 after it into a clean roland ss amp. i didnt mean to make anyone not build it. just keep in mind that depending on your amp, guitar etc your milage may vary!



Brent

[Khas Evets]

I just got mine running. I had slight problem with a mislabelled resistor (68k instead of 680k), but the good news is that I found it quickly. I'm slowly getting better at this. I think mine sounds consistent with the samples at Roger Mayer's site, although his lead tone sounds fuller to my tin ears.

Everyone's descriptions are pretty accurate. It's a very sharp sounding fuzz that benefits by following it with a second stage of softer clipping (preamp tubes or overdrive/distortion pedal). The noise level of the pedal is surprisingly quiet given the gain available (and I haven't boxed it up yet). It does get a little noisey with everything dimed, but it's not bad. I find it gets a little mushy at the high drive settings, but the range of the fuzz control lets you find your spot.

The 'smooth' control is not very noticeable, but I think it sounds a little better with it taming the high end a bit. I'm keeping it in there.

Replacing the 47k resistor with the 100k pot 'feedback' control is a nice option for cleaning it up a bit. I find the sweet spot is between ~20k - 50k. Over 50k is subtile, and I may add a 100k resistor across the outer lugs of the 100k pot to make the range more useful.

I'm preferring the Axis Fuzz in the lower register of the guitar and especially on power chords. Rich harmonic content when combined with a dirty Marshall. I'm not crazy about the lead tone on the upper part of the neck with moderate gain settings. It gets a little thin and buzzy (which may be solved by Eb7+9's 200k mod). With the gain all the way up, the lead tone gets thicker, but then you're in mush on the lower part of the neck. I'm also going to try Eb7+9's 4.7uF mod, as it can get pretty mushy with humbuckers and full gain. So far I'm happy with it and got what I expected. I'm sure there are many more tonal colors to be discovered using the guitar's volume and tone controls. Build one!

[brianwenz]

Hello Hello--
    That's why I replaced that 47K  with a 22K in my first build a few years ago.  It took some of the "mush" out of the low strings.  This was before I put a 50K pot in there instead.
Brian.

[brianwenz]

Hello Hello Again--
       Try substituting a BS 170 in place of the NPN  [3904] !  You might like it.
Brian.

[Khas Evets]

Interesting. Does it require rebiasing?

[Khas Evets]

I just plugged one in. The pins read...

D  5.61
G  4.66
S  2.76

Great idea, Brian! Now I'm wondering if a BS250 in place of the 2N3906 would work. Unfortunately, I don't have one handy.

It definitely softens up the distortion and removes some of the grind. It sounds great in the upper register too. There's a little harshness (is fizz the word?), kind of high-end sharp clipping. This is a great one to try. I'll look into making it switchable with a DPDT switch (switch C/D and B/G, tying down E/S).

[brianwenz]

Hello Hello--
    Great!
Yeah, I just popped it in with no adjustments and it sounded fine........different but good.
Brian.

[Doug_H]

The upper register can sound kind of wheezy with it, a subtle upper octave can be heard. Backing the guitar volume down reduces some of this and there are a lot of juicy fuzz and overdrive tones that can be found over the range of the guitar volume control. This pedal is probably the most responsive to guitar volume changes than any I've heard so far. That's one of the things I love about it- radically controllable from the handy volume knob.

I'll have to check mine and see if I used a 220 or 220k like JC suggested. I can't remember.

Doug

[Eb7+9]

I found that too ...

I hard wired the gain knob to max on mine because of the way the guitar volume controlled the gain so well ...

I came across a ref somewhere that said RM used low gain devices to advantage in this circuit ... just a thought

jc

[powerplayj]

So, the Axis Fuzz only sounds good through a dirty Marshall?  Dang!!!

My amps are a Fender Twin Reverb, a Peavey Classic 50/410 and a Marshall solid-state 5210 112 combo.  I just put in an order at Small Bear for the stuff I need to build myself an Axis Fuzz.  Oh well, maybe it'll work if I put a decent overdrive box after the Axis Fuzz.  I have many good dirt boxes such as a Tonebone Classic, Marshall Guv'nor(original), etc.......

 

I actually run mine through a Classic 30 and it sounds good, even at lower volumes.  Maybe it has something to do with the weird input config that Peavey employs.  After trying a few transisitors mentioned in other Axis threads, the original transistors sounded the best.  Plus, they cost all of 8 cents.