Building a Roger Mayer Stone Fuzz

[lcv]

Just guessing: it would be really funny if the voltages on the schematic we are discussing were just copied from the sixth post of this old thread:
https://www.diystompboxes.com/smfforum/index.php?topic=51107.0
From a reportedly "fried" unit , at that?

Luca

[Rob Strand]

https://www.diystompboxes.com/smfforum/index.php?topic=51107.0
From a reportedly "fried" unit , at that?

You never know.  It's the internet.

[jgenet99]

I know exactly the post you are talking about, and another reason for my skepticism. It seems obvious that the schematic was developed second hand, so I'm not sure the numbers are reliable. I've not had the collector on Q2 come in anywhere close to 4.5v, but every time I've tried, the .74v on the emitter is spot on. It is reasonable to expect that the voltage would be near 4.5v (although the Jimi Hendrix Dunlop (the better of the two) has a much higher bias, like 6.5 or something), so at any rate if we can't trust the schematic, then what should the voltage be? I find it hard to believe the low voltage I had is correct, because it really didn't sound good (but then it's hard to determine from the two source videos I could find exactly what it should sound like!) Unfortunately still a few missing pieces.

While I'm complaining about YouTube videos demonstrating amps and pedals...like 9/10 videos, I can hear the pick on the guitar strings...and not subtle! It's not that hard to place your mic so you only get the amp tone! I just find it completely unbelievable...

[Rob Strand]

It is reasonable to expect that the voltage would be near 4.5v (although the Jimi Hendrix Dunlop (the better of the two) has a much higher bias, like 6.5 or something),

I'd say 4.5V is very typical for a FF.    I don't mind the sound of an FF with 6.5V it's has a different character which is totally useable.   One FF I built has a switch for 4.5 and 6.5 (can' t rememeber exact voltages).

If your Q1 collector resistor is set to quite a low value the base voltage on Q1, the emitter voltage on Q2 and the base voltage on Q2 will all rise-up.

The circuit is obviously a bit fickle in terms of voltages.   If you set the Q1 collector resistor to 47k then adjust the uncertain 1K/10K resistor to get 4.5V I think you would be getting very close to something representative of the real unit.

[lcv]

The official sound of the stonefuzz is presented here:   
https://www.roger-mayer.co.uk/juke3.htm
BTW the setup with 10n+47k output sounds quite different to my ears, so I tend to favor  the 100nF option.

If you set the Q1 collector resistor to 47k then adjust the uncertain 1K/10K resistor to get 4.5V I think you would be getting very close to something representative of the real unit.

I agree on this.  I would add that one should use the same (or very similar)  transistors to start with.

Also, I would just listen to the sound, varying Q1 resistor (or Trimpot value) , keeping 10K+10K, to see if something similar can be obtained.

Luca





 

[jgenet99]

Thanks, I had looked on the site before but couldn't find the stone fuzz clips...I couldn't navigate the page, it sort of looks like the pages we used to host on angelfire back in the day! It does sound like it's using the 10n...you can hear where it boosts and distorts the lowest frequencies, but then rolls those down and favors the higher frequencies. I personally really like it! It depends so much on the amps though, I assume with a dark Marshall amp, it balances pretty nicely.

I recently read that Hendrix actually modded the Marshall amps to boost the high frequencies, does anyone know anything about that? I've really become a fan of the tone of the fuzz with the neck pickup and the volume turned back.

[willienillie]

...with a dark Marshall amp...

I recently read that Hendrix actually modded the Marshall amps to boost the high frequencies

I often hear people refer to "dark" Marshalls, and they can be if you plug into the bassier channel.  But the "High Treble" channel is extremely bright, and the Presence control is very effective.  I can't see doing a mod to boost high frequencies when you can jumper the two channels and get a mix that includes all the highs any guitarist could possibly want.*  I've heard that Hendrix preferred KT88 or 6550 power tubes to get more bottom end, but I don't know if that's true.

*[edit:  I should be more specific, I was referring to the typical 4-input Super Lead model (ie 1959, 1987) from the late 60s to mid 70s.  The earlier plexis with the tied V1 cathodes, probably most Marshalls that Hendrix used, will have a less dramatic difference between the two channels, though still no shortage of high end if you set the controls for it.]

[Rob Strand]

I was trying to compare the Classic Fuzz (which does have 10n and a 47k pot) to the Stone Fuzz.  The samples just don't have enough low-end to work out the cap.

[Gus]

this might help
https://www.diystompboxes.com/smfforum/index.php?topic=114153.msg1059002#msg1059002
note how almost all the resistors adjust the transistors operating points

Look at an older VOX fuzz the 1K gain has a 820 ohm "under it"

Look at the older Si FF like circuits

Something I built years ago(90's) the values were selected so you could buy most of the parts from radio shack. The 5k is an external bias control.
https://www.diystompboxes.com/pedals/gusFuzzFace.gif
In the 90's I looked at a number of FF circuits and traced them as well as looking at schematics on the web

DON'T get stuck on the collector at 4.5VDC nonsense you read on the web. It limits what you can do. Sometimes it is OK
If you want to adjust the 2nd transistor collector voltage it is often better to adjust the first collector resistor up or down.

https://www.diystompboxes.com/smfforum/index.php?topic=114210.msg1059667#msg1059667

[Rob Strand]

If you want to adjust the 2nd transistor collector voltage it is often better to adjust the first collector resistor up or down.

Adjusting the collector resistor on Q1 is only effective when Q1's gain is low.   For high gain Q1 it's a very ineffective method.     Basically you are relying on the voltage drop across the 100k to cause a voltage drop across the Q2 emitter resistor, then finally affect the voltage on Q2's collector.  So it's relying on an indirect effect.  When Q1's gain is high the voltage drop across the quite small unless Q1's collector resistor small; which makes Q1's base current high and increases the drop across the 100k.   Adjusting  Q2's collector resistor is a very direct cause and affect and guaranteed to affect Q2's collector voltage.   For low-gain germaniums you can get away with adjusting Q1's collector resistor because they are low gain and already rely on the voltage drop across the 100k to bias correctly.

[jgenet99]

I can't be certain, but I think that's why this circuit uses the trim pot... in the classic fuzz, it was acceptable because of the AC128. As observed previously, I think he just repurposed the board to make it work with silicon, and then eventually redesigned the board so that all the components were there and replaced the trimmer. That's just a guess of course, but you're right that on the germanium it would have worked, but it has much less of an impact on these silicon ones.

[Rob Strand]

I can't be certain, but I think that's why this circuit uses the trim pot... in the classic fuzz, it was acceptable because of the AC128. As observed previously, I think he just repurposed the board to make it work with silicon, and then eventually redesigned the board so that all the components were there and replaced the trimmer. That's just a guess of course

It makes perfect sense to me.

For fuzz-face there's kind of two factions: those who adjust RC1 and those who adjust RC2.   For most other circuits people tend to adjust the equivalent of RC2 (but not always).