MARSHALL: 1974X AKA the Marshall 18w one knob tone controls

Another simple one knob tone control, this time from the Marshall 18w amplifier, or the model 1974x, whatever you prefer...   

Marshall used two different tone controls in the amp.  A lot of folks build the 18w lite, which is just the normal channel of the amp, and they often use the trem channel tone control as it has a bit more range to it (18w lite II, it's a great little amp).  The trem channel is basically a Fender one knob tone control using 500k pots.

Normal channel - a little less bass than the trem channel, no top boost

Trem channel - more bass than the normal channel, a bit of top boost

Combined

MARSHALL: Blues Driver / Boss Discrete Opamp

If you've read any recent posts, you may have noticed that I'm experimenting with discrete opamps - specifically replacing standard opamps with simplified discrete opamps - I've only played with the Joe Davisson diode compression opamp so far (with some success).

While I was checking out DIY Stompboxes, I saw Rob Strand was experimenting with the Boss discrete opamp that's used in the Blues Driver.  I've occasionally looked at it but have never gotten around to playing with it.  

Anyway...  it appears to work as a drop-in, and I thought a Blues Driver / Blues Breaker hybrid might be fun.  They are totally different circuits by the way - similar in name only.

More to follow on the Boss discrete opamp.  Feel like I need to spend some time playing with this and the Blues Driver.  I have an interesting idea in mind for the Blues Driver; I'll post it if it's successful.  

MARSHALL BLUES BREAKER X BOSS BLUES DRIVER DISCRETE OPAMP

It looks like a Blues Breaker on LTspice.  But I'm always a little suspicious of anything using JFETS on spice as they're notoriously variable beasts in real life.

BOSS BLUES DRIVER SCHEMATIC - OPAMP MARKED

Here's the Blues Driver schematic, with the discrete opamp marked. A good analysis can be found here if you are interested in further reading.  

MARSHALL: Blues Breaker, Discrete Opamp Mod

I've had some success with the Joe Davisson diode compression discrete opamp on some other circuits (namely the Cornish SS-2), so I thought I might try a Blues Breaker - it works...  sounds good.  

Apart from the lack of IC, there are a couple of other minor mods to a stock Blues Breaker, mainly a larger input cap (Morning Glory) and some larger resistors on the inverted input of the first opamp (King of Tone).  I'd also recommend a C250k pot for gain, if you want more dirt (which is what I'm using).

I've increased the bass response on mine - all you have to do is change the 10n cap on the bottom row to something bigger, and you'll get some low-end back.  I like 220n, as it seems to keep all of the bass that I started with, but some might find this too much.  Could be cool to have this on a switch.  

For anyone with a keen set of eyes, you might notice the RC networks on the inverting input of the first stage going to ground instead of vbias - it was convenient to do on the layout, and it makes zero difference to the sound.   

DISCRETE OPAMP - BLUES BREAKER - VERO LAYOUT

DISCRETE OPAMP - BLUES BREAKER - SCHEMATIC

And here's it looking a mess, but 100% working, attached to the breadboard - I made a couple of errors initially, so this is not exactly the same layout as above, as I fixed it on the fly.   Note to self: don't attempt creating layouts followed by soldering them when really tired.

 

 

VISUAL ANALYSER

440hz sine wave input

Scope taken on the output, with the volume turned down a bit.   Starts with gain on 100%, sweeps tone control up and down a couple of times, and then reduces gain.

 

The output of the first boost stage, gain 100% (note the input signal sits at -25)

 

The output of the clipping stage (gain 100%), before any filtering from tone controls and caps to vbias - it has a lot of harmonics / high-frequency content, and it's bordering on being a square wave.

MARSHALL: Shredmaster

The Marshall Shredmaster is a somewhat classic distortion, even if it has a cringe worthy name.  I don't think I would have even looked at this back in the day as a result of the name.  Shredding is not my thing.

I think it's fair to say that Radiohead using the Shredmaster has certainly helped its image in more recent times.  

This version has diode clipping options, which will help open it up a bit.   I'm sure you can work out how to build the stock version or change diodes to suit your taste from this layout - there's certainly enough room to play with around that section.  Note that it has been said that jfet based ICs are not great for this circuit, so I'll let you decided on that side of things.

MARSHALL SHREDMASTER - VERO LAYOUT (WITH DIODE CLIPPING MOD)

MARSHALL: Blues Breaker

Nothing to see here - just a Marshall Blues Breaker...   quite the classic today, bit did not do well when it was released.  

MARSHALL BLUES BREAKER - VERO LAYOUT

And more Tone Bending...   just make sure you add a decent size capacitor to decouple the output, I doubt anyone wants / needs stray DC escaping your fuzz.

MARSHALL SUPAFUZZ PROTOTYPE - TAG BOARD LAYOUT

MARSHALL SUPAFUZZ PROTOTYPE - VIDEO

Just to continue on Tone Benders for a little longer...    Marshall SupaFuzz on tag board.

MARSHALL SUPAFUZZ - TAG BOARD LAYOUT

MARSHALL: SupaFuzz Prototype, 0.15" Vero Layout

This is the early version of the Marshall SupaFuzz based on a Tone Bender MKI - it's close to the original layout used.  

Strangely it never had a coupling cap on the output, so best to add something decent sized there.  I've also seen a couple of photos with a power cap across V+ / V-  I couldn't see a value, but on an old pedal like this I would expect maybe a 25uf or 50uf - running of battery this is not exactly essential either.

Transistors are OC75, but use whatever you can get with decent amounts of leakage and moderate hfe, which is typical of OC75s.

Being a Tone Bender MKI variant, you can easily add a fuzz pot.  In this circuit the 470k resistor and the 47k resistor bias the base of Q2 (the middle one). 

The 47k resistor from the base of Q2 to ground, is normally where the fuzz pot sits in a MKI circuit.  Just check the standard Sola Sound Tone Bender MKI to see how this works - pretty easy to convert if you want to try this.  Alternatively, one could use a trimmer instead of the 47k resistor to dial it in and leave it set in a happy place.  This might be the best option.  

Unlike the other Marshall Supa Fuzz, the filter control here is actually a filter control...  

MARSHALL SUPA FUZZ PROTOTYPE - VERO LAYOUT

MARSHALL SUPA FUZZ PROTOTYPE - SCHEMATIC

Had to use AC128 in here, as I don't have an OC75 model in LTspice - anyway, you get the idea.

This is with a 100n cap on the output - makes no difference to the response.  Filter takes off a lot of high-end.

MARSHALL: SupaFuzz, 0.15" Vero Layout

This is the original 1968 0.15" vero layout for the Marshall SupaFuzz (which may be disputed by some).  

The cuts may not be in the exact positions as the original, but no matter...  same with Filter pin 3 connection.  This may be on the other side of the board, but that makes no real difference.  I have not included shielding, but that's easy enough to work out from the photo.  

MARSHALL SUPAFUZZ - VERO LAYOUT

MARSHALL SUPAFUZZ - ORIGINAL BOARD

MARSHALL SUPAFUZZ - ON THE BENCH

MARSHALL: SupaFuzz, Schematic

Basically the Marshall Supa Fuzz is a Sola Sound Tone Bender MKII, but with some different capacitor values and it uses Mullard OC75s for all three transistors.  It sounds killer, and cleans up really nicely using the volume control on the guitar.  When you hear the fuzz, it's hard to imagine the clean sound, the fuzz is supa...  This is really something that MKII Tone Benders are known for.

Like all Tone Benders using germanium transistors, there can be a bit of a process involved in selecting and/or biasing transistors.  There's quite a lot of opinions out there on various forums - if you have the chance, get it on a breadboard first and try a few different combinations.  Doesn't take long, and it removes all the guess work prior to soldering.  Apart from that, once you get it on a breadboard and hear it for yourself and notice how it responds to your guitar, the opinions of people on forums disappear.  You like what you hear, not what you read.

If you're suffering low volume, but like the sound, the standard Fuzz Face volume mods will work.

MARSHALL SUPA FUZZ - POINT TO POINT LAYOUT

 

MARSHALL SUPA FUZZ - SCHEMATIC

MARSHALL SUPA FUZZ - VISUAL ANALYSER

Signal: 440hz sine wave, approx 130mv TRMS

GAIN 100

 

STRAIGHT OFF Q3 COLLECTOR