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.