"Marshall 6100" Preamp

 A few years ago, I used to have a Marshall 6100 30th anniversary amp, which I really liked. I liked it so much that I went the extra mile on the restoration of it and changed all its pots, tubes and electrolytic caps. I also fixed a problem that is common with these amps in the midi circuit, where the channel leds stay lit and channels won't change. Then I converted its power stage back to the original EL34 specs (mine was the 5881, non-LM version) and also added a choke to beef up the low end and remove a little more of the supply ripple and noise that comes with it and lastly I added a switch on the back to allow switching between the stock and the LM mod. The reason I did all these things was because that amp deserved it in my opinion - once correctly set up it was one of the very best channel-switching amps that I have heard - from crystal clean, to crunch, to liquid lead with a push of a button and responsive and uncompressed (unlike most other channel-switching amps in the market). The reason is simple actually - apart from a few compromises, the stages of the 3 channels are designed separately; that means that the channels are indeed as independent from each other as possible (again, there were a few compromises but not many) and this can be easily observed by counting the preamp tubes (there is a total of seven 12ax7s - six of them belong to the preamp and the last is the phase inverter). At some point I had to let the amp go as it became a luxury in the sense that I already had the amps that it was mostly replicating (JTM45, Plexi/JCM800), plus the complexity of the amp was overwhelming (many techs won't even touch it). Oddly enough though, what I enjoyed the most in this amp was the Clean (1st) and the Lead (3rd) channels and that is how this project came to life.

First of all, I have to admit that I enjoyed the input options of this amp (the Compensation switches) and that is why I chose to keep those instead of implementing a more tried and tested input scheme. The only difference is that I didn't find any reason to make two separate switches - instead, with a little clever wiring, I made the Compensation a three-way switch.

The Clean channel is fantastic in my opinion and it provides all the sounds that I could want. It is big and spanky and the switches (in form of push-pull pots) work very well. The Bright switch adds shimmer, while the Mid Shift switch makes the mids more hollow and guitars with humbuckers can be greatly benefited - especially when strumming chords.

The Lead channel on the other hand is the total opposite. It can go from crunchy to liquid lead or tight rhythm easily. The eq is very responsive and touching a few knobs makes a world of difference. And on top of that there are also the switching options. The Gain Boost (implemented as a push-pull switch on the Gain knob),well, adds more gain (duh!) and juice, while the LM switch does the same but at the same time changes the voicing as well (think of it as a vintage vs modern switch). The LM mod along with the Gain Boost is an overkill though when running the gain knob above 4, so this is not advisable. Lastly, the Contour switch completely changes the response of the Middle knob. When engaged, it feels like the Middle knob turns into a fixed-Q band-cut parametric filter on the mids where the Middle knob changes the center frequency. The interesting thing about it is that unlike the mid-heavy sound that is associated with Marshall in general, this switch allows one to go to mid-scooped territory. One of my favorite sounds from this channel actually was a dry, cutting, tight and chuggy rhythm sound (think Metallica) - a sound that I didn't expect to hear from a stock Marshall - and the Contour knob helped a lot to that.

To make this project real, I had to redesign the power supply from scratch and solve the channel switching challenges. Hopefully, everything worked out as planned.

Lastly, because of the high-gain nature of the preamp, I had to take extra care to ensure it would remain silent. That means that I had to use shielded wire in a few places and, as always, take good care of the lead dressing and grounding. As far as parts are concerned, I used Cliff jacks, Miyama switches, Alpha pots, Xicon 1W Carbon Film resistors, Mallory M150, Solen Fast, CDE DME and Orange Drop 6PS film caps and F&T electrolytic caps. I also couldn't resist using the power indicator light that came with Marshall Vintage Modern amps (another amp I had to let go at some point) - purple is just so nice!

UPDATE: Small improvements to the channel switching system for quieter operation