Background
Around the same time as the guitar renovation project, I thought I’d carry out a long-desired modification to my guitar practise amplifier. The Marshall Lead12 is perhaps unusual among guitar amps in that it features a headphone⁄lead-out jack socket. (It’s much more common to mic guitar amps.) To be able to play loud late at night in a built up area (and, to be honest, to mitigate self-consciousness while practising), I wished there was a switch to cancel the output to the speaker while retaining the output to the headphone⁄line-out jack.
Plan
As with the guitar customization project, once the kernel idea caught hold, I thought I may as well renovate the equipment while I was at it. As with the guitar, I had long been fond of the amp but there were a few things I wanted to improve.Over the years, the potentiometers had become inconsistent with use: the most used ones has practically no resistance left and would occasionally crackle just at my preferred knob settings. The electrical contact from the jacks was not as clean as it should be.
I’d bought it second hand at the same time as the guitar and, although never abused, it had become just a tad tatty over the three decades or so of service. I decided I was OK with this. The guitar was sporting a home-made grungy look, so the amp didn’t have to look brand shiny new either.
Where to put such a switch? There is not a lot of spare space on the front panel. Beside the headphone socket would be the most suitable place. This leaves only two realistic choices: either rudely cut into the Lead12 decal or squeezed into the space between the socket and the power switch. Space permitting, I chose the latter.
Execution
I went for a rocker switch, similar to the main power switch (as opposed to a toggle, paddle or other kind of switch; unlike the power switch, it’s plain black and doesn’t light up). It squeezes in snugly.
Even as a 12-watt practise amp, this thing was really built to last internally. One can see that Marshall’s enduring reputation is not for nothing: every component is solid.
The plan was to have the switch replace the (impedance of the) speaker with some static discrete resistance. I reckoned that rudely cutting the speaker out as an open- or short circuit could potentially harm the circuit board. Measuring the level of power showed that it was too high for ordinary resisters though. It was rather more cost effective to fabricate a “battery” of parallel resistance than to buy a single (high-) power resister. In the photos, this home-made compound power resister is soldered together under heat shrink.
While I was at it, all the pots & jack sockets got replaced with new ones. I couldn’t find a replacement for the particular switching-on-jack-insertion arrangement for one of the jack sockets. With a little DIY-engineering, I was able to reconstruct it successfully though.
Epilogue
It works well. With the new switch in one position, the amp is just like it was before. In the other position, the main speaker is disengaged, replaced internally by a resistance bank, and the headphone⁄line-out socket works as ever.I might do more, such as add another jack socket to allow foot-pedal switching between the low & high inputs, but these days I play much more often through an Eleven Rack, so this old amp is OK for now.
