A few month's ago, we decided to run an MI DIY challenge on our Facebook page. We asked for amateur DIY-ers to submit some examples of their work so we could select three challengers to be amongst the first to build the MI DIY Dual Drive Kit.
One of the challengers selected was Neil Briones from the Philippines. He currently makes custom pedals under the name Subzero FX and has made some interesting effects that can be seen on his blog. Neil and the other two chosen DIY-ers were asked not only build the kit but to add something special that they could share with the other MI DIY-ers... and Neil met that challenge. Let's hear it in action!
Neil built both the overdrive and distortion circuit provided with the MI DIY kit but chose to add two additional circuits to make this pedal his own. The circuits added are two favourites from the DIY community; the Matsumin Valvecaster and the 1/2W Little Gem power-amp. This inspired design is a small amplifier of sorts - it has an initial valve gain stage then uses both MI Dual Drive circuits as preamps running into the 1/2W power amp. This setup makes for one versatile drive box! Below is a circuit flow diagram:
The Matsumin Valvecaster is THE DIY valve overdrive circuit that has been made many times over with great success. A YouTube search will show you just how many and the possibilities this simple circuit has for modifications. DIY guru, Aron Nelson, hosts a website with thousands of DIY circuits and modifications - and the Valvecaster is no exception. Neil used a 12AT7 preamp tube and omitted the tone control from the original Valvecaster design. Any 12A_7 tube can be used with each having a unique gain and tonal character.
Important note: Neil is running his pedal off a 12VDC power supply. The tube boost circuit requires a minimum of 12VDC to operate efficiently. However, both the tube boost circuit AND the MI DIY Dual Drive can take up to 18VDC giving a more responsive drive section, less gain but more headroom.
Below is the amended circuit Neil used:
Following the 12AT7 tube boost stage is a 1/2W power amp design based off Runoffgroove's Little Gem which is a variant of another DIY classic - the Smokey Amp. Neil's take on the Little Gem sees the removal of the GAIN pot and MASTER VOLUME, favouring a MASTER VOLUME control at the front end of the circuit. Pins 1 and 8 of the LM386 amplifier have been left open to reduce the output gain. The addition of this small power amplifier circuit means this MI DIY Dual Drive can directly drive a speaker cabinet! Here is the circuit Neil used in his Dual Drive:
An interesting feature Neil implemented was the addition of a SEND and RETURN [FX LOOP] via the 3.5mm jacks before the power amp circuit. This allows you to use the Dual Drive with your favourite drive/modulation/delay pedals to add some grit, drive and/or boost.
Also, the 1/2W power amp is switchable: When on, it enables the output jack to run directly to speaker or a 4x12 cabinet rated at 16/8 ohms, turning the whole pedal into a 1/2 watt "head". In this mode, the Dual Drive becomes the preamp. The MASTER VOLUME knob is only active in this mode.
HOW CAN I ADD THESE CIRCUITS TO MY OWN MI DIY DUAL DRIVE???
-- You will need to get some vero/stripboard or perfboard from any number of electronics suppliers (I have linked to Mammoth Electronics for examples). You could also search eBay too.
-- Vero/perfboard layouts can be found at both Aron Nelson's great DIY gallery, and of course the DIY Stompboxes Forum.
-- The Little Gem power amp circuit can be found at www.runoffgroove.com.
-- Check out these sites for many more DIY circuits for you to experiment with.