This is one of my preamp designs, I call it the Overdrive Extreme. It is designed to be a small pre-amp capturing the big tones of the Dumble ODS amp. The circuit is a mix of solid state (JFET) and vacuum tube. With many controls and features the tones are highly flexible.
Clean Channel – the clean channel is run at 40V using two JFETs; two gain stages. The circuit uses a tweaked Dumble ODS tonestack. The tonestack is very flexible and can cover a wide range of tones. There is three switching options as well (push-pull pots).
Deep – provides very low rich bassy tone.
Shift – changes the slope resistors for either classic clear tone or a more middy Marshall type tone.
PAB – pre-amp boost. This disconnects a portion of the tonestack and frees up a bunch of gain. This gives a large volume boost, essentially like a really good clean boost.
Finally after the tonestack there is a volume control for the clean channel. There is also an internal Presence control trimpot to tweak the high end. The Presence trimpot is very useful for getting that smooth warm clean tone but getting the clarity too, ideal for switching between singles and humbuckers. The clean channel is set up so it stays clean and wont clip on it’s own.
Reverb – the reverb is achived by using the Belton reverb brick. The circuit that works around the “brick: uses a quad FET op-amp (TL074). I used the short module version of the brick, though there is lots of deep natural reverb on tap. I am not a fan of “over the top surf” reverb. It sounds really good with the only control being a Reverb Level pot. I also added a Reverb Tone switch on the back. It’s a three position switch that affects the reverb tone’s high end.
Overdrive Channel – the overdrive is provided by a single 12AX7 running at 150V. The circuit is a tweaked ODS’ style overdrive channel. The sound is very smooth sounding with good compression, the circuit provides a great bluesy/jazz overdriven tone. There is an input gain trim pot inside to tweak for best sounding overdrive. That said I’m running that trimpot almost fully open. Other than that the typical Drive and Level control are present. The circuitry uses “low plate” topology. I spent a lot of time tweaking the plate/cathode resistors to my liking. Eventually settled on 100K/1K5 (“low plate”). Metal film ended up sounding best for the plate load resistors. I used mini Orange Drops for coupling, NOS Philips for bypass caps and ceramic caps for snubbers. With the PAB engaged the overdrive circuit is hit with a much hotter signal. That’s when the preamp really starts to sing. The sustain and harmonics are amazing with the added boost.
Both the overdrive and reverb are externally footswitchable. LEDs on the front of the preamp and on the footswitch tell whether or not the overdrive/reverb is on or not. This switch is done with two 12V relays. Another cool little thing is the tube glows blue when the preamp is turned on. I mounted a super bright blue 3mm LED in the tube socket to do this.
Very minimal noise coming from this unit. My grounding schem with lots of filtering really seems to keep the noise out. Noise was one thing I was worried about with such a tight build. It was a very tight fit in the enclosure but I made it all work.