Overdrive Extreme Preamp

7 May 2010

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.

Features:

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.

 

Schematic:

schematic1

Clean Channel, Overdrive Channel and Power Supply circuits.

schematic2

Reverb circuit

 

 

 

 

 

 

Build:

{ 9 comments… read them below or add one }

Ed Goforth May 8, 2013 at 18 h 25 min

Hi, any sound clips? Very nice work there! How does the Fet clean side sound? I have a Low plate classic clone based on the Dumble #124. I have also built the fet version of the preamp (low plate) with good results, but there is a difference using a real tube for OD, The fet sounds great just different like different tubes can sound different. There is a thickness to the 12ax7 where the fet has a clean, articulation and more available gain that some 12ax7’s. I am curious how the Fet CLEAN sounds compared to a 12ax7 clean? Very tight fit indeed, but very nice work as well! Your clean fet circuit is bit different from the Runoffgroove site, I might have to try this circuit :^)

Thanks
Ed

Reply

cmoir May 8, 2013 at 20 h 31 min

Hi Ed,

No sound clips at the moment, I will work on getting some up though.

I really tried hard to make the clean sound as good as possible because after all if you don’t have a good clean you can’t expect your overdriven sound to be that great. As you noted the setup of my FET stages is very different than ROG’s. To be honest their Dumble style circuit sounds good but nothing like a Dumble. If you search around on the circuit you will see Dumble aficionados echo this sediment.

It’s defiantly worth a try on the bread board!

Reply

Brian Miler November 14, 2013 at 12 h 36 min

Is Fet J201 the same as the 1st Fet 2N5457?

Reply

cmoir November 14, 2013 at 12 h 46 min

Hey Brian,

The J201 is similar to the 2N5457 but not the exact same. The J201 has more gain than the 2N5457. If you used the J201 for the first stage you could expect a bit more volume, dirt/crunch and a little less clarity.

Reply

Mariano July 8, 2014 at 15 h 22 min

Hi

Did you use the same transformer to bring th 120v to 12 and then rhe 12v to 120v?

Reply

cmoir July 9, 2014 at 17 h 30 min

Yes
The schematic shows two back to back transformers (120V to 12V then 12V to 120V). This is all done with one transformer.

One transformer but with dual 120V and 12V primaries and secondaries respectively.

Reply

Peter June 17, 2015 at 12 h 42 min

Hi, Great site you have put up:-)

I would like to try only the clean FET channel of your Overdrive Exstreme Preamp (without the OD tube) for an amp project of mine – feeding a transistor poweraamp.

Looking a the diagram I have to power it with 40V DC. How do I do that conserning transformer and PSU? What circuit do I need?

Reply

Peter June 17, 2015 at 17 h 38 min

HI! Great site with great projects :-)

I will try to build your Overdrive Exstreme Preamp, but only need the FET clean stage for an transistor poweramp.

Quistion is how to supply the 40 V DC? With a regulator like LM317? Or just a simple RC-stage?

Can I modify the schematic to run it on fx. 15V DC?

Best, Peter

Reply

Sjoerd February 17, 2017 at 16 h 15 min

Hello Chris,

Great site and work! Thanks.
I’m building the Overdrive Extreme and a question about that one:

On the schematic you are writing ‘- Bias JFET drains to apex 5/8 B+2.’ and drawing of a new little B+2 schematic (with 47mF, 33k and 1k2) in the top right. Is this meant as a modification of the B+2 (with the 22mF and the 470mF)?

Best, Sjoerd

Reply

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