Another One of my Preamps....Overdrive Extreme

[kurtlives]

Another one of my preamps, I call this one the Overdrive Extreme.

I am really proud of this one though. I've been working on it off and on since September. University kind of made it difficult to get a whole lot of work done on it at one time. I tried to work on it a bit on weekends I was home, redesign and tweaking the schem in lecture. It spent a lot of time on the breadboard during the year, tweaking it to my liking. I finished school last week so I finally got to wire it all up. I'm really happy with the end result.


Features:

Clean Channel – the clean is run at 40V using two JFETs, so two gain stages. The circuit uses a tweaked Dumble 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 – very low rich bassy tone.
Shift – changes the slope resistors for either classic clear tone or a more middy Marshall type tone.
PAB – preamp 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. Also there is an internal Presence trimpot to tweak the high end. Very useful for getting that smooth warm clean tone but getting the clarity, ideal for switching between singles and humbuckers. The clean channel is set up so it stays clean and wont clip on it's own. You can see this in the schem, look after the volume control.

Reverb – the reverb uses the Belton reverb brick. The circuit that works around the brick uses a quad 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" 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. The switch affects the reverb tone's high end. But when set in the middle position the reverb is set flat and makes the effect more subtle.

Overdrive Channel – the overdrive is provided by a single 12AX7 running around 150V. The circuit is a tweaked ODS' style overdrive. 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. 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 and ceramic 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 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. That 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. Minimal debugging needed actually. Only a bit of smoke and a few bugs to work out.


So overall a cool little preamp, with a ton of tones on tap. Hoping to record some clips soon.




Pics...

















Schem...
http://img.photobucket.com/albums/v610/kurtlives/OD1.jpg
http://img.photobucket.com/albums/v610/kurtlives/OD2.jpg

[Taylor]

Impressive. Where's the Belton brick? Tucked under one of the perf boards?

[Brymus]

Wow thats really nice   
So the snubbers you are refering to are the 330p from plates to cathodes ?
I dont see any on the PS.
And did you try the filaments unregulated at first ? Or did you start with them regulated ?
Also wheres the other 12v transformer ?
Thats a super nice looking build.
I love the blue LED for the tube, nice touch !!!

[kurtlives]

Impressive. Where's the Belton brick? Tucked under one of the perf boards?

Thanks,
Should have mentioned, hard to see. The brick is under the power supply boards. Was a tight fit getting it in there. Drilling this thing was a bi tch!

Wow thats really nice   
So the snubbers you are refering to are the 330p from plates to cathodes ?
I dont see any on the PS.
And did you try the filaments unregulated at first ? Or did you start with them regulated ?
Also wheres the other 12v transformer ?
Thats a super nice looking build.
I love the blue LED for the tube, nice touch !!!

Thanks!

The snubbers are indeed the 330pF from plate to cathode.

The filaments were always regulated. I did it like this because regulating for one tube is not a big deal and
in my experience it seriously cuts down on noise. The heaters, relay and reverb are all powered by the regulated 12.6V
so it was important it was clean. The 2200mF filtering cap probably helps as well .

The transformer has dual 120V primaries and dual 12V secondaries. So it makes the filament and HT easy.

[frequencycentral]

Looks great Chris, and a real tight squeeze to get it all in the enclosure. 

[Paul Marossy]

Oooo, nice work! 

[davent]

Wow Chris really impressive! Out of school a week... are you getting bored yet... and you manage to get this together.  Just how small is the enclosure  you used, there can't be any room left for anything else or is there something else up you sleeve?

Take care,
dave

[Auke Haarsma]

Wow! Very nice build, thanks for sharing!

Can't wait to hear the soundclips

[kurtlives]

Thanks for all the comments

Wow Chris really impressive! Out of school a week... are you getting bored yet... and you manage to get this together.  Just how small is the enclosure  you used, there can't be any room left for anything else or is there something else up you sleeve?

Take care,
dave

I had the enclosure all done and all the parts mounted about a month or so ago. So I really only had to wire it up and work out the bugs. A few rainy days and it was done!

Enclosure is a Hammond "DD". It measures 7.4 x 4.7 x 1.3. I wanted to fit an FX loop but three more pots, two jacks and some active circuitry would not fit . I also kinda wanted to add a relay for the PAB. But then I would have to get a 4pin or 5pin DIN jack and cable which I really didn't want to do. O well, those ideas are for another project

[modsquad]

Wow  ...did I say WOW   

Very nice, I aspire to be able to build something like this from the schematics.  Again 

[defaced]

Nice build!  I'm working on doing something with a tube preamp of some high gain amp channels.  I can't tell from the picture, but what power transformer are you using?  That's the one aspect of my build I haven't settled on yet. 

[kurtlives]

Nice build!  I'm working on doing something with a tube preamp of some high gain amp channels.  I can't tell from the picture, but what power transformer are you using?  That's the one aspect of my build I haven't settled on yet. 

Dual primaries are 120V, dual secondaries secondaries are 12V. Run the "two" transformers back to back. Tap off 12V in the middle, HT is provided at the end (120V).

[defaced]

I get it conceptually (it's pretty ingenious provided you account for half of the incoming VA not being there), I mean the actual model number of the transformer. 

[joegagan]

chris, kudos, that is beautiful work. can't wait to hear it.