Tube Sound OD / Briggs PAF Overdrive

This little circuit is part of DIY pedal designs early beginnings. I first ran into it when I was surfing the early DIY forums in the early 2000s but never really paid it much attention, I wish I had done because it sounds great!

Here's the original newsgroup posting in which Arsenio Novo presents his circuit idea (from the 4th July 1995!);

"********************
From: Arsenio.Novo@mba.org (Arsenio Novo)
Date: 04 Jul 95 23:04:50
Newsgroups: rec.music.makers.guitar
Subject: New Overdrive Circuit
Organization: MtlNet (MBA.org) MBA [514-465-8524] Brossard, QC

Hi,

I've noted that talk on this echo always comes back to the subject of
overdrive distortion. Whether generated by a vacuum tube amplifier or
a transistor amplifier there seems to be undeniable differences to me
as well.

Lately, I was tinkering with an unusual transistor circuit
configuration I had come upon a few years ago and made a few
modifications to the circuit that turned it into one beautiful
screaming "tube-like" overdrive but without the wall of noise these
things usually make. When pushed it even makes that distintive
"zoo-zoo" sound...!!!

The original circuit was simply a complementary matched pair of
transistors connected so that all the terminals overlapped. i.e. both
bases tied together, emitter of the PNP connected to the collector of
the NPN, and the collector of the PNP connected to the emitter of the
NPN. 

This transistor pair is then biased by 2 equal value resistors in each
of the compound legs, one to the positive supply and the other to
ground common. The signal is coupled to the base pair leg and the
output is picked off either of the other 2 legs.

The result is that the above circuit exhibits the behaviour of a
multiplier over a range of signal values. It basically performs a
sin function: in other words a frequency doubler.

This doesn't have a very good distortion sound though because it is
rather "burpy and buzzy". However, lately I was toying with the
circuit when I offhandedly decided to try doing something to it just
to see what would result.

After adding a large cap from the NPN's emitter to ground the thing
went wild on me... WANGO ZE TANGO! SUPREMO DISTORTION! I then
proceeded to refine the circuit a little more and got a better
understanding of what it was doing.

The final schematic follows but first a couple of notes on the
circuit. The "bias balance" trimmer should be adjusted for a
symetrical clipping threshold of the output signal as viewed on a
scope. Short of this it can be easily set by "ear" for the most
sensitivity somewhere around mid-turn.

The input should be driven by a lo-z stage if your electric guitar
doesn't have a built-in pre-amp. You can alter the emitter capacitor
value in a range from 0.1uF to 1uF in order to obtain various basement
characteristics but I found the indicated value is a good compromise.
The input capacitor should not vary much either though because if it
is made too large the circuit goes balistic and cuts out on the tutti.

The operation of the circuit more closely resembles a vacuum tube than
a diode clipper does because of the strong square law characteristic.
This is due to the negative feedback around both base-emitter pairs.
This feedback accentuates the junction non-linear behaviour manyfold.

Thus each transitor drives the other even harder so that the transfer
curve ends up more logarithmic than is typical of a single transistor.
In other words: the clipping is gradual and not abrupt like it is in
the case of a silicon diode. Typically a lot more 2nd harmonic is
produced as well. As a bonus the waveform folds over on itself when
the circuit is overdriven!

Now in the interest of the common good I donate this design to the
public domain for personal use but retain copyright and reserve all
rights for any commercial purpose. In other words build one for you
and your friend but if you have it massed produced for profit I only
ask a fair share.

Please, do try the circuit and leave any comments in private at my
e-mail address: arsenio.novo@mba.org
****************"

The original message had ASCII artwork depicting the circuit schematic but it was hard to read, here's an image of the schematic instead;

As you can see it's a strange arrangement for the two transistors, something (as far as I can remember) that I've not seen done before or since.

The sound is excellent for such a small parts count and can be described as "COT" like (as in similar to the Lovepedal COT/Electra Overdrive) but I find the clipping to have a much more natural feel and a less drastic on set than the Electra circuit. You can get all those low to medium gain blues tones from the circuit and much more if you play with your guitar volume knob. I especially liked the tone with single coils where the break up felt very natural and dynamic. This circuit certainly does give you something a little different from all those Tubescreamer type controls.

It sounds really good even if you just build it as it is, there are a few modifications that you can make to the circuit to improve the tone even more;

1) The input impedance of the circuit is low and it loads your guitar down - the addition of an input buffer sorts this right out.

2) There's not gain control! Not a massive problem in itself as you can control all the gain you need from your guitar volume pot but some people like a gain control on an overdrive.

3) No tone controls. Again, not a massive problem but it would be nice to have a little control over the top end content.

So, here's the circuit with a few mods and additions;

So there you have it - controls for Bass, Gain, Treble, Boost and Volume. The 20k trim provides bias (I actually used a 50k trim in my build), you can bias by ear - just tweak until it sounds best! In reality I wouldn't build this up with all of those controls. I'd build it with the 2n2 cap subbed for a 33nF with a 33k resistor in series with it and remove the bass control completely. Also I'd bypass the gain control so the circuit is running at full gain all the time. That would leave you with the Boost, Treble and Volume controls - the standard 3 knob overdrive layout (you could even have the boost knob as an internal trim pot). I also had good results subbing in the Big Muff type tone control from the BSIABII (The same control that is in the Zvex Box of Rock) in place of the treble control and omitting the Bass control.

[EDIT] Here's another few mods I'd like to suggest for this little circuit. Basing these mods on the first schematic of the Tube Sound OD;

"The TubeSoundOverdrive is a nice sounding little circuit. There are a few mods I make to this thing when I build them;
1 - I add in a 100kB (Linear) pot wired are a variable resistor in series after the input cap, this allows you to smooth the tone out, reduce the gain a little and take away some of the cutting highs (in some builds where I don't want too many knobs I just use a 10-15k resistor instead of the pot),

2 - I raise the input cap to 22/33nF (depending on how much bass is wanted),

3/4 - I add in a 4n7 cap in parallel to a 500kA (Log) volume pot, this further reduces the overpowering top end,

5 - Instead of using a 10uF bypass cap on the 10k resistor from the NPN transistors emiitor I use a 470nF cap as the bypass, I then wire up a series configuration of a 10uF cap with a 4k7A (Log) pot, this is then wired in parallel to the 470nF bypass cap. This mod allows you to control the overall gain/low end saturation of the build. With this mod you can go from treble boost right through to a full range boost."

If you're sick of tubescreamer type overdrives I'd strongly recommend you give this little circuit a build - you won't be disappointed.