Dolly the Tube Screamer

All this talk of guitars set me thinking about the old days when, as a kid, I used to make replicas of my MXR Distortion Plus ...

Unlike most other areas of audio engineering, guitar amplification is NOT about sound reproduction - rather it is about sound production. The distinction is important; sound reproduction (as in monitoring and what was once called "Hi-Fi") is about generating a faithful acoustic representation of an electronic signal. Guitar amplification, in marked contrast, is part of the system creating the signal. The sound of the guitar amplifier is not (necessarily) at all neutral, introducing components of the overall sound that are important. Most important of these is the family of sounds when (particularly valve) amplifiers are pushed into overdrive, when the resulting distortion can have a pleasant, musical sound in the right context.

Recognition of this fact led to the production of effects pedals (like my old MXR, above) to create (or, at least, emulate) the sound without having to go to the inconvenience (and noise exposure) of over-driving an amplifier. It also led to the appearance of a whole lot of amplifier emulations, exemplified by those built into my new amplifier. Between these extremes of the early, simple distortion (or "fuzz") box and modern complex DSP emulations of classic amplifiers in overdrive there was a whole generation of more complicated distortion effects which I missed over the past few decades. I decided to "catch up" by making a clone of an Ibanez Tube Screamer - thus "Dolly" was born.

The Tube Screamer has three controls; "Drive", "Tone" and a final "Level" control, as seen on my cloned unit...

The schematic can be found at lots of sites over the 'net - start your search here. The most important part of the Tube Screamer (and many other distortion pedals) is the exploitation of the clipping effects of a pair of back-to-back diodes, as seen here in the "clipping" stage of my "clone"...

I used 1N4148 diodes and am (at present) using a TL072 op-amp (I'll change it for one of the original JRC4558s when I get round to making a final version).

Where the Tube Screamer is distinguished from other distortion pedals (including, for example, my old MXR) is in partnering the diode clipping section with filtering circuits - including an adjustable shelving HF section, giving lift or cut to high frequency components both of the original signal and of the harmonics generated by the clipping. The tone section and other aspects of the circuit are seen "round the back"...

Having made the unit, I thought I'd take the opportunity to show some waveforms illustrating its effect on a sinusoidal input (at 2kHz).

Here's the response of the device in "Bypass" mode (the original Tube Screamer has electronic "bypass" switching implemented by some n-channel FETS in a configuration I used before for the Tx audio mute of my Funster Plus rig) controlled by a push button (Dolly uses a 555 in a toggling bistable circuit, whereas the original uses a discrete flip-flop)...

Turning on the unit, with the "Drive" control on max (giving greatest clipping effect from the diodes) and the "Tone" on max (giving HF shelving boost) gives this response (in all the following images I adjusted the "Level" control to keep the output amplitude roughly constant)...

Rolling off the "Tone" to minimum, whilst keeping the "Drive" at maximum makes the waveform "smoother" - and has exactly the same effect on the sound...

Turning the "Drive" to minimum reduces the distortion effect, influencing the sound only when the guitar is played loudly - this sounds more like an amplifier close to its limit, being pushed over the edge into saturation only by the loudest signal components. It sounds nice!

With tone setting on "Max", the effect on the 2kHz sinewave is seen as a tendency toward a triangular wave...

Backing off the tone to minimum, all but removes the visible distortion of the 2kHz waveform...

In truth, all the sounds available from this clone of the Tube Screamer are pretty close to sounds already well-covered by the amplifier emulations built into my new Roland Cube amplifier - but it has been fun to clone Dolly. I might make a PCB and knock up a few copies for friends, just as I did in the old days!

 ...-.- de m0xpd

Cube 40XL Footswitch

I came home from yesterday's G-QRP Mini Convention at Rishworth with (amongst other things) some pushbutton switches from Bowood...

intending to make a footswitch for my new Roland Cube 40XL guitar amplifier. The project was not quite as trivial as expected and other Cube owners may be interested to hear my findings. But first, a little background...

2012 has been for me thus far what Liz might call annus horribilis. Illness has robbed me of some of the energy and enthusiasm for leisure projects and work has been even more of a distraction than usual. Even the virtual organ project (Blogs Passim) has taken a back seat - it is upstairs and it requires that you sit up straight on the uncomfortable, hard bench - hardly the thing when you're not feeling 100%. I did, however, remember that there is one thing that a couch potato can do instead of watching (or even whilst watching) the idiot's lantern - play guitar!

I treated myself to a nice new guitar in celebration of this re-discovered pastime - a Peerless New York.

Then thoughts turned to amplifiers. I already have more than my fair share of guitars and amplifiers, but wanted something that included some of the interesting developments of the past few decades (on-board effects, "modelling" etc). I looked at Fender Super Champs, Line 6 Spiders and the Roland Cube 40XL. I settled on the latter, not least because I could make a simple footswitch to control the many features (rather than pay the manufacturer for one).

So, yesterday I started to knock up a trial footswitch, taking the chance to experiment with an idea for an "enclosure" made of scrap MDF. The piece of scrap that first came to hand was about right for three switches, so that's what I made...

Here's the circuit I used...

(I hope regular readers of this irregular blog will excuse the naive schematics in this post - this material might be of interest to other readers for whom electronics is not the vernacular).

The footswitch worked perfectly in controlling the Effects and Reverb. It also worked perfectly in switching between channels and selecting the alternative "Solo" settings. However, it refused to do anything in controlling the Delay / Looper functions - a particular disappointment as this was the feature I most urgently needed foot control for!

A quick sniff around the net led me to fellow radio amateur Steve, GW1XVC's excellent page on QRZ.com. Steve has also homebrewed a footswitch for the Cube 40XL and his switch works, as he proves in a video on YouTube. His QRZ page included the magic phrase "normally closed", describing the switches he used in his successful footswitch. Mine from Bowood (like all the other momentary action single pole switches in my junk boxes) are "normally open".

I tried "simulating" a normally closed switch (by holding down both switches and momentarily opening one of them) and - hey presto - the looper functions worked under control of my switches. So - I think I need to make an important announcement...

Owners of Roland's Excellent Cube XL amplifiers who, like me, are too cheap to buy expensive commercial footswitches, can use normally open switches for FX/Reverb and Channel/Solo functions BUT MUST USE NORMALLY CLOSED (/"PUSH TO BREAK") SWITCHES FOR THE DELAY/LOOPER functions. 

So - having got that off my chest, what do I do with the apparently useless normally open switches?

I measured the electrical behaviour at the footswitch sockets - open circuit, the control inputs sit at about +3.5V relative to the reference voltage on the sleeves of all the sockets (the footswitch inputs are on 1/4 inch Tip, Ring, Sleeve ('TRS') jack sockets - more commonly known as 'stereo' jacks). When shorted to the sleeve, a current of 450microAmps flows.

I could easily see how to control the looper with the required "normally closed" behaviour if I had some power available, but I didn't want to add a battery to complicate the footswitch - so I came up with a little twist to change the switching behaviour so as to emulate the "normally closed" function using a 2n3904 and a 10k resistor...

For those who don't know, both 2n3904 transistors and 10k resistors are widely available for less money than the switches.

The modified footswitch works perfectly. I just made the change on the two switches used for the Delay/Looper functions - the other switch on my prototype controls the Effects in "normally open" mode.

It works perfectly for me but now for the caveats:

1. It hasn't been tested on another Cube 40XL, so I can't guarantee it will work perfectly for you and...
2. It might not like the EMC-hostile conditions of live stage applications (but I'm sure that any issues could be resolved with a single additional capacitor for each transistor) and... 
3. My footswitch wasn't designed to survive the violent attentions of thrash metal players in Jack Boots. Steve, GW1XVC's rugged construction is more equal to that task!  

Now I'm off to play some guitar

...-.- de m0xpd