Thus far my effects building experience has all been limited to assembling pre-made boards and soldering the components. Along the way I’ve tried to study the schematics of the original effect, such as the Tubescreamer, but none of it was really sinking in. To remedy this I have decided to go back to basics and learn more about the principles of audio electronics from the bottom up. I have defined ‘the bottom’ as a guitar tone control.
My starting point then is some kind of boost circuit. Many great tones from the 60’s were achieved by driving the front end of a tube amp quite hard using something like the Rangemaster. Certainly Clapton was rumoured to have used one to brighten up his Les Paul in the Beano sessions.
So I looked for boost circuit schematics. One of the first ones I found was a really good single page explanation on how a boost works:
This was perfect as it explained it in dumb-musicians terms. For more detail (and for later consumption) I am reading this:
Things of note:
- Notice how similar a basic common-emitter transistor amp is to a common-cathode tube amplifier?
- All boost circuits are basically the same. They might bias the transistor differently, or they might have extra components (for protection or good behaviour) but fundamentally a clean boost is a common-emitter amp.
- I can probably build one of these quite quickly.
Which one to build first?
My choices were between the ZVEX Super hard-on (the SHO) or the Electro-harmonix LPB-1 as they have plenty of reputation in the industry and they’re incredibly simple devices. I decided on the LPB-1 for no apparent reason other than it featured in an article on Beavis Audio’s site which helped me greatly in knowing how to approach this. Here’s that article, go and read it if you’re interested in doing this.
As encouraged by the beavis article I downloaded DIY Layout Creator, and literally created my own veroboard layout using the LPB- 1 schematic. This was loads of fun and surprisingly simple. It forced me to know the schematic in great detail. I tried to keep in mind the how-it-workz article so I had a feel for why each component was there. Here’s the resulting layout:
As you can see, it is pretty simple and even though I haven’t done this in years I was able to assemble all the on-board components in about half an hour. I bet I can do it in half that time now as I kept screwing up.
I made a part substitution as I failed to order some 2N5088 transistors. I was under the impression I already had some, but didn’t. Instead I used a 2N4401.
So, once I’d wired up power, switching and signal jacks I plugged it into my amp to try it out, but all I got was hiss and distortion. So I unplugged and checked all the solder joints, reflowed a few and tried again. No different. I began to suspect the transistor as that was the only deviation from the schematic. I tried a 2N3904 this time, with much better results. I do get a boost (not as much as expected) and also lots more bass, which isn’t necessarily a good thing. At this point I was discouraged.
Time to re-evaluate. After some thought, I recalled the Beavis document, and some other discussions surrounding the SHO boost; check your transistor pin-outs. I had stupidly placed the transistors pins into the overboard without honestly checking that the collector, emitter, base pins were actually in the right place. I couldn’t tell what brand I was using, so I used a technique I found online here to derive the pins using the diode test function of my multimeter. And sure enough, I had it wired incorrectly. Literally 180 degrees wrong.
Alright, so put the transistor in the right way around and it now works. There’s definitely lots of gain in this device, and lots of bottom end boost too. And its more than happy driving the hell out of a tube-amp.
The following pictures is before I trimmed the board down to its final size. You can also see I went for an on-board trimmer (as I didn’t have a 100k pot to hand). I may well leave it like this for when it gets integrated into my OD2.