Monthly Archives: June 2013

Replacing the Preamp Input Resistors

The existing 10k input resistors need to be replaced by 1M resistors to better match the impedance of the guitar pickups. (I suppose you don’t need to change the second stage but I replaced both input resistors anyway).

On the underside of the pcb near the input phono sockets you will find two very small surface mount (SMT)  resistors. These are clearly labelled 10k on the screenprinted pcb. Unsolder these and remove them completely then bend and trim the leads of two conventional 1Meg ohm resistors and solder them carefully to the small pads where the SMT resistors were.



A Look Inside the Finished Unit

I’ve just completed the wiring and testing of the MIDI functionality. In my previous automation Mark II box the MIDI hardware was mounted on a small veroboard and soldered directly onto the MIDI sockets. In this Mark III version I wired up the MIDI circuit on an Adafruit Protoshield pcb together with the circuitry for an I2C EEPROM which would allow more patches to be stored but is not yet supported by the software.


This shows the ‘stack’ of Arduino-style Shields – saves a lot of extra wiring doing it this way.


The MIDI sockets and the Audio Out socket on the back panel.


This is the MIDI/EEPROM protoshield. I haven’t plugged in the EEPROM yet as there is no support for it in the software.

The only remaining job is to wire the foot-switch socket for up/down preset selection.


It Works … Works … Works… Works ….

… after an initial period when I got lovely sound but no echoes! The only mode that gave the right sound was Tremolo with Reverb. After checking the voltages sent by the Arduino to the SKRM module on the Eccamatic board (the three control Pots P0, P1 and P2 and the Mode selector SW0, SW1 and SW2) and found them to be working as expected I was stumped for a while 😦

Anyway, to cut a long story short, I found I needed to join the signal from the input jack to both the Left and Right inputs on the Eccamatic/SKRM board (previously I only joined to the Left input). Once this was done – success, we had lovely echoes and a quick check showed all the emulations and presets were working as expected – happy days 😀

Some sound samples will follow as soon as I get a chance to record them.

Let There Be Sound …

… and there was sound!

Completed most of the remaining wiring and measured the HT and heater supply voltages before plugging in the valves:

HT = 197 V dc

Heater supply: 6.00 V dc

Next steps:

  • switch off and wait a while for the HT caps to discharge – I’m rather allergic to high voltages ;-).
  • Now plug in the two 6N3 valves and switch on
  • Yes!!!! The valves are heating up
I love the glow of hot valves :-)

I love the glow of hot valves 🙂

Before plugging in the SKRM-eTap2 module (which actually produces those luscious vintage echo sounds, courtesy of the FV-1 chip designed by the late Keith Barr (of Alesis fame) and carries the DSP program by my friend Piet Verbruggen) I did a loop through test of the audio path.

First, connect a jumper lead from the Left O/P of the SKRM (pin 13 ) to the left I/P (pin 2)

NB I couldn't find a supplier for the required 16 way (2x8) socket for the SKRM so I had to improvise by cutting the last four pins off a 20-way connector. The four holes at the bottom of the connector are empty, don't get confused like I did!

NB I couldn’t find a supplier for the required 16 way (2×8) socket for the SKRM so I had to improvise by cutting the last four pins off a 20-way connector. The four holes at the bottom of the connector are empty, don’t get confused like I did!

Now connect guitar to the front panel input jack and connect the rear output jack to a guitar amp. Switch on, wait for the valves to warm up, pluck those strings and – hey Presto – we got sound 😀

First impression – very little hum and hiss – and that’s without the case being screwed back together. The Arduino hardware is not powered up so I’m hoping that won’t generate much noise.

Front Panel is Done!

I removed all the front panel controls, deburred any holes or cutouts that needed it, cleaned the panel with white spirit and applied the print. Again there are a few blemishes. I’m beginning to think that the adhesive layer is not uniformly applied to the vinyl sheet and these blemishes are where there is less/no adhesive – so not my fault after all 🙂


Rear Panel Completed

It was quite tricky attaching such a big self-adhesive vinyl print. For my first attempt I cut the sheet exactly to the size of the panel but then I couldn’t avoid getting fingerprints under the edge where I peeled off the backing paper 😦

So, print another and this time leave plenty of extra material around the edges, especially at the short sides. It’s important to try to get the vinyl stuck down straight and not to let it develop any creases – stick one edge down and peel the backing off as you go, smoothing the vinyl down with a soft cloth. Even then I got a couple of air bubbles but managed to smooth most of these out at the edges; one or two little blemishes but I can’t be bothered to do another one!


Back Panel Drilled

Can you spot the balls-up? Next time I will measure actual the DIN sockets and not rely on the hole diameter in the ‘FrontDesigner’ model. Oh well, one good thing is, it prompted me to be a bit more creative with some ‘design features’ on the printed panel – you’ll just have to wait and see 😉