The Long and Winding Road

Part the Third

“You certainly usually find something, if you look, but it is not always quite the something you were after.”
J.R.R. Tolkien, The Hobbit

In this the penultimate part of the journey the path forks and I take a look at some experiments with using a valve preamp and improvements to the original FET preamp.

Opening the Valves

Eric Thacker, better known as Ecca, had demonstrated on the Shadows Music forum that Piet’s FET and op-amp preamp design could be replaced with a valve (tube) preamp that was available from a number of eBay sellers in Hong Kong and China. (Here, for example).

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The valve preamp and Eccamatic pcb mounted in an enclosure.

The circuit was originally designed to be used as a Hi-Fi stereo preamp using two 6N3 dual triode valves, one valve for each stereo channel. For the eTap2HW module we only require a mono audio pathway so the left channel of the stereo pair was used as the input preamp feeding the echo module and the right channel became the recovery amp, taking the echo module output and acting as a buffer and output level control.

Eric had produced a PCB for his ‘Eccamatic’ design which allowed the echo module to be plugged in, provided a step-up power supply to generate the HT voltage and the LT heater supply for the valves and had connections for the three pots that adjust the echo settings.

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The bits for Eric Thacker’s Eccamatic pcb.

Having already designed the automation circuitry it was easy to adapt the design so that the Arduino Uno would feed the pot inputs on the Eccamatic board, reading the required settings from pots mounted on the front panel.

The front panel also had input and output pots, controlling the signal levels.

I have described this valve EchoTapper build in a series of earlier posts and if you’d like more details just click on the Valve entry in the Categories list on the right of this page.

When the new unit had been completed, the sound was quite pleasing with less hiss and noise than the FET preamp but rather ‘sterile’ as might have been expected from it’s origins as a Hi-Fi design. As I am sure you will know, guitar amplifiers and the early Meazzi echo units have circuits that are far from Hi-Fi for a very good reason (at least in the case of the typical valve-based guitar amp) – the almost perfectly flat frequency response of a Hi-Fi design and it’s deliberate lack of harmonic distortion (achieved largely through the use of negative feedback) is really not what guitarists want or indeed need to get the sound they desire. The original Meazzi preamp is hardly a classic of valve design but somehow has that magic sound that we have come to love.

Piet’s FET preamp design had made use of the ‘Fetzer Valve’ circuit which uses an FET to emulate the sound of a triode valve so it does create harmonically rich sound when the input level control is set correctly to match the guitar pickup level to ‘push’ the Fetzer ‘valve’ into generating the pleasant sounding harmonics we all like.

Steve Mitchell soon had his thinking cap on and he came up with a number of modifications to the valve circuitry that, according to his simulations of the circuit which he had made using the TINA SPICE modeller, should tailor the frequency response better to the typical guitar pickup signal and also generate those desirable harmonics to give that rich sound we all love.

Steve and I came up with a three stage process that culminated in a final design that sounded great.

Stage 1: Remove the built-in stereo volume control pot and increase the input impedance

Stage 1 was essential to allow the input and output level controls to be separated and to provide a better match to the typical guitar pickup’s high impedance.

Stage 2: Remove the negative feedback from the preamp and add Meazzi-like tone shaping

Stage 2 was regarded as highly recommended to obtain a better tone and more gain which would encourage the desirable valve harmonics and tailor the frequency response which was very flat and resulted in a ‘lifeless’ sound, thus improving the sound for guitar purposes.

Stage 3: Add a separate Gain pot and three-way tone switching

The Stage 1 and Stage 2 modifications had resulted in a really good sounding preamp but Steve came up with Stage 3 which would take the whole thing to another level. Whilst not regarded as essential, these mods did make the unit even more flexible. There would now be three ‘level’ controls, the new Gain pot controlling the drive to the second valve stage in the left channel of the amp. The Volume pot controls the signal to the echo module and the Master pot controls the level from the output socket.

Modified Vavle Preamp Flowchart

Flowchart by Steve Mitchell (SCM)

Modified Valve Preamp Freq Responset

Predicted frequency response of the modified circuit

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Front Panel of the final Valve-based Echotapper

The 3-way tone switch provided ‘normal’, ‘vintage’ and ‘warm’ options by tailoring the bass roll-off. On the unit in the picture (built for Mario Voltolini in Italy) I also added, at his request, a Cutting Edge Filter (CEF) which could be switched in and out if required.

Biting the Bulletins

In parallel with the improvements to the valve preamp, Steve was also working on a series of modifications to Piet’s FET preamp design.

One of the problems with any FET is that individual devices, even from the same batch, can have parameters that vary over quite a wide range and this makes it difficult to design a circuit that will allow the FET to operate in the optimum part of its characteristics.

Steve and Rolf Holmberg collaborated and came up with a spreadsheet that would calculate the best resistor values to bias the FET into the optimum operating point. This would be a bit complex for the average user to get to grips with so Steve and I came up with a simpler ‘flowchart’ approach. This would allow the best operating point for the FET to be found using a simple voltage measurement and a little trial an error with a few preferred resistor values.

Steve put together a series of ‘bulletins’ that were published on Piet’s EchoTapper blog which also included simple modifications to improve positive signal headroom, improving the FET gain and reducing white noise (hiss). These mostly involved removing or changing some resistor values and adding some extra capacitors.

Piet has since modified his original preamp PCB design to accommodate these improvements and this is documented on his blog.

What Next?

Now I had two excellent sounding echo units but I had an urge to see if it would be possible to squeeze all this technology into a smaller package, one that would fit on a pedal board or sit neatly on the floor at my feet like a typical guitar effects pedal.

Thus began the germ of an idea that would result in the Blue Nebula. It had to be compact, so that ruled out the use of valves and it needed to be much easier to build, with minimal off-board wiring so that, hopefully, anyone with sufficient experience would be able to put one together from a kit or buy a completed built-to-order unit at reasonable cost.

We had to go from this …

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My first automated eTap2hw

 

to this …

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The final Blue Nebula Design

 

To be continued.

Another New Build

This unit was built for Mario Voltolini in Italy. It uses a new motherboard pcb and LCD display board designed by Eric Thacker and a number of new subassembly pcbs designed by yours truly. These new boards support the modifications devised by Steve Mitchell and myself to improve the sound and suitability of the Yuan Jing 6N3 valve preamp for use with a guitar input, rather than its original design purpose as a HiFi preamp.

There is also a neat little MIDI interface board and a switchable Cutting Edge Filter (CEF) as per the design by Charlie Hall.

Mario requested the ability to turn off the echo by using a remote footswitch so I added a bypass circuit using a signal relay to bypass the echo module. It’s not true bypass as we wanted to retain the tone of the valve preamp when bypassed.

Click on the thumbnails below for a larger picture and explanatory captions.

Test recordings


Valve Echotapper Recordings

I’ve just completed what I’m calling ‘Stage 3’ of the modifications to the Yuan Jing 6N3 valve (tube) pre-amplifier that Steve Mitchell and I have been working on. As you probably know, Eric Thacker (aka Ecca) originally came up with the idea to use a fairly low-cost ready-made Chinese valve preamp that is available from various sellers on eBay, as a front end to the eTap2HW module programmed by Piet Verbruggen to emulate the vintage echo machines used by  Hank Marvin, the lead guitarist of the British group The Shadows.

Based on the ‘Matisse’ circuit, it’s design is intended as a HiFi preamp and as such the original sound was not ideal when it came to be used as a preamp ahead of the SKRM-eTap2 echo module. Working with a simulation of the circuit, Steve came up with a number of changes to make it more guitar and eTap2 friendly. I then tested these in the real world and gave feedback to Steve which enabled him to make further tweaks until we were happy with the result; though no doubt he’s busy thinking up a few more for me to try even as I’m typing these words 🙂

I will shortly be providing the details of how to carry out these modifications but in the meantime I feel the time is right to publish a couple of test recordings that I have made using the ‘Stage 3 Modified’ Valve Echotapper so others can judge the results for themselves.

In both these recordings (which are made for the sole purpose of education and research) I have used the original recordings by The Shadows but have been able to turn Hank’s lead guitar parts on and off as and when I wish. I recorded myself playing the lead part to this ‘backing track’ and rendered the finished tracks you will hear below and alternated the lead part between Hank and myself. (There is a short section just after the start of “Theme for Young Lovers” where we both play together).

No additional effects were added to Theme for Young Lovers. Blue Star had some EQ added to the recorded track before rendering it to the final mp3.

eTap2HW Poll Results

Thanks to everyone who took the time to listen to the samples from my three eTap2HW-based EchoTapper variants and cast a vote in my poll on which sound they preferred.

There was a very clear preference for Track ‘C’ which polled 55% of the votes, in second place was Track ‘B’ which polled 30% and coming well up the rear was Track ‘A’ with a mere 15%.

Now it’s time to reveal the details of the ‘competitors’…

  • Track ‘A’ was the unit with the valve preamp (15%)
  • Track ‘B’ was the unit with the unmodified FET preamp (30%)
  • Track ‘C’ was the unit with a modified FET preamp with optimized bias (55%)

Which really makes me wonder about all the fuss and hullabaloo on some of the Shadows forums regarding how ‘good’ the valve preamp-based units sound. My personal opinion is that the valve preamp sounds too ‘mellow’ with not enough bite to get the sound of the early Shadows recordings. It does sound nice but comes nowhere near ‘that sound’ but maybe with some modifications to the circuit it could get closer – we shall see.

Incidentally, I also prefer the sound of the modified FET preamp, it gives more gain and punch though Piet’s original FET design is also excellent and the results of individual units can vary depending on the wide manufacturing spread of the specs for individual FETs.

The original sound samples are still available here if you’d like to audition them (again).

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.

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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.

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

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

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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.

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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.