venerdì 15 luglio 2011


Much progress has been made and the NSS-Valvecaster is...ready :)

A *huge* "thank you!" and a hug goes to my father Mario who basically did all the wiring (as he's so much much better than me at soldering in narrow spaces!). NSS-Valvecaster would not be here without him!

I will soon post the pics of the final build. The voltage doubler works, is absolutely not noisy and to my view also very stable: your mileage as well as your opinion may vary but for this project and for my build I get a steady and consistent 22.7 V out of the 12V from the switching PSU. The "blue" channel runs with a 12AU7, the 12AT7 is already too "hot" for the crunch/semi-clean tone I want for this part of the circuit. For the second stage, so far the best candidate is a 12AX7. When driven hard by the 12AU7 it gives *a lot* of compression and a *true* distortion. This is something I would not have expected but the red channel has a really sweet distortion (not crunch) with lots of sustain and squealers. Bliss :) Next to come

1. Pictures of the build
2. Sound clips!
3. Bias points for reference
4. Study of the response of the circuit

For the moment, here is the layout I am creating

martedì 5 luglio 2011

Voltage doubler

This is my voltage doubler, based on a LT1054, drawn following the reference design provided by Linear Technologies:

I have implemented the circuit on a (tiny) Vero stripboard, ending up with the following thing

Honestly, it is not the neatest design ever :) But it works, and it has been fast enough to prototype and test (one lunch break). To demonstrate the functioning of the circuit, I recorded this little clip - with my phone handheld...bear with me!

There is a parasitic voltage drop, of about 1V at 12V input (theoretical 24V output). I think I will be fine with that. In the final part of the video, you can see the slow voltage drop after the input voltage is shut off. This is due to this rather large capacitor (100 uF) filtering the output of the circuit:

lunedì 4 luglio 2011

What is a Valvecaster?

But how does a Valvecaster work?

This is an awful sketch of the first Valvecaster, composing the "green" channel or the booster for the "red" channel.

It is built around the 12AT7 tube system, composed of two triodes packaged together, each of which amplified its input signal. The circuit is very easy to follow, at least in its basic functioning. The input signal is fed into the first stage, dubbed 12AT7(a), via a decoupling input filter (to prevent DC components and low-frequency hum). This stage amplifies the signal with a gain controlled by the variable resistor VR1. Subsequently, this amplified signal is injected into the 12AT7(b) stage, held at the maximum gain. A decoupling filter is in order since we just want the alternating signal (music) and not the DC bias applied to the anode. The output from this stage is again decoupled. A simple voltage control is provided with a partitioning VR2 resistor.

There it is! Neat and effective!

Happy family!

Indeed, talking of "The" Valvecaster is rather limitative. Indeed, this project is so simple that has led to all sorts of modifications and extensions...Just google "Valvecaster" and see for yourself!

Although I am rather pleased by my Valvecaster, I decided to create a better version of it. Many reasons led me to this.
  1. Design: the socket-mount is not exactly a neat work, especially given my poor soldering skills.
  2. Quality: raising the anodic voltage to 12V provided a terrific boost in performance (as opposed as simple 9V anodic voltage/12B heaters). So I want more! :)
  3. Performance: the top gain of the circuit is not that high. Even though this design does not lend itself to blistering saturations, I would like it to explore hotter territories than a simple crunch.
  4. Versatility: I would like to be able to switch between a mellower "green" channel for rhythm and a hotter "red" channel for lead.
What I came up with, is this:

I will build two Valvecaster stages. The first one will run on a 12AT7 tube, providing middle gain and nice creamy saturation. The second stage will sport a high-gain 12AX7 tube. A foot switch will route the signal after the first stage, either straight to the output ("green" channel), or into the second stage ("red" channel).

To reach higher anodic voltages, I will employ a charge-pump voltage doubler, built on the LT1054 chip. The circuit is very easy and is basically the reference design provided by the producer.

All the build, which will start in the next days, will be performed on Vero stripboards.

In the beginning...

...there was Matsumin. This distinguished Japanese guitarist decided to create a simple Tube stompbox with a "starved plate" design based on the 12AU7 sual triode.

What he ended up with it now known as the "Valvecaster". It is such a simple project which can be essentially mounted on a noval tube socket. Anybody who is able to handle a solder can build his/her own Valvecaster. Including me! This is my very own Valvecaster

The sample was recorded with my iphone sitting near the speaker of my Vypyr. The guitar goes through the valvecaster, then into the amp. Not bad, given the recording "technique" and the fact the circuit was running off a 9V battery. Yes, because this circuit can even be fed with a simple battery. Two issues arise, however.
  1. The battery will be drained at a stupidly fast rate. This is because the heaters suck up a lot of current. Even more, a tube is designed to work with a precise temperature of the filament: electrons literally boil off the filament...But only if this is hot enough. This happens when 12V are fed to the heaters (when connected in series). At 9V the filament is simply not hot enough. Indeed, the 12AU7 emits a faint glow when it is hot enough. With 9V the glow is not even there. This impacts sound quality.
  2. The anode voltage is again way too low. Although this is a specific characteristic of "starved plate" projects, 9V is really too low.
For the above reasons, I dug into computer cemeteries and I found a wonderful stabilized 12V PSU which I adapted to the project. Now this led to a huge improvement in the sound quality. Which was topped only by the usage of a sweet vintage Telefunken tube. My Valvecaster 1.0 was born!